mark kibort

I think there is a little projection going on your end. why so angry? We all were joking around. you said "torque to the wheels" and i just pointed out the next level of clarification being "force at the wheels". If you read the exchange of the texts, mine are right in line... you are a bit sensitive today! ironically, you get it and I'm on your side on this one.. Sorry if you took it any other way.

333pg333

I agree that skipping a gear is fine so long as you heel and toe appropriately. Watching this video is a bit misleading. In the cases where this guy does it he's almost finished the corner and has slowed considerably before hurriedly swapping 2 cogs. I don't think this is the perfect example.

Most of the time it seems to be more natural to row but there are sometimes a couple of corners where skipping works better at your local track. I say do it!

mark kibort

But my point, as we discussed a while ago in great detail, is that if you skip a gear, the wear goes up to almost 5x (in some cases) by skipping a gear, on the synchro. you can heal toe until the cows come home and this fact doesnt change. removing the engine from the driveline has nothing to do with the speed differentials of the driveline (aka drive shaft, input shaft, clutch discs, layshaft, etc) vs wheel speeds. not as much of an issue with low weight clutch disc and a 911 short driveline size, but still, the forces are due to the differential of speeds............this is the synchros function ........to use the friction surface to speed up the driveline components before the new gear can be selected. One technique is to use the "dipping " clutch technique to use the engine and clutch to speed up the driveline with a blip, but it means a partially depressed clutch pedal at the "blip" time.

of course if you are only using braking to slow to a very low engine RPM, a blip and shift skipping a gear, is not a lot of force.. But, at a cost of being in a gear on approach to a turn where rear wheel forces due to engine braking, can add to control, and have power available during such an approach, that you wouldn't have by skipping gears. in other words, its usually best to row the gear box, because you are always in the right gear in the HP range best to accelerate or control the car if needed.

mark kibort

This guy would be a LOT faster in those turns with proper shifting selection. he ends up at 1000rpm on some turns, trail braking and you dont get any of the positive effects of rear wheel forces by taking advantage of the LSD. Great laps, lots of good racing, but that is not one of his strong suits. It doesnt matter what he has "going on" he should be rowing the gears for both better control forces and wear. go to 1:50 on the video. His hands are over the wheel. shifting in the middle of turns is not optimal and , even though this guy is quick, he could be a little quicker with those things changed. a good example is at 7:25, where he is approaching the mustang. he doesnt downshift on the turn approach, grabs two gears , and looses a lot of exit power to better keep the race closer. Huge mistake to use that technique at 9:50 as well. the lapped car gets a better run out of the turn sequence.... The even almost stallls the car ! .. this is a great example of why NOT to skip gears .

the only clear example of this where it doesnt seem to hurt much , is the video i saw of Winders doing it once at Sears point at the end of a very fast straight, into a slow turn skpping a gear... with his close ratio gear box and time braking .. it didnt seem to be an issue, other than wear, but with close ratios, that is not much of an issue.. and he did this so infrequently.

stownsen914

I'd always assumed that the LSD works based only on the forces between the left and right driven wheels/tires, with the other main variable being whether the car is in "power mode" vs. "braking mode" (and not necessarily how much engine braking is going on). Is that not the case? Or maybe I misunderstood your point ...

mark kibort

[QUOTE=stownsen914;14172058][QUOTE=mark kibort;14171505]This guy would be a LOT faster in those turns with proper shifting selection. he ends up at 1000rpm on some turns, trail braking and you dont get any of the positive effects of rear wheel forces by taking advantage of the LSD.
When you are off-throttle going into a turn, trail braking even just deceling with engine braking, and there are uneven forces on tires, the LSD can be a stabilizing factor. the engine acts as a kind of ABS and slip control for the rear tires with differential lateral slip angle and longitudinal slip %. As additional benefit, you can apply much more accelerative forces and deceleration forces by having the engine in the proper RPM range for a given turn. Im sure most racers have seen the effect and differences of stability and control of coasting through a turn vs being in the proper gear around it.

333pg333

I'll defer to your better understanding of engineering Mark...however I would also guess that the difference when skipping a shift if done properly maybe once a lap...twice at most, is virtually negligible. Again, subjective but you're hitting the brakes really hard and washing off a ton of speed. The extra perceived damage to the syncros would be not a lot. I always had a gut feeling that it was faster too.

mark kibort

Yeah, you are probably right... there are so many things going on and so many factors, we are only talking about one wear element here. but, the reason that there is a high wear potential with a skip shift, no matter how someone perceives as "correctly" , is because of the driveline speed vs the tire speed/gearselected speed.

Here is a quick example of someone we call " joe" doing a high speed to low speed decel from 130 to 45mph 2nd gear turn. he is at 6,000rpm in 4th approaching a turn.... he mashes on the brakes all the way to 45mph then he, pushes the clutch in, pulls out of gear, going to 3rd. gives a quick "blip"and selects 2nd gear. what happens? 2nd gear at 45mph is 5,000rpm, the RPM before depressing the clutch at the entrance of the turn is at 1700..... the snychros have friction surfaces that have to spin up the difference of 1700rpm to 5000rpm (new 2nd gear engine speed) in less than a fraction of a second. nothing else is assisting this force on the synchro . and this is regardless of how you blip the engine with the clutch in.................so, in the end, normally, the spin up would be from say, 1/2 the speed difference (just to use round figures), so since the force goes up with the square of speed, there is 4x the wear than if you just rowed the gear box normally.

the point here is that every shift wear adds up.. there is a reason that the 2nd gear/ or 3rd gear synchros fail .. they have the most use and the most abuse. (faster shifts make more wear too). you mention that you are only doing this 2 times a lap.... but you might only use 2nd 2 times a lap anyway, so all the wear you would normally have is just due to this gear selection. if you expect the failure of your transmission to be synchos in 8 years... this technique, might make the repair cycle after 2.

the one thing you can do to prevent unusual wear , is to learn to dip the clutch for the skip gear shift...it takes practice , but is possible, and uses the engine to spin up the driveline as you pass through neutral. but this takes some practice to do more good than harm as engine blip speed vs shift timing can be a friend or foe in this situation.

Personally, i don't like to see people skip gears, because usually, you end up with shift points like our "BMW" friend in the video.. in the wrong gear, shifting too late, getting on the power late, etc.... it just is one more set of reasons that can cost lap time. (and as i mentioned before, control sacrifices on the approach to a turn during the decel phase by not keeping the rear wheels connected to an engine at the higher RPM ranges.) If noticed a LOT of crashes where folks have been in too tall of a gear entering a turn and didnt have the power control of the rear end because of being in the wrong gear and too low of RPM, less stability, engine braking, etc.

AO

I used to skip gears, Mark told me not to, but I did anyway. Trans started making weird noises. Swapped it out before it blew up (still need to tear that apart). But the only thing I can think that is causing the noise is either a bearing or or a synchro. No matter, I decided maybe I shouldn't skip gears after all. So I don't do it anymore.

I've also really worked hard on my Heel-Toe. It's finally coming together - quite nicely too I might add. It's becoming very natural.

One thing I still don't get, if I have to drop 2 gears (like China Beach at Mid-O), would I blip for each gear? I think one would, right? I'm trying to imagine what it would be like to shift 2 gears but blip once... feel weird.

Der ABT

Isn't the real question....what does pdk do?

Sorry had to

jscott82

To make the coolest sound.. ya gota blip twice...

(Forget the word "blip".. think "rev match"... anytime the clutch is coming up, you want the input and output shafts spinning at the same speed.)

edit: For bonus points you want the intermediate shaft at the right speed too... but that's double declutch, downshifting, and takes a little longer to execute... plus its like patting your head and rubbing your tummy... And its doesn't sound as cool anyway so why do it.

mark kibort

Andrew, try and read over what i wrote above. I was trying to make clear that the blipping has no bearing on the spinning up of the layshaft, thats only to make a clean release of the clutch which mates the engine to driveline speed without undue forces. if you are going to drop two gears, you want to push clutch in , pull out of gear, blip whiile going through neutral, and fit into next gear...release clutch....... repeat for the next gear. each shift is an indivitual load on the "next gear's" synchro...... i think what was talked about ,was skipping a gear, (eg shift from 4th to 2nd) and then you would blip once with a higher blip than usual, (because you are starting from a lower RPM) and the synchros would spin up the layshaft and driveline (with clutch discs) to the syhcnronous speed. Now, becuase you have a dual disc clutch with an intermediate plate, the drag of a partiallly depressed clutch pedal is enough to go thorugh neutral on a shift, and blip, and this will spin up the driveline before the shift. this can be done by skipping gears too. But again, if you get it wrong, its even worse on the synchros. (thats why i mentioed dipping or dragging the clutch between shifts can be a friend or foe)

the 928 synchros are VERY sensitive and wear out very easily. (even in street use) this is becuase there is a fairly massive torque tube that joins the transmission and the transmission itself ,and some pretty massive gears internal. (given the low numerical rear end of 2.2:1 vs most cars in the 3.3 to 4:1 range)

so, being too quick to shift or skipping gears can accelerate the wear even more ... those wear parts fill the fluid with metal that wears the bearings and then things go south from there. there is a reason ive been able to race a 928 stock transmission given to me being 15 years old to start and i raced it for over 150 race days with no issues until 5th gear just broke due to fatigue. synchros were perfect. i still have the gear box and when i get a 5th gear, thats alll ill replace probably!.............maybe..


Yep! But, dipping the clutch has the same effect as double clutching. its a technique that really helps speed the shift and save wear on the transmission. but in all cases, there is little of a case to skip gears. to many risks, wear and sacrifices to make it a regular practice.........especially if you dont have a close ratio transmission... best always to blip, shift and go through the gears.......always.

winders

Not really. "Dipping", as you like to call it, doesn't work well with high grip clutches that are basically on or off. If you dip the clutch and there is friction, how can there be no acceleration forces when you blip the throttle?

You seem the whole world has your transmission and clutch....

More bits I will never get back......sigh.

CrookedCommie

Mid Ohio China Beach double downshift you say?

mark kibort

agreed.... to a degree.. but you are right, most of the very high grip clutches dont have much slip before max grip..but, ive been able to prove a change in synchro performance even with a pure racing clutch. one quick test , is if no blipping gives you a grind, and blipping doesnt on a simple downshiift test say at mid RPM levels.
there is a point where even a racing clutch will have a little bit of drag. its enough to spin the driveline, but not enough to move the car. (or accelerate it) Its much harder, but can be done.
by the way... "dippiing" was peter's term, not mine... i call it that because he didnt like "dragging " the clutch. Also, my advice to AO about "dipping " was due to him having a "transmission and clutch like mine" ... he has a 928..

the main purpose of the replies really is about not skipping gears to reduce synchro wear and little do to with "dipping".

do you have to try and be such a jerk on your posts or does that just come naturally?