Why engine driveline components break with mis-matched RPM downshifts
08-29-2014, 09:08 PM
#47
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I agree. Jackie liked to row .. Senna thought it was a waste of time. As cars and brakes and technology have improved techniques have evolved. With a click-box sequential, downshifts are fine, but with a conventional box they become a distraction from the braking event.
If you think I'm full of BS try testing yourself. Drop a line on some pavement and perform ten braking events from 100 to 0, starting at that line while downshifting. Mark these stops in one color. Repeat the process without the downshifts. Mark these stops in a different color. If you don't believe the data try repeating the exercise in the reverse order. You might be faster if you you row. I'm not, I've tested it.
I am not Jackie and I am definitely not Senna. But I do read, study and try to learn as much as possible .. including sources other than just Internet forums where the old guard bestow their ordained knowledge from post counts on high to the helpless, clueless newbies.
If you think I'm full of BS try testing yourself. Drop a line on some pavement and perform ten braking events from 100 to 0, starting at that line while downshifting. Mark these stops in one color. Repeat the process without the downshifts. Mark these stops in a different color. If you don't believe the data try repeating the exercise in the reverse order. You might be faster if you you row. I'm not, I've tested it.
I am not Jackie and I am definitely not Senna. But I do read, study and try to learn as much as possible .. including sources other than just Internet forums where the old guard bestow their ordained knowledge from post counts on high to the helpless, clueless newbies.
09-01-2014, 09:36 AM
#49
uninformed gas bag
(contemplating on whether gas bag is one or two words)
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(contemplating on whether gas bag is one or two words)
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I agree. Jackie liked to row .. Senna thought it was a waste of time. As cars and brakes and technology have improved techniques have evolved. With a click-box sequential, downshifts are fine, but with a conventional box they become a distraction from the braking event.
If you think I'm full of BS try testing yourself. Drop a line on some pavement and perform ten braking events from 100 to 0, starting at that line while downshifting. Mark these stops in one color. Repeat the process without the downshifts. Mark these stops in a different color. If you don't believe the data try repeating the exercise in the reverse order. You might be faster if you you row. I'm not, I've tested it.
I am not Jackie and I am definitely not Senna. But I do read, study and try to learn as much as possible .. including sources other than just Internet forums where the old guard bestow their ordained knowledge from post counts on high to the helpless, clueless newbies.
If you think I'm full of BS try testing yourself. Drop a line on some pavement and perform ten braking events from 100 to 0, starting at that line while downshifting. Mark these stops in one color. Repeat the process without the downshifts. Mark these stops in a different color. If you don't believe the data try repeating the exercise in the reverse order. You might be faster if you you row. I'm not, I've tested it.
I am not Jackie and I am definitely not Senna. But I do read, study and try to learn as much as possible .. including sources other than just Internet forums where the old guard bestow their ordained knowledge from post counts on high to the helpless, clueless newbies.
09-02-2014, 11:02 AM
#50
Three Wheelin'
thats a 8:1 reduction coming from a 11:1 reduction in the gear box. the force starts at the rear tires and their Cf. (mu) if it takes 4000lbs to make a tire break free, thats the force at which its spinning up the engine's inertia.... that force would be 4000ft-lbs / 8:1 or near 500ft-lbs.
if you measured how fast the engine spun up, the torque would be 500ft-lbs. the driveline could survive. more than likely.
now, do the same in 4th to 3rd. get a chirp at 3rd gear by dumping the clutch and the REFLEcTED inertial of the entire car is at the tire on the road. even though the break away force is still 4000lbs, you are going much faster! (much higher KE). the gear ratio is less, but the reflected torque to the driveline is 4000lbs / 5:1. now you are talking 800ft-lbs. this force is what breaks the driveline components. the CV joiints, half shafts are not seeing anything higher than what they see when breaking the wheels free under acceleration.
if you measured how fast the engine spun up, the torque would be 500ft-lbs. the driveline could survive. more than likely.
now, do the same in 4th to 3rd. get a chirp at 3rd gear by dumping the clutch and the REFLEcTED inertial of the entire car is at the tire on the road. even though the break away force is still 4000lbs, you are going much faster! (much higher KE). the gear ratio is less, but the reflected torque to the driveline is 4000lbs / 5:1. now you are talking 800ft-lbs. this force is what breaks the driveline components. the CV joiints, half shafts are not seeing anything higher than what they see when breaking the wheels free under acceleration.
You're good with base-ik fiziks. Try this one. T=I*a (a is alpha)
09-02-2014, 01:48 PM
#51
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I used a common 300ftlbs of torque 300hp engine, and a general range of gear ratios. +/- 10% gives you 45, 75, 105, and 135mph for 1-4th gear.
tossing a few number out there and using that, (and ill save you the very complicated math TBwise), thats 10:1 / 8:1 / 5:1/ 3.5:1 for 1st through 4th, now what does that get you?
what does 8 x 300? = 2400ft-lbs.
so if it takes 2nd gear to get a chirp of the tires, thats 2400lbs of force or 2400ftlbs on the CV joints. It actually, be a lot more, because we are generally talking about a speed shift, or a fast shift with fast clutch release. that tends to take the higher engine rpm and drop them down quickly for an inertial moment effect. in other words, you might not be able to get wheel spin by just flooring a slick clad car in 2nd gear at max torque. so , if that number is higher, the number is even higher on the downshift.
that same chirp on a mismatched downshft, from 4th to 3rd, will be 2400ft/lbs divided by 5:1 (3rd gear ratio) .... or near 500ft-lbs on the driveline. you just nearly doubled the force that the driveline sees.
Now, is this that complicated for you to grasp???
Last edited by mark kibort; 09-02-2014 at 04:07 PM.
09-02-2014, 01:57 PM
#52
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Thread Starter
I agree. Jackie liked to row .. Senna thought it was a waste of time. As cars and brakes and technology have improved techniques have evolved. With a click-box sequential, downshifts are fine, but with a conventional box they become a distraction from the braking event.
If you think I'm full of BS try testing yourself. Drop a line on some pavement and perform ten braking events from 100 to 0, starting at that line while downshifting. Mark these stops in one color. Repeat the process without the downshifts. Mark these stops in a different color. If you don't believe the data try repeating the exercise in the reverse order. You might be faster if you you row. I'm not, I've tested it.
I am not Jackie and I am definitely not Senna. But I do read, study and try to learn as much as possible .. including sources other than just Internet forums where the old guard bestow their ordained knowledge from post counts on high to the helpless, clueless newbies.
If you think I'm full of BS try testing yourself. Drop a line on some pavement and perform ten braking events from 100 to 0, starting at that line while downshifting. Mark these stops in one color. Repeat the process without the downshifts. Mark these stops in a different color. If you don't believe the data try repeating the exercise in the reverse order. You might be faster if you you row. I'm not, I've tested it.
I am not Jackie and I am definitely not Senna. But I do read, study and try to learn as much as possible .. including sources other than just Internet forums where the old guard bestow their ordained knowledge from post counts on high to the helpless, clueless newbies.
Watch any video of folks that know how to race a race car and drive fast without trashing the equip. the ALL downshift, as i explain and practice myself . why? because rowing the gears is easier on the driveline as we discussed all the reasons earlier. And as you do downshift approaching braking zones. the rear braking of the engine does provide substantial force that adds to stability of the rear of the car. It also fights lockup in a way that ABS cannot.
Ive done the tests with my car, and its pretty obvious. i can clearly stop from 100mph to 0 much better without engaging my very old style ABS fully. i used it for an early indicator, and just stay above its engagement. however, fullly disconnected i am still a little better, but a small mistake can cause a lockup and further stopping distances.
Newer ABS systems are better.....agreed.
much of the engine braking is good in stability into and around turns, where feathering the throttle on and off can enhance control and allow better rotation control, rather than if you were left foot braking around a turn (not that anyone does that around tight turns). you get the effect of a non -locking up or ABS engaging braking, and its only being applied to the rear of the car. its yet another tool racers use for control and to achieve fast lap times.
by the way, i dont think many of us on most of the tracks we visit and the speeds we get too, dont "row" the gear box that much. even the main example here of 130mph to 40mph is only a 2 gear shift exercise however, i will say that the first shift happens pretty quick, and all that braking force of the engine takes me down to turn in, where just before that point, i blip and hit second. additional braking force, but hitting the turn in point in gear, with the ability to maneuver much better and with much more control than if someone tried to coast in and fumble around with the gears at that point.
09-02-2014, 02:03 PM
#53
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09-02-2014, 02:18 PM
#54
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Half of me knew this.. 
did you see my response to this?? its all about unbalance of force... or net force...the force is acting on the rear tires in a downshift. its the road acting on the tire relative to the speed of the car. on an upshift, its the tire acting on the road, based on the force that the engine and driveline are contributing. big difference in force.
the problem with this is that if you optimize for 100% braking for one straight, it will be a lock up problem for another. or as weight is changing in the car, how much, if any brake fade, etc etc. lots of factors. then couple that with non straight line braking, and you then have REAL big braking problems, not to mention that the amount of engine braking is real and you cant avoid it , unless you drive incorrectly and not downshift.
that force also changes radically with each gear and through the RPM ranges. i mentioned that up to 200hp or 200ftlbs of torque can be produced by engine braking at the driveline. again, if you look at a threshold braking slow down, there is not much weight on the rear wheels unless you are in a 911............ so, engine braking and bias set up is a balancing dance. its one of the reasons that F1 cars are constantly playing with thier settings . with a front engine car, and sticky tires, its hard to ask too much of the rear brakes, even with engine braking, due to maybe only 300lbs sitting on each wheel during a hard slow down.
No. It's a resultant of the driveline inertia; having to spin the entire driveline to match the engine speed. That is what produces the force that breaks the driveline. This makes it more probable that failure occurs in the lower gears than the higher as the inertial force from the speed mismatch is higher at the lower speeds.
I have excellent help here.
Another possibility occurs to me. There could be a car set-up problem.
If a car is gaining a benefit from engine braking, assuming the driver is making maximum use of all the braking force available through the brake pedal, then the braking system of the car is not at its optimal configuration.
I'd look to brake bias, pads & rotors (sizes, compounds), wheel & tire set-up, springs & dampers, etc.
In any braking event, 100% of the time at effective maximum braking capacity should be more efficient then maximum effective braking capacity interrupted when where's no engine braking (during downshifts). Phiziks, right?
Another possibility occurs to me. There could be a car set-up problem.
If a car is gaining a benefit from engine braking, assuming the driver is making maximum use of all the braking force available through the brake pedal, then the braking system of the car is not at its optimal configuration.
I'd look to brake bias, pads & rotors (sizes, compounds), wheel & tire set-up, springs & dampers, etc.
In any braking event, 100% of the time at effective maximum braking capacity should be more efficient then maximum effective braking capacity interrupted when where's no engine braking (during downshifts). Phiziks, right?
that force also changes radically with each gear and through the RPM ranges. i mentioned that up to 200hp or 200ftlbs of torque can be produced by engine braking at the driveline. again, if you look at a threshold braking slow down, there is not much weight on the rear wheels unless you are in a 911.
........... so, engine braking and bias set up is a balancing dance. its one of the reasons that F1 cars are constantly playing with thier settings . with a front engine car, and sticky tires, its hard to ask too much of the rear brakes, even with engine braking, due to maybe only 300lbs sitting on each wheel during a hard slow down.
09-02-2014, 03:53 PM
#55
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Not sure what that has to do with real-life track experiences, though.
09-02-2014, 04:36 PM
#56
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yes, T=Ia. alpha= d/dt w (w = angular velocity)
(T= is the net torque, or sum of "torques" ). (I = moment of inertia) and you got it..... alpha is your angular acceleration
that's why when i say, 2400lbs of force, its 2400ft-lbs, because im assuming a 24" diameter.
(T=rF as long as r is perpendicular to the force, which it is)If you are still debating where the force is generated from or "If" the force is greater on a downshift, up shift, then use this and then plug in the numbers for the engine RPM being rapidly being brought down due to acceleration clutch release VS the engine bring brought up more quicky on a mismatched downshift. The force is greater on the downshift due to the gearing reflects the entire inertia (mass of car) to the engine, which its force will go up with speed...... the force of breakaway force of the tire is constant.
Now, to use T=Ia you need to think about what resultant force you are looking at. is it the car moving at a velocity, connected to the tires and road, or is it the sump of all the changes of angular momentum of the engine and drive-line? the point here is that because we can clearly see the force to accelerated the speed of the engine, goes up with velocity of the car, ( car KE) vs the engines ability to accelerate the mass of the car via bringing its RPM down (engine KE) can be grossly different.
Last edited by mark kibort; 09-02-2014 at 05:01 PM.
09-02-2014, 04:58 PM
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Mark, I I think we are talking about our braking situations from very different points of view. I was not talking about planting my foot until the ABS engages. And yes, the best modern ABS systems are excellent. I was talking about threshold braking without engaging the ABS under the very best possible on-track conditions, without being hampered by compromises.
If I'm understanding you correctly, you are talking about using engine braking to aid with stability and add braking force when the car's brakes can't take maximum advantage of the rear tires. Depending on the car, its set-up, and the situation that obviously makes sense.
I also am familiar with how much things change even within a single braking event, let alone from corner to corner. And I know how much a car's braking behavior changes as tires, track surfaces and temperatures change over the course of a day. And based on debris, altered line, trail braking and traffic related issues. Not every corner can be attacked in the same way, and even the same corner is not consistent. Please also give me a bit of a break on the "rowing" term. I know that most corners don't require more than one or two downshifts.
Another thing we haven't talked about is how class rules can limit car set-up, and how that can bring different driving techniques back into the equation. When someone is required race with a set-up that could be more optimal if the rules allowed for it, specialized techniques need to be developed and applied to adapt to those limitations. I have no idea if this is a factor for you or the other racers commenting in this thread.
Another portion is the driver's inner ear and natural sensitivity to change along with their experience and seat time. All of this helps them to understand what the car and tires are are doing. The way I set-up a car always feels stable to me, but when I've had to share it with another driver, I usually need to dial in some understeer to help them feel comfortable. This usually shows up as slower lap times for me.
I think we just may be approaching (or looking at) the problem from opposite directions. I'd also be willing to bet that our car set-ups would feel at least slightly different. At this point I'm not convinced it's a matter of right or wrong, just different paths towards the same goal. Basically each of us making our case for opposite ends of the "situation" spectrum.
If I'm understanding you correctly, you are talking about using engine braking to aid with stability and add braking force when the car's brakes can't take maximum advantage of the rear tires. Depending on the car, its set-up, and the situation that obviously makes sense.
I also am familiar with how much things change even within a single braking event, let alone from corner to corner. And I know how much a car's braking behavior changes as tires, track surfaces and temperatures change over the course of a day. And based on debris, altered line, trail braking and traffic related issues. Not every corner can be attacked in the same way, and even the same corner is not consistent. Please also give me a bit of a break on the "rowing" term. I know that most corners don't require more than one or two downshifts.
Another thing we haven't talked about is how class rules can limit car set-up, and how that can bring different driving techniques back into the equation. When someone is required race with a set-up that could be more optimal if the rules allowed for it, specialized techniques need to be developed and applied to adapt to those limitations. I have no idea if this is a factor for you or the other racers commenting in this thread.
Another portion is the driver's inner ear and natural sensitivity to change along with their experience and seat time. All of this helps them to understand what the car and tires are are doing. The way I set-up a car always feels stable to me, but when I've had to share it with another driver, I usually need to dial in some understeer to help them feel comfortable. This usually shows up as slower lap times for me.
I think we just may be approaching (or looking at) the problem from opposite directions. I'd also be willing to bet that our car set-ups would feel at least slightly different. At this point I'm not convinced it's a matter of right or wrong, just different paths towards the same goal. Basically each of us making our case for opposite ends of the "situation" spectrum.
09-02-2014, 05:05 PM
#59
Three Wheelin'
Not sure I agree with you here. Yes, the inertia of the vehicle is an input, but if the static friction of the tire doesn't change then you're threshold doesn't change. What I'm saying is that inertia of the driveline is a big factor. If your driveline inertia is high relative to high static friction at the tire then the resultant torque must be reacted thru the driveline components. The weakest one fails first.
09-02-2014, 06:10 PM
#60
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Thread Starter
thats a good point. But the discussion was a round those that broke clutches, driveline, input gears and were wondering why..... watching videos and figuring things out, it all makes sense now. 
yes, i never claimed that assumption, i just thought that was obvious that it was one. again, as you said, you know i was using it not to make a point but to describe the concept.
That is why i was adding up things to show why this is true. Using KE is a good way too do this. if you think about it, the KE is so high as the velocity goes up, that you can see that the potential for damage is great.
calculate that angular momentum, and that rate of change, and you can see that forces acting on the driveline are much higher than any that can be thought about at the engine, because of the gear box. Its reverse if you think about the engine doing the acceleration, and using the engines KE in anyway conceivable. It just doesnt and never can add up.
Now, are you still not in agreement. again, just think of the engine as a brake only. like my example... say it just siezes! infinite inertia. whats going to break? what are the forces?? again, it all starts at the road and tire, and is divided (not multiplied ) to the driveline. on acceleration, it takes RATE OF CHANGE OF KE to accelerate the engine , even dumping the clutch on a redline shift, to transfer force to the driveline. you are limited to the sum of angular momentum of the engine and driveline as a source of force. the point to your push back is this. On a downshift, it doesnt matter what the engine interial compoent is. as you said, the force for "breakaway at the tire" is constant. if that is so, the max force on the driveline is going to be breakaway force at the tire / gear ratio. PERIOD.
what if we were able to go 700mph, and you had a 10 speed gear box and you downshifted 3 or 4 gears and got a chirp at the rear wheels. because the gear ratos would be in the .5 or .2 range, those forces acting on the driveline would be that same 2400lb force / .5 or .2. that would now be 5000 or 10,000 ft lbs of torque on the driveline. sometimes its good to look at extremes to see the concepts.
I think i mostly agree with all you have said. we have gotten off track enough here to forget the main point of all this. But, i will say, in a perfect world, i think you want that just perfect amount of rear brakes and the other stuff wont matter much. since ive become one of the better at working around certain deficiencies. (dubious honor), it has taught me a lot. Most of what i do is not optimal, but it works for all the right reasons. as i have said before, if i had a cup car, none of the work arounds would matter. you would just set the car up for what you want it to do and be on your way.
But the discussion was a round those that broke clutches, driveline, input gears and were wondering why..... watching videos and figuring things out, it all makes sense now. Yet, nowhere do you claim that assumption. You don't need to use numbers to make your point, but when you chose random ones to do it you can make an point you want. I'm sure I could prove the that sun spins around the earth and the earth is stationary if I make up numbers to fit my argument. Variables can be quite useful even if they have no inherent value associated with it.
Not sure I agree with you here. Yes, the inertia of the vehicle is an input, but if the static friction of the tire doesn't change then you're threshold doesn't change. What I'm saying is that inertia of the driveline is a big factor. If your driveline inertia is high relative to high static friction at the tire then the resultant torque must be reacted thru the driveline components. The weakest one fails first.
Not sure I agree with you here. Yes, the inertia of the vehicle is an input, but if the static friction of the tire doesn't change then you're threshold doesn't change. What I'm saying is that inertia of the driveline is a big factor. If your driveline inertia is high relative to high static friction at the tire then the resultant torque must be reacted thru the driveline components. The weakest one fails first.
That is why i was adding up things to show why this is true. Using KE is a good way too do this. if you think about it, the KE is so high as the velocity goes up, that you can see that the potential for damage is great.
calculate that angular momentum, and that rate of change, and you can see that forces acting on the driveline are much higher than any that can be thought about at the engine, because of the gear box. Its reverse if you think about the engine doing the acceleration, and using the engines KE in anyway conceivable. It just doesnt and never can add up.
Now, are you still not in agreement. again, just think of the engine as a brake only. like my example... say it just siezes! infinite inertia. whats going to break? what are the forces?? again, it all starts at the road and tire, and is divided (not multiplied ) to the driveline. on acceleration, it takes RATE OF CHANGE OF KE to accelerate the engine , even dumping the clutch on a redline shift, to transfer force to the driveline. you are limited to the sum of angular momentum of the engine and driveline as a source of force. the point to your push back is this. On a downshift, it doesnt matter what the engine interial compoent is. as you said, the force for "breakaway at the tire" is constant. if that is so, the max force on the driveline is going to be breakaway force at the tire / gear ratio. PERIOD.
what if we were able to go 700mph, and you had a 10 speed gear box and you downshifted 3 or 4 gears and got a chirp at the rear wheels. because the gear ratos would be in the .5 or .2 range, those forces acting on the driveline would be that same 2400lb force / .5 or .2. that would now be 5000 or 10,000 ft lbs of torque on the driveline. sometimes its good to look at extremes to see the concepts.
Mark, I I think we are talking about our braking situations from very different points of view. I was not talking about planting my foot until the ABS engages. And yes, the best modern ABS systems are excellent. I was talking about threshold braking without engaging the ABS under the very best possible on-track conditions, without being hampered by compromises.
If I'm understanding you correctly, you are talking about using engine braking to aid with stability and add braking force when the car's brakes can't take maximum advantage of the rear tires. Depending on the car, its set-up, and the situation that obviously makes sense.
I also am familiar with how much things change even within a single braking event, let alone from corner to corner. And I know how much a car's braking behavior changes as tires, track surfaces and temperatures change over the course of a day. And based on debris, altered line, trail braking and traffic related issues. Not every corner can be attacked in the same way, and even the same corner is not consistent. Please also give me a bit of a break on the "rowing" term. I know that most corners don't require more than one or two downshifts.
Another thing we haven't talked about is how class rules can limit car set-up, and how that can bring different driving techniques back into the equation. When someone is required race with a set-up that could be more optimal if the rules allowed for it, specialized techniques need to be developed and applied to adapt to those limitations. I have no idea if this is a factor for you or the other racers commenting in this thread.
Another portion is the driver's inner ear and natural sensitivity to change along with their experience and seat time. All of this helps them to understand what the car and tires are are doing. The way I set-up a car always feels stable to me, but when I've had to share it with another driver, I usually need to dial in some understeer to help them feel comfortable. This usually shows up as slower lap times for me.
I think we just may be approaching (or looking at) the problem from opposite directions. I'd also be willing to bet that our car set-ups would feel at least slightly different. At this point I'm not convinced it's a matter of right or wrong, just different paths towards the same goal. Basically each of us making our case for opposite ends of the "situation" spectrum.
If I'm understanding you correctly, you are talking about using engine braking to aid with stability and add braking force when the car's brakes can't take maximum advantage of the rear tires. Depending on the car, its set-up, and the situation that obviously makes sense.
I also am familiar with how much things change even within a single braking event, let alone from corner to corner. And I know how much a car's braking behavior changes as tires, track surfaces and temperatures change over the course of a day. And based on debris, altered line, trail braking and traffic related issues. Not every corner can be attacked in the same way, and even the same corner is not consistent. Please also give me a bit of a break on the "rowing" term. I know that most corners don't require more than one or two downshifts.
Another thing we haven't talked about is how class rules can limit car set-up, and how that can bring different driving techniques back into the equation. When someone is required race with a set-up that could be more optimal if the rules allowed for it, specialized techniques need to be developed and applied to adapt to those limitations. I have no idea if this is a factor for you or the other racers commenting in this thread.
Another portion is the driver's inner ear and natural sensitivity to change along with their experience and seat time. All of this helps them to understand what the car and tires are are doing. The way I set-up a car always feels stable to me, but when I've had to share it with another driver, I usually need to dial in some understeer to help them feel comfortable. This usually shows up as slower lap times for me.
I think we just may be approaching (or looking at) the problem from opposite directions. I'd also be willing to bet that our car set-ups would feel at least slightly different. At this point I'm not convinced it's a matter of right or wrong, just different paths towards the same goal. Basically each of us making our case for opposite ends of the "situation" spectrum.