Downshifting after a long straight... which way is best?
05-11-2017, 11:55 AM
#61
Supercharged
Rennlist Member
Rennlist Member
Join Date: May 2002
Location: Back in Michigan - Full time!
Posts: 18,925
Likes: 0
Received 59 Likes
on
33 Posts
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..
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..
Mid Ohio China Beach double downshift you say?
https://www.youtube.com/watch?v=h3rPOJeQtGY
https://www.youtube.com/watch?v=h3rPOJeQtGY
05-11-2017, 01:18 PM
#62
Three Wheelin'
this is a bit contradictory. downshifting usually puts you in a worse place on the torque curve, but a better place on the available at the wheels torque curve due to gear multiplication. you have to factor in the total torque available at the wheels. The next higher gear has less torque multiplication, so with very few exceptions, you are nearly always better off in a lower gear for acceleration (even past the torque peak). As an example, a 997 GT3 has 37% more torque multiplication in 2nd vs. 3rd gear.
05-11-2017, 01:40 PM
#63
Three Wheelin'
I agree with Mark on going down step-wise, but that VIR (love that track) video is intriguing. You can always learn...
Going step-wise, I don't have to think about missing my speed/gear ratio/RPM timing, so "get it just right" doesn't have to be in my head (not much room in there on the track). Also, it looks like VIR/BMW guy is shifting really late after trailbraking (often near the apex). That's a dangerous spot NOT to have quick throttle access in a 911...
On the other hand, I don't agree there is 5x more wear in skipping gears. That doesn't make sense. If you shift into 2nd gear at 45mph, you have to match the lay shaft speed to the drive shaft speed, and the collars don't "know" whether you have come from 4th or 3rd. The work of the synchro (spinning up the collar/dog teeth to match the gear speed) is exactly the same. You have to match the engine speed in 2nd gear to the car speed - that equation doesn't change.
I do agree that you would see increased wear if you skip-shift and let out the clutch too early (over-rev and extra wear on the synchros)
Going step-wise, I don't have to think about missing my speed/gear ratio/RPM timing, so "get it just right" doesn't have to be in my head (not much room in there on the track). Also, it looks like VIR/BMW guy is shifting really late after trailbraking (often near the apex). That's a dangerous spot NOT to have quick throttle access in a 911...
On the other hand, I don't agree there is 5x more wear in skipping gears. That doesn't make sense. If you shift into 2nd gear at 45mph, you have to match the lay shaft speed to the drive shaft speed, and the collars don't "know" whether you have come from 4th or 3rd. The work of the synchro (spinning up the collar/dog teeth to match the gear speed) is exactly the same. You have to match the engine speed in 2nd gear to the car speed - that equation doesn't change.
I do agree that you would see increased wear if you skip-shift and let out the clutch too early (over-rev and extra wear on the synchros)
05-11-2017, 02:46 PM
#64
Rennlist Member
As I understand from college physics, as long as torque is rising, so is acceleration. Once you pass the peak of the torque curve, acceleration slows down. You are still speeding up as you approach redline, but less so.
If downshifting puts you in a better place on the torque curve, you would accelerate faster...
Hypothetically...
If downshifting puts you in a better place on the torque curve, you would accelerate faster...
Hypothetically...
so no, downshifting doesnt put you on a better place on the torque curve, just the opposite. it puts you in a better place on the HP curve so that you can make more rear wheel forces using less torque fromm the engine, but higher HP.
always try and think that HP is the vehicles capacity to accelerate at any given speed.
Last edited by mark kibort; 05-11-2017 at 03:02 PM.
05-11-2017, 03:00 PM
#65
Rennlist Member
I agree with Mark on going down step-wise, but that VIR (love that track) video is intriguing. You can always learn...
Going step-wise, I don't have to think about missing my speed/gear ratio/RPM timing, so "get it just right" doesn't have to be in my head (not much room in there on the track). Also, it looks like VIR/BMW guy is shifting really late after trailbraking (often near the apex). That's a dangerous spot NOT to have quick throttle access in a 911...
On the other hand, I don't agree there is 5x more wear in skipping gears. That doesn't make sense. If you shift into 2nd gear at 45mph, you have to match the lay shaft speed to the drive shaft speed, and the collars don't "know" whether you have come from 4th or 3rd. The work of the synchro (spinning up the collar/dog teeth to match the gear speed) is exactly the same. You have to match the engine speed in 2nd gear to the car speed - that equation doesn't change.
I do agree that you would see increased wear if you skip-shift and let out the clutch too early (over-rev and extra wear on the synchros)
Going step-wise, I don't have to think about missing my speed/gear ratio/RPM timing, so "get it just right" doesn't have to be in my head (not much room in there on the track). Also, it looks like VIR/BMW guy is shifting really late after trailbraking (often near the apex). That's a dangerous spot NOT to have quick throttle access in a 911...
On the other hand, I don't agree there is 5x more wear in skipping gears. That doesn't make sense. If you shift into 2nd gear at 45mph, you have to match the lay shaft speed to the drive shaft speed, and the collars don't "know" whether you have come from 4th or 3rd. The work of the synchro (spinning up the collar/dog teeth to match the gear speed) is exactly the same. You have to match the engine speed in 2nd gear to the car speed - that equation doesn't change.
I do agree that you would see increased wear if you skip-shift and let out the clutch too early (over-rev and extra wear on the synchros)
However, as far as increased wear factors for a skipped shift, you might be missing one key factor. when you are in 4th, and slowing down to a 45mph turn (following the example) the drive shaft, and layshaft are slowing down too based on the ratios dictated by 4th gear. say you end up at 45mph and that is 2000rpm on the engine ... you quickly push in the clutch and select 2nd.... the engine speed required for 2nd say, is 5000rpm. for a given shift time, you need forces (synchros provide this force) to speed up the entire driveline/layshaft to match this new synchronous speed. the "collars" put force to speed up the driveline to the new speed. If you were to go to 3rd before hand, the RPM difference between 4th and 3rd is closer, and if the speed differential is 1500rpm vs 3000rpm, and the time is the same for the shift, the forces would be 4x on the skipped gear vs no skip.
so yes, the "collars" do KNOW if you are coming from 3rd or 4th. absolutely! because the speeds are dictated by wheel speed and gear ratios. driveline /layshaft speeds need to speed up before the shift can occure... this is what the sychros do when you start to enter the next gear shift gate. (engine is out of equation)
letting the clutch out fast or slow, is only forces on the clutch , engine and transmission gears and shafts.. the synchrs are not in the equation anymore after the gear has been selected.
05-11-2017, 04:45 PM
#66
Three Wheelin'
Doesn't the layshaft "coast" down pretty rapidly when decoupled from the prior gear and with the clutch in?
I'm surprised the freewheeling inertia of the layshaft has that much impact, but I guess it could (particularly with a "dual mass" flywheel).
I'm surprised the freewheeling inertia of the layshaft has that much impact, but I guess it could (particularly with a "dual mass" flywheel).
05-11-2017, 05:08 PM
#67
Rennlist Member
The "coast down" is a characteristic that is a factor for upshifts to be smooth and lower synchro wear. so yes, in that regard you are correct. we are talking about downshifts, and in this case, you need to spin up the layshaft to meet the new, higher RPM of the lower gear to mesh... this means, the layshaft, half the gears, and the driveline, including the clutch discs, all need to be spun up...........the synchros do this. the majority of their wear is caused by this. so when the speed target is double, the forces go up squared. thats where the 4x or 5x wear factor comes from.
05-11-2017, 05:42 PM
#68
Three Wheelin'
Mark,
I'm not an expert, but when the layshaft is decoupled, it likely slows down. So if you step downshift from 4th to 2nd, the 3rd gear syncho brings the layshaft up to 3rd gear driveshaft speed, then the 2nd gear synchro brings the layshaft up to 2nd gear driveshaft speed. I believe you may be assuming that the layshaft continues to spin at the higher gear speed after it is decoupled from that gear. That seems unlikely, but I don't really know.
We can speculate on layshaft inertia on the internet, or we could test this really easily. Just put your car in neutral, rev to 6k rpm, push in the clutch and let off the gas, then listen for how long it takes for the layshaft/clutch to stop spinning!
I'm not an expert, but when the layshaft is decoupled, it likely slows down. So if you step downshift from 4th to 2nd, the 3rd gear syncho brings the layshaft up to 3rd gear driveshaft speed, then the 2nd gear synchro brings the layshaft up to 2nd gear driveshaft speed. I believe you may be assuming that the layshaft continues to spin at the higher gear speed after it is decoupled from that gear. That seems unlikely, but I don't really know.
We can speculate on layshaft inertia on the internet, or we could test this really easily. Just put your car in neutral, rev to 6k rpm, push in the clutch and let off the gas, then listen for how long it takes for the layshaft/clutch to stop spinning!
05-11-2017, 06:25 PM
#69
Rennlist Member
Mark,
I'm not an expert, but when the layshaft is decoupled, it likely slows down. So if you step downshift from 4th to 2nd, the 3rd gear syncho brings the layshaft up to 3rd gear driveshaft speed, then the 2nd gear synchro brings the layshaft up to 2nd gear driveshaft speed. I believe you may be assuming that the layshaft continues to spin at the higher gear speed after it is decoupled from that gear. That seems unlikely, but I don't really know.
We can speculate on layshaft inertia on the internet, or we could test this really easily. Just put your car in neutral, rev to 6k rpm, push in the clutch and let off the gas, then listen for how long it takes for the layshaft/clutch to stop spinning!
I'm not an expert, but when the layshaft is decoupled, it likely slows down. So if you step downshift from 4th to 2nd, the 3rd gear syncho brings the layshaft up to 3rd gear driveshaft speed, then the 2nd gear synchro brings the layshaft up to 2nd gear driveshaft speed. I believe you may be assuming that the layshaft continues to spin at the higher gear speed after it is decoupled from that gear. That seems unlikely, but I don't really know.
We can speculate on layshaft inertia on the internet, or we could test this really easily. Just put your car in neutral, rev to 6k rpm, push in the clutch and let off the gas, then listen for how long it takes for the layshaft/clutch to stop spinning!
curious, why do you think i assume the layshaft spins at a higher speed when it decoupled from any gear? i dont. when you go from 4th to 3rd and pass through neutral, at that point, the driveline, layshaft, associated gears, start to slow slightly . its freewheeling................ 3rd gear will require a faster spinning layshaft, so it needs to speed up... the synchro hubs do this.. thats the wear factor.
yes, you can estimate the moment of inertia for the driveline, layshaft clutch discs, etc by doing what you said. ... then, do the calculations for bringing that inertia up to 5000rpm, from 2000rpm in a fraction of a fraction of a second. its a substantial force. one that wears out the synchros...... and the force is much higher 4x higher when you skip gears. (i.e. going from 4th directly to 2nd)