Why engine driveline components break with mis-matched RPM downshifts
08-29-2014, 08:58 AM
#16
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Since I've missed all the other threads, and have no desire to go read them, I'll add my two cents here. Both cents include the assumption that the car in question is a 5 speed. I'm also agreeing with Mark.
Theory:
Ultimately, from a purely physics point of view, the forces going into the transmission will always follow the same curves based on vehicle speed and engine RPM. It doesn't really matter if you're doing an upshift or downshift, the rotating mass of the engine will exert the same force (or resistance to change) at the same rpm, and the vehicle will also always exert the same force (resistance to change) depending on current road speed. The net difference between those two levels of force is what the transmission & clutch must dissipate; typically as heat.
The rate of change does matter. The rate of change matters in that when smoothly accelerating while in gear, the change in force over time is is much lower than when releasing (or especially dumping) a clutch .. either on an upshift or downshift. The rate of change also matters from a materials standpoint, in that it may be possible to exceed a specific component's ability to absorb those forces when there is a large spike in localized force. The rate of change also changes the quantity of heat being placed into a specific component over a specific amount of time. Another factor that matters is the design of the components taking the load. It is possible for a component's design to be asymmetrical, or able to withstand more force in one direction than another. A typical ring & pinion and typical transmission gears are examples of this; they are designed to exhibit more strength (resist more force) when the car is going forward. This design consideration can also carry into the differential or transmission housing and resistance to loads and flexing of the case under those conditions.
Conclusions:
Design considerations: I have never looked at the internals of a manual 928 gerarbox & hopefully I never will have that need. However, I am going to assume that the gears inside the 928 gearbox are more able to resist force under acceleration than deceleration. The case and bearing carriers may also be more resistant to flexing under extreme loads when loaded under acceleration then deceleration.
Physics: In my go-to equation for thumbnail physics guess-timations, E=MC^2, velocity is a squared value. That means that road speed will be a huge factor in the amount of energy that needs to be dissipated in a 5th gear downshift.
Probable effects: Downshfting from 5th to 4th at speed (with a low engine rpm) will put the greatest differential of force possible into the transmission. It will also be loading the gears and transmission in the direction they are weakest.
Examples:
Lets look at two situations. The first is a car at 120 mph, the second is a car at 60 mph. All we we want to see is the amount energy involved. Remember, E=MC^2 shows velocity as a square, and we are interested in the energy we need to dissipate. Using super-cheesy math and ignoring units:
120^2 = 14,400
60^2 = 3600
From a straight velocity standpoint, the 120 mph car is putting 4 times the energy into the transmission of the 60 mph car.
If you are using the transmission as a brake, it only gets worse. You need to look at the difference between the squares of the starting and ending values of each speed differential.
(Suggested) Practice:
Avoid the 5-4 downshift when possible. Use rev-matching if a 5-4 downshift is necessary. 5-4 downshifts while doing heal-toe work are likely to be hard on the transmission. It is probably worth mentioning that using the transmission to make the engine function as a brake is likely generally a bad idea. Its primary design goal is typically to turn engine rotation into forward motion, not be used for braking loads.
Footnotes:
I'm one of those guys who hits the clutch in a braking zone & just stands on the brakes until the car is at the speed I want. I'll also typically move the car into neutral while braking, and usually will partially release the clutch. As I turn in towards my transition point, I'll almost simultaneously blip the throttle, add clutch, and select he desired gear. You should know the rest...
I never was much of a heal-toe downshifter. Once I achieved "smooth enough" with the technique, I stopped using it. It didn't improve my lap times, and arguably used up concentration and potential physical motion that could be spent in other areas if necessary. Not long after that, I also transitioned to a much faster car. One with really good brakes and plenty of power. In that car, the brakes generally out-performed the engine's ability to keep pace with heal-toe downshifts even with a light flywheel. In that car, there was no point in even considering the use of heal-toe.
Theory:
Ultimately, from a purely physics point of view, the forces going into the transmission will always follow the same curves based on vehicle speed and engine RPM. It doesn't really matter if you're doing an upshift or downshift, the rotating mass of the engine will exert the same force (or resistance to change) at the same rpm, and the vehicle will also always exert the same force (resistance to change) depending on current road speed. The net difference between those two levels of force is what the transmission & clutch must dissipate; typically as heat.
The rate of change does matter. The rate of change matters in that when smoothly accelerating while in gear, the change in force over time is is much lower than when releasing (or especially dumping) a clutch .. either on an upshift or downshift. The rate of change also matters from a materials standpoint, in that it may be possible to exceed a specific component's ability to absorb those forces when there is a large spike in localized force. The rate of change also changes the quantity of heat being placed into a specific component over a specific amount of time. Another factor that matters is the design of the components taking the load. It is possible for a component's design to be asymmetrical, or able to withstand more force in one direction than another. A typical ring & pinion and typical transmission gears are examples of this; they are designed to exhibit more strength (resist more force) when the car is going forward. This design consideration can also carry into the differential or transmission housing and resistance to loads and flexing of the case under those conditions.
Conclusions:
Design considerations: I have never looked at the internals of a manual 928 gerarbox & hopefully I never will have that need. However, I am going to assume that the gears inside the 928 gearbox are more able to resist force under acceleration than deceleration. The case and bearing carriers may also be more resistant to flexing under extreme loads when loaded under acceleration then deceleration.
Physics: In my go-to equation for thumbnail physics guess-timations, E=MC^2, velocity is a squared value. That means that road speed will be a huge factor in the amount of energy that needs to be dissipated in a 5th gear downshift.
Probable effects: Downshfting from 5th to 4th at speed (with a low engine rpm) will put the greatest differential of force possible into the transmission. It will also be loading the gears and transmission in the direction they are weakest.
Examples:
Lets look at two situations. The first is a car at 120 mph, the second is a car at 60 mph. All we we want to see is the amount energy involved. Remember, E=MC^2 shows velocity as a square, and we are interested in the energy we need to dissipate. Using super-cheesy math and ignoring units:
120^2 = 14,400
60^2 = 3600
From a straight velocity standpoint, the 120 mph car is putting 4 times the energy into the transmission of the 60 mph car.
If you are using the transmission as a brake, it only gets worse. You need to look at the difference between the squares of the starting and ending values of each speed differential.
(Suggested) Practice:
Avoid the 5-4 downshift when possible. Use rev-matching if a 5-4 downshift is necessary. 5-4 downshifts while doing heal-toe work are likely to be hard on the transmission. It is probably worth mentioning that using the transmission to make the engine function as a brake is likely generally a bad idea. Its primary design goal is typically to turn engine rotation into forward motion, not be used for braking loads.
Footnotes:
I'm one of those guys who hits the clutch in a braking zone & just stands on the brakes until the car is at the speed I want. I'll also typically move the car into neutral while braking, and usually will partially release the clutch. As I turn in towards my transition point, I'll almost simultaneously blip the throttle, add clutch, and select he desired gear. You should know the rest...
I never was much of a heal-toe downshifter. Once I achieved "smooth enough" with the technique, I stopped using it. It didn't improve my lap times, and arguably used up concentration and potential physical motion that could be spent in other areas if necessary. Not long after that, I also transitioned to a much faster car. One with really good brakes and plenty of power. In that car, the brakes generally out-performed the engine's ability to keep pace with heal-toe downshifts even with a light flywheel. In that car, there was no point in even considering the use of heal-toe.
08-29-2014, 09:17 AM
#17
Mr. Excitement
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Since I've missed all the other threads, and have no desire to go read them, I'll add my two cents here. Both cents include the assumption that the car in question is a 5 speed. I'm also agreeing with Mark.
Theory:
Ultimately, from a purely physics point of view, the forces going into the transmission will always follow the same curves based on vehicle speed and engine RPM. It doesn't really matter if you're doing an upshift or downshift, the rotating mass of the engine will exert the same force (or resistance to change) at the same rpm, and the vehicle will also always exert the same force (resistance to change) depending on current road speed. The net difference between those two levels of force is what the transmission & clutch must dissipate; typically as heat.
The rate of change does matter. The rate of change matters in that when smoothly accelerating while in gear, the change in force over time is is much lower than when releasing (or especially dumping) a clutch .. either on an upshift or downshift. The rate of change also matters from a materials standpoint, in that it may be possible to exceed a specific component's ability to absorb those forces when there is a large spike in localized force. The rate of change also changes the quantity of heat being placed into a specific component over a specific amount of time. Another factor that matters is the design of the components taking the load. It is possible for a component's design to be asymmetrical, or able to withstand more force in one direction than another. A typical ring & pinion and typical transmission gears are examples of this; they are designed to exhibit more strength (resist more force) when the car is going forward. This design consideration can also carry into the differential or transmission housing and resistance to loads and flexing of the case under those conditions.
Conclusions:
Design considerations: I have never looked at the internals of a manual 928 gerarbox & hopefully I never will have that need. However, I am going to assume that the gears inside the 928 gearbox are more able to resist force under acceleration than deceleration. The case and bearing carriers may also be more resistant to flexing under extreme loads when loaded under acceleration then deceleration.
Physics: In my go-to equation for thumbnail physics guess-timations, E=MC^2, velocity is a squared value. That means that road speed will be a huge factor in the amount of energy that needs to be dissipated in a 5th gear downshift.
Probable effects: Downshfting from 5th to 4th at speed (with a low engine rpm) will put the greatest differential of force possible into the transmission. It will also be loading the gears and transmission in the direction they are weakest.
Examples:
Lets look at two situations. The first is a car at 120 mph, the second is a car at 60 mph. All we we want to see is the amount energy involved. Remember, E=MC^2 shows velocity as a square, and we are interested in the energy we need to dissipate. Using super-cheesy math and ignoring units:
120^2 = 14,400
60^2 = 3600
From a straight velocity standpoint, the 120 mph car is putting 4 times the energy into the transmission of the 60 mph car.
If you are using the transmission as a brake, it only gets worse. You need to look at the difference between the squares of the starting and ending values of each speed differential.
(Suggested) Practice:
Avoid the 5-4 downshift when possible. Use rev-matching if a 5-4 downshift is necessary. 5-4 downshifts while doing heal-toe work are likely to be hard on the transmission. It is probably worth mentioning that using the transmission to make the engine function as a brake is likely generally a bad idea. Its primary design goal is typically to turn engine rotation into forward motion, not be used for braking loads.
Footnotes:
I'm one of those guys who hits the clutch in a braking zone & just stands on the brakes until the car is at the speed I want. I'll also typically move the car into neutral while braking, and usually will partially release the clutch. As I turn in towards my transition point, I'll almost simultaneously blip the throttle, add clutch, and select he desired gear. You should know the rest...
I never was much of a heal-toe downshifter. Once I achieved "smooth enough" with the technique, I stopped using it. It didn't improve my lap times, and arguably used up concentration and potential physical motion that could be spent in other areas if necessary. Not long after that, I also transitioned to a much faster car. One with really good brakes and plenty of power. In that car, the brakes generally out-performed the engine's ability to keep pace with heal-toe downshifts even with a light flywheel. In that car, there was no point in even considering the use of heal-toe.
Theory:
Ultimately, from a purely physics point of view, the forces going into the transmission will always follow the same curves based on vehicle speed and engine RPM. It doesn't really matter if you're doing an upshift or downshift, the rotating mass of the engine will exert the same force (or resistance to change) at the same rpm, and the vehicle will also always exert the same force (resistance to change) depending on current road speed. The net difference between those two levels of force is what the transmission & clutch must dissipate; typically as heat.
The rate of change does matter. The rate of change matters in that when smoothly accelerating while in gear, the change in force over time is is much lower than when releasing (or especially dumping) a clutch .. either on an upshift or downshift. The rate of change also matters from a materials standpoint, in that it may be possible to exceed a specific component's ability to absorb those forces when there is a large spike in localized force. The rate of change also changes the quantity of heat being placed into a specific component over a specific amount of time. Another factor that matters is the design of the components taking the load. It is possible for a component's design to be asymmetrical, or able to withstand more force in one direction than another. A typical ring & pinion and typical transmission gears are examples of this; they are designed to exhibit more strength (resist more force) when the car is going forward. This design consideration can also carry into the differential or transmission housing and resistance to loads and flexing of the case under those conditions.
Conclusions:
Design considerations: I have never looked at the internals of a manual 928 gerarbox & hopefully I never will have that need. However, I am going to assume that the gears inside the 928 gearbox are more able to resist force under acceleration than deceleration. The case and bearing carriers may also be more resistant to flexing under extreme loads when loaded under acceleration then deceleration.
Physics: In my go-to equation for thumbnail physics guess-timations, E=MC^2, velocity is a squared value. That means that road speed will be a huge factor in the amount of energy that needs to be dissipated in a 5th gear downshift.
Probable effects: Downshfting from 5th to 4th at speed (with a low engine rpm) will put the greatest differential of force possible into the transmission. It will also be loading the gears and transmission in the direction they are weakest.
Examples:
Lets look at two situations. The first is a car at 120 mph, the second is a car at 60 mph. All we we want to see is the amount energy involved. Remember, E=MC^2 shows velocity as a square, and we are interested in the energy we need to dissipate. Using super-cheesy math and ignoring units:
120^2 = 14,400
60^2 = 3600
From a straight velocity standpoint, the 120 mph car is putting 4 times the energy into the transmission of the 60 mph car.
If you are using the transmission as a brake, it only gets worse. You need to look at the difference between the squares of the starting and ending values of each speed differential.
(Suggested) Practice:
Avoid the 5-4 downshift when possible. Use rev-matching if a 5-4 downshift is necessary. 5-4 downshifts while doing heal-toe work are likely to be hard on the transmission. It is probably worth mentioning that using the transmission to make the engine function as a brake is likely generally a bad idea. Its primary design goal is typically to turn engine rotation into forward motion, not be used for braking loads.
Footnotes:
I'm one of those guys who hits the clutch in a braking zone & just stands on the brakes until the car is at the speed I want. I'll also typically move the car into neutral while braking, and usually will partially release the clutch. As I turn in towards my transition point, I'll almost simultaneously blip the throttle, add clutch, and select he desired gear. You should know the rest...
I never was much of a heal-toe downshifter. Once I achieved "smooth enough" with the technique, I stopped using it. It didn't improve my lap times, and arguably used up concentration and potential physical motion that could be spent in other areas if necessary. Not long after that, I also transitioned to a much faster car. One with really good brakes and plenty of power. In that car, the brakes generally out-performed the engine's ability to keep pace with heal-toe downshifts even with a light flywheel. In that car, there was no point in even considering the use of heal-toe.
MK drop the calulator, you are not going to win the internet proving simple givens 6 places to the right of the dec point. People understand the phizix. Another good way to break things is hop a wheel at WOT. Now you need to bust out your flashiest abacus move to calc the increase of rpm over the time the wheel is off the ground in order to know how much energy will be added to the system and then spiked when the tire regains contact and quickly returns to the rpm of the one on the other side of the car.
Or not, you see, the numbers do not matter. Knowing what to and not to does matter. A thread about what to do to reduce driveline shock might be of use to more than just those trying to win the internet.
08-29-2014, 09:22 AM
#18
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08-29-2014, 09:28 AM
#19
Physics: In my go-to equation for thumbnail physics guess-timations, E=MC^2, velocity is a squared value.
...
Examples:
Lets look at two situations. The first is a car at 120 mph, the second is a car at 60 mph. All we we want to see is the amount energy involved. Remember, E=MC^2 shows velocity as a square, and we are interested in the energy we need to dissipate. Using super-cheesy math and ignoring units:
120^2 = 14,400
60^2 = 3600
From a straight velocity standpoint, the 120 mph car is putting 4 times the energy into the transmission of the 60 mph car.
...
Examples:
Lets look at two situations. The first is a car at 120 mph, the second is a car at 60 mph. All we we want to see is the amount energy involved. Remember, E=MC^2 shows velocity as a square, and we are interested in the energy we need to dissipate. Using super-cheesy math and ignoring units:
120^2 = 14,400
60^2 = 3600
From a straight velocity standpoint, the 120 mph car is putting 4 times the energy into the transmission of the 60 mph car.
E=MC^2 is Einstein's equation for the energy released for a given mass destroyed in a nuclear reaction. "C" is the speed of light. I think you were going for (1/2)MV^2
.No matter, it doesn't really affect your conclusion because the ratios are the same.
I never was much of a heal-toe downshifter. Once I achieved "smooth enough" with the technique, I stopped using it. It didn't improve my lap times, and arguably used up concentration and potential physical motion that could be spent in other areas if necessary. Not long after that, I also transitioned to a much faster car. One with really good brakes and plenty of power. In that car, the brakes generally out-performed the engine's ability to keep pace with heal-toe downshifts even with a light flywheel. In that car, there was no point in even considering the use of heal-toe.
A "cool" heel-toe has a large engine blip and associated sound. A "proper" heel-toe is done at the end of the braking zone where the blip is very small. A really good heel-toe sounds like no blip at all and there are no drive line jolts because the revs are matched nearly perfectly. Ideally, if one was not looking at the driver while in the passenger seat they may fail to detect the shift at all.
If one is rowing down the gearbox (multiple downshifts while braking), each heel-toe is timed to the end of each gear.
Again, nothing to do with using the engine for braking. A very old and relevant piece of advice still holds, brakes and rotors are cheaper than clutches, diffs, and transmissions. Drive line parts are for making the car go, brakes are for stopping.
-Mike
Last edited by TXE36; 08-29-2014 at 09:46 AM.
08-29-2014, 10:23 AM
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lol .. serves me right for getting drawn into a huge mess after being up all night playing in the garage.Yes, I know about the phyziks (1/2 MV^2 .. yeah), and knew I was butchering them. Yes the ratios are constant. And yes there are a bunch of "extras" tossed into my response. They usually come up one way or another in these discussions. And yes, I've ridden with too many people who use heal-toe inappropriately and then go on to argue its benefits as a braking assist. And done wrong it can, and sometimes does, damage engines and/or transmissions. IIRC transmission damage was the point of this thread.
Obviously my one post on an Internet forum cannot be considered a comprehensive treatment on the subject. It also cannot take into account every permutation on the subject someone hell bent on slicing pixels can can invent. I'm ok with that, it sounds like you guys have it covered.
Take this all for what it's worth, lol.
I totally deserved that...
08-29-2014, 10:34 AM
#21
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snowball rolling down a long hill...
08-29-2014, 10:43 AM
#22
I'm in....
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Footnotes:
I'm one of those guys who hits the clutch in a braking zone & just stands on the brakes until the car is at the speed I want. I'll also typically move the car into neutral while braking, and usually will partially release the clutch. As I turn in towards my transition point, I'll almost simultaneously blip the throttle, add clutch, and select he desired gear. You should know the rest...
.
08-29-2014, 11:13 AM
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08-29-2014, 11:32 AM
#25
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Not mentioned yet is how hard unmatched downshifts are on your clutch drive straps.
They work in tension, not so great in compression (as caused by large engine braking inputs).
I'm not sure how Pcar clutches are made but there is usually a quarterly thread on the BMW forums with some guy posting pics of his clutch with snapped or taco'd drive straps wondering how on earth it happened. Well buddy...
They work in tension, not so great in compression (as caused by large engine braking inputs).
I'm not sure how Pcar clutches are made but there is usually a quarterly thread on the BMW forums with some guy posting pics of his clutch with snapped or taco'd drive straps wondering how on earth it happened. Well buddy...
08-29-2014, 11:51 AM
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...
08-29-2014, 01:29 PM
#28
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OMG... yes, this has gone nuclear.
Kurt, people DONT understand physics. even someone that partially gets it, is quoting Einsteins equations and confusing them with newton/watt.
KE does not equall E.
(MC^2 vs MV^2)
Pretty soon, dogs and cats will be sleeping together and folks will be quoting KE values as VR's instead of Joules.
Yep, wheel hop or anthing that causes big vibration or moments should be avoided. the entire point was more regarding something that goes unappreciated. downshifts withoiut engine matching causes forces that are not really known to be greater than the forces of "known" activities, like speed shifts. instantaneous speed variations can really chew things up, for sure. Im talking about moments.
Mike, using engine braking is a fact of racing. the more experience you will get, the more you will realize this. waiting until the end of a decel zone to get your shifting done, is no where as fast and controllable as braking in gear to speeds where high RPM downshifts can be made. there is very little wear on the engine if done correctly as the braking forces of the engine are fractions of its fulll output. usually only 1/2 to 1/4 of its dyno'ed output).. its forces are very stabilizing and allows for control my modulating throttle. since you are just going in your racing pursuits, you might want to practice this on the track. and GOD please teach anyone you might be instructing this as I do. by coasting, to a stop, in traffic, you might find that "things change" (a guy cuts you off, or you get bumped from behind, etc) you need to ALWAYS be in the right RPM for max acceleration at any point on the race track. using the throttle to control acceleration and braking is not only better for fast lap times, but safer as well. please dont advocate coasting down to a sto and letting the engine run at low RPM. the rpm target range should be what you have from redline to post shift RPM levels --- always. look at my video or look at patrick long's video. does he "coast to the turn in points"???? no he certainly, does not, nor does anyone with any relivant racing experience.
lol .. serves me right for getting drawn into a huge mess after being up all night playing in the garage.
Yes, I know about the phyziks (1/2 MV^2 .. yeah), and knew I was butchering them. Yes the ratios are constant. And yes there are a bunch of "extras" tossed into my response. They usually come up one way or another in these discussions. And yes, I've ridden with too many people who use heal-toe inappropriately and then go on to argue its benefits as a braking assist. And done wrong it can, and sometimes does, damage engines and/or transmissions. IIRC transmission damage was the point of this thread.
Obviously my one post on an Internet forum cannot be considered a comprehensive treatment on the subject. It also cannot take into account every permutation on the subject someone hell bent on slicing pixels can can invent. I'm ok with that, it sounds like you guys have it covered.
Take this all for what it's worth, lol.
I totally deserved that...
no worries, you got a little equation incorrect, but thanks for validating the main point. there is a huge differnce on downshifting vs upshifting. the forces are sourced by the engine or the KE of the car. heal toeing aside, that is a way to keep braking constant, and get the engine in the proper gear on the way down from speed. having the car in the proper gear is important for not only braking forces of the engine, but control of the car, and the antisipation of having to use the engine at max output on demand, as welll as matching the revs at the higher rpm being actually easier and smoother than at the lower scales. as a side benifit, it saves synchros if done right. if you think about it, with a 30% drop in RPM for a shift, its easier to do that at 6k to 4500rpm, 2500rpm drop vs at 3000rpm to 2000rpm, only a 1000rpm drop. more margin for error.. thats my opinion. but if you notice, i never have gear grind, and ive raced transmissions for 8 full racing seasons, 130 race days, with no sychro issues.... (even though i was a little sloppy with the gear box back in 2001 to 2002 )
????
can you produce a comment with any value at all?
ha ha ha........ einstein vs Newton/watt???
why correct it though, we all know what he means! oh yeah, i forgot i got lambasted for typos and spelling errors consistantly. You're and your....etc.... focus on the topic and not tying to embarrass anyone.
clutch drive straps?? large decel forces from engine braking are using the clutch in a fraction of what is used by it , on acceleration . UNLESS you have a miss matched downshift at high speed.
clutches are clutches from this respect. the forces im talking about are KE from the engine vs KE from the car at speed. big difference. the drive line can usually handle most anything the engine can dish out, but CV joints can always snap on clutch dumps in 1st. however, i cant get a driveline to break if I dump the clutch on a miss matched RPM from 4th to 3rd because the KE is the car, not the engine and its inertia, for the reasons i stated earlier.
think about this. lets say you have an engine lock up (siezed) in your favorite gear....... whats the forces to the drivline if max traction of the rear tires is 4000lbs.. ( on a 24" tire, thats 4000ft-lbs on the CV joints)?
reflected back to the driveline is 4000lbs divided by the gear ratio !!! (no multiplication here folks) DIVIDED!!!! BY!!!!!
so, in 5th or 6th gear, you might have a final drive of 2.2 to 3:1 ....... thats 4000/3 or near 1300ft-lbs on the driveline. where as if you busted your engine in 1st gear, and wheels locked up, the forces are 4000/10, or only 400ft-lbs.
thats the best i can explain it. if you dont agree, please, by all means let me know why.
Kurt, people DONT understand physics. even someone that partially gets it, is quoting Einsteins equations and confusing them with newton/watt.
KE does not equall E.
(MC^2 vs MV^2)Pretty soon, dogs and cats will be sleeping together and folks will be quoting KE values as VR's instead of Joules.
This is "shock" well said.
MK drop the calulator, you are not going to win the internet proving simple givens 6 places to the right of the dec point. People understand the phizix. Another good way to break things is hop a wheel at WOT. Now you need to bust out your flashiest abacus move to calc the increase of rpm over the time the wheel is off the ground in order to know how much energy will be added to the system and then spiked when the tire regains contact and quickly returns to the rpm of the one on the other side of the car.
Or not, you see, the numbers do not matter. Knowing what to and not to does matter. A thread about what to do to reduce driveline shock might be of use to more than just those trying to win the internet.
MK drop the calulator, you are not going to win the internet proving simple givens 6 places to the right of the dec point. People understand the phizix. Another good way to break things is hop a wheel at WOT. Now you need to bust out your flashiest abacus move to calc the increase of rpm over the time the wheel is off the ground in order to know how much energy will be added to the system and then spiked when the tire regains contact and quickly returns to the rpm of the one on the other side of the car.
Or not, you see, the numbers do not matter. Knowing what to and not to does matter. A thread about what to do to reduce driveline shock might be of use to more than just those trying to win the internet.
Oh, boy. Another innocent is drawn in.
It's official, one of Mark's threads has gone nuclear.
E=MC^2 is Einstein's equation for the energy released for a given mass destroyed in a nuclear reaction. "C" is the speed of light. I think you were going for (1/2)MV^2.
No matter, it doesn't really affect your conclusion because the ratios are the same.
Heel-toe done properly has nothing to do with using the engine to slow the car down. It is simply a way to rev match the transmission during a braking zone and is a technique to manipulate 3 pedals with 2 feet.
A "cool" heel-toe has a large engine blip and associated sound. A "proper" heel-toe is done at the end of the braking zone where the blip is very small. A really good heel-toe sounds like no blip at all and there are no drive line jolts because the revs are matched nearly perfectly. Ideally, if one was not looking at the driver while in the passenger seat they may fail to detect the shift at all.
If one is rowing down the gearbox (multiple downshifts while braking), each heel-toe is timed to the end of each gear.
Again, nothing to do with using the engine for braking. A very old and relevant piece of advice still holds, brakes and rotors are cheaper than clutches, diffs, and transmissions. Drive line parts are for making the car go, brakes are for stopping.
-Mike
It's official, one of Mark's threads has gone nuclear.
E=MC^2 is Einstein's equation for the energy released for a given mass destroyed in a nuclear reaction. "C" is the speed of light. I think you were going for (1/2)MV^2
.No matter, it doesn't really affect your conclusion because the ratios are the same.
Heel-toe done properly has nothing to do with using the engine to slow the car down. It is simply a way to rev match the transmission during a braking zone and is a technique to manipulate 3 pedals with 2 feet.
A "cool" heel-toe has a large engine blip and associated sound. A "proper" heel-toe is done at the end of the braking zone where the blip is very small. A really good heel-toe sounds like no blip at all and there are no drive line jolts because the revs are matched nearly perfectly. Ideally, if one was not looking at the driver while in the passenger seat they may fail to detect the shift at all.
If one is rowing down the gearbox (multiple downshifts while braking), each heel-toe is timed to the end of each gear.
Again, nothing to do with using the engine for braking. A very old and relevant piece of advice still holds, brakes and rotors are cheaper than clutches, diffs, and transmissions. Drive line parts are for making the car go, brakes are for stopping.
-Mike
lol .. serves me right for getting drawn into a huge mess after being up all night playing in the garage.Yes, I know about the phyziks (1/2 MV^2 .. yeah), and knew I was butchering them. Yes the ratios are constant. And yes there are a bunch of "extras" tossed into my response. They usually come up one way or another in these discussions. And yes, I've ridden with too many people who use heal-toe inappropriately and then go on to argue its benefits as a braking assist. And done wrong it can, and sometimes does, damage engines and/or transmissions. IIRC transmission damage was the point of this thread.
Obviously my one post on an Internet forum cannot be considered a comprehensive treatment on the subject. It also cannot take into account every permutation on the subject someone hell bent on slicing pixels can can invent. I'm ok with that, it sounds like you guys have it covered.
Take this all for what it's worth, lol.
I totally deserved that...
) can you produce a comment with any value at all?
ha ha ha........ einstein vs Newton/watt???
why correct it though, we all know what he means! oh yeah, i forgot i got lambasted for typos and spelling errors consistantly. You're and your....etc.... focus on the topic and not tying to embarrass anyone.
Not mentioned yet is how hard unmatched downshifts are on your clutch drive straps.
They work in tension, not so great in compression (as caused by large engine braking inputs).
I'm not sure how Pcar clutches are made but there is usually a quarterly thread on the BMW forums with some guy posting pics of his clutch with snapped or taco'd drive straps wondering how on earth it happened. Well buddy...
They work in tension, not so great in compression (as caused by large engine braking inputs).
I'm not sure how Pcar clutches are made but there is usually a quarterly thread on the BMW forums with some guy posting pics of his clutch with snapped or taco'd drive straps wondering how on earth it happened. Well buddy...
clutches are clutches from this respect. the forces im talking about are KE from the engine vs KE from the car at speed. big difference. the drive line can usually handle most anything the engine can dish out, but CV joints can always snap on clutch dumps in 1st. however, i cant get a driveline to break if I dump the clutch on a miss matched RPM from 4th to 3rd because the KE is the car, not the engine and its inertia, for the reasons i stated earlier.
think about this. lets say you have an engine lock up (siezed) in your favorite gear....... whats the forces to the drivline if max traction of the rear tires is 4000lbs.. ( on a 24" tire, thats 4000ft-lbs on the CV joints)?
reflected back to the driveline is 4000lbs divided by the gear ratio !!! (no multiplication here folks) DIVIDED!!!! BY!!!!!
so, in 5th or 6th gear, you might have a final drive of 2.2 to 3:1 ....... thats 4000/3 or near 1300ft-lbs on the driveline. where as if you busted your engine in 1st gear, and wheels locked up, the forces are 4000/10, or only 400ft-lbs.
thats the best i can explain it. if you dont agree, please, by all means let me know why.
08-29-2014, 01:54 PM
#29
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clutch drive straps?? large decel forces from engine braking are using the clutch in a fraction of what is used by it , on acceleration . UNLESS you have a miss matched downshift at high speed.
clutches are clutches from this respect. the forces im talking about are KE from the engine vs KE from the car at speed. big difference. the drive line can usually handle most anything the engine can dish out, but CV joints can always snap on clutch dumps in 1st. however, i cant get a driveline to break if I dump the clutch on a miss matched RPM from 4th to 3rd because the KE is the car, not the engine and its inertia, for the reasons i stated earlier.
think about this. lets say you have an engine lock up (siezed) in your favorite gear....... whats the forces to the drivline if max traction of the rear tires is 4000lbs.. ( on a 24" tire, thats 4000ft-lbs on the CV joints)?
reflected back to the driveline is 4000lbs divided by the gear ratio !!! (no multiplication here folks) DIVIDED!!!! BY!!!!!
so, in 5th or 6th gear, you might have a final drive of 2.2 to 3:1 ....... thats 4000/3 or near 1300ft-lbs on the driveline. where as if you busted your engine in 1st gear, and wheels locked up, the forces are 4000/10, or only 400ft-lbs.
thats the best i can explain it. if you dont agree, please, by all means let me know why.
clutches are clutches from this respect. the forces im talking about are KE from the engine vs KE from the car at speed. big difference. the drive line can usually handle most anything the engine can dish out, but CV joints can always snap on clutch dumps in 1st. however, i cant get a driveline to break if I dump the clutch on a miss matched RPM from 4th to 3rd because the KE is the car, not the engine and its inertia, for the reasons i stated earlier.
think about this. lets say you have an engine lock up (siezed) in your favorite gear....... whats the forces to the drivline if max traction of the rear tires is 4000lbs.. ( on a 24" tire, thats 4000ft-lbs on the CV joints)?
reflected back to the driveline is 4000lbs divided by the gear ratio !!! (no multiplication here folks) DIVIDED!!!! BY!!!!!
so, in 5th or 6th gear, you might have a final drive of 2.2 to 3:1 ....... thats 4000/3 or near 1300ft-lbs on the driveline. where as if you busted your engine in 1st gear, and wheels locked up, the forces are 4000/10, or only 400ft-lbs.
thats the best i can explain it. if you dont agree, please, by all means let me know why.
Say you are at Summit Main, or LS, or pick your track where you need a 6-4 or 5-3 downshift. Say you decide to skip shift, botch your blip, and dump the clutch at 3000 engine rpm less than desired. That puts one hell of a shock through the drivetrain (though whatever means you have stated above or whathave you) in the opposite direction than some components were designed, chief of which, I feel, are the clutch drive straps.
Screw that shift up and I highly doubt you are going to grenade hardened axle splines, CVs, U joints, diff, or tranny gears first. However you are reverse loading (in compression) a piece of glorified sheet metal that in many cases is already bent in a subtle "Z" shape. Bad news.
I'm not talking minorly botched shifts, I'm talking gross errors in rev matching (or none at all).
None of this applies in accel, where the clutch was made to handle Much more force than it can otherwise (tension on the drive straps).
08-29-2014, 03:16 PM
#30
Rennlist Member
Thread Starter
Say you are at Summit Main, or LS, or pick your track where you need a 6-4 or 5-3 downshift. Say you decide to skip shift, botch your blip, and dump the clutch at 3000 engine rpm less than desired. That puts one hell of a shock through the drivetrain (though whatever means you have stated above or whathave you) in the opposite direction than some components were designed, chief of which, I feel, are the clutch drive straps.
Screw that shift up and I highly doubt you are going to grenade hardened axle splines, CVs, U joints, diff, or tranny gears first. However you are reverse loading (in compression) a piece of glorified sheet metal that in many cases is already bent in a subtle "Z" shape. Bad news.
I'm not talking minorly botched shifts, I'm talking gross errors in rev matching (or none at all).
None of this applies in accel, where the clutch was made to handle Much more force than it can otherwise (tension on the drive straps).
Screw that shift up and I highly doubt you are going to grenade hardened axle splines, CVs, U joints, diff, or tranny gears first. However you are reverse loading (in compression) a piece of glorified sheet metal that in many cases is already bent in a subtle "Z" shape. Bad news.
I'm not talking minorly botched shifts, I'm talking gross errors in rev matching (or none at all).
None of this applies in accel, where the clutch was made to handle Much more force than it can otherwise (tension on the drive straps).