noturavgm

Say you are driving along at 5k rpm in 4th and need to downshift to 3rd for the upcoming corner. The equivalent rpm for that road speed in 3rd is, lets just say, 6k rpm.

If you downshift and put the clutch in, all your engine is ever doing from that moment on, without a rev, is slowing. Even if your clutch is dragging (a problem), your layshaft will never be spinning greater than 5k (the moment of shift initiation). That 1k rpm difference needs to be made up my a) synchros in a synchromesh box, or b) a rev in neutral in a non-syncrho/worn synchro box.

Agree?

ProCoach

Real performance driving is a) solving your defective or inadequately cooled braking system, b) matching revs as best you can secondarily to the best possible execution of maximum deceleration, i.e. maximum braking, and c) blending the end of braking into the cornering phase in order to AVOID the push by maintaining that forward weight distribution bias you have so carefully calculated. Keep working on those three things, Mark.

Scott!

There lies the crux of the matter. Think of synchros as FRICTION BRAKES.

One shaft in the gearbox has gears fixed to the shaft, the other shaft has gears that rotate freely on the shaft.

The gears all move at different speeds because they are all of different, but complimentary and matched radius (center to center), tooth counts. They ALL move, ALL the time.

The way particular gears are "engaged" easily is by a synchro ring that sits between two sets of gears, which through a "hub" locked to the shaft that the gears rotate freely on can, when beginning to become engaged with the gear, compress the synchro mechanism within the gear to "match" the shaft speeds of each, then the synchro ring can complete the engagement to "lock" the free-wheeling gear to the shaft. Shift done.

Now, this synchro "ring" is equipped with small engagement "dogs" on the inside of the hollow ring. These "dogs" mechanically engage with the small engagement "dogs" which are part of the desired, selected gear.

When you hear the "crunch", it's the dogs clashing. A worn synchro is one that does not, under engagement, squeeze the gear "brake" enough to get the gear up to speed, the SAME speed, as the shaft, hence the hub, hence the synchro ring is turning!

Again, the synchro ring squeezes on the "brake" that is built into the synchro mechanism of the gear and quickly (when they are new) and less quickly (when they are old) matches the gear speed to the shaft speed.

As the ring and brake mechanisms gain clearance through normal wear, they no longer act on the gear to "match" the shaft speed of the synchro ring.

I was an Alfa mechanic for thirty years. I KNOW about synchros and how gearboxes work.

It was GRAVY for me when someone complained about clashing gears due to worn syncros. The first fix was to put the proper OIL in it!

Oddly, Alfa called it a "Porsche style, baulk ring synchronizer."

Ah, no.

Not true on the bold observation. If you don't let the revs drop too quickly, you don't "lose" the shaft speed (which occurs when you depress the clutch and disengage the transmission from the engine) and "lose" the gear.

Timing is everything. No need for letting the clutch out to speed up the shafts between downshifts. Not even in a dog (non-synchro) box...

Good MAN! The light bulb is coming ON!

TXE36

Veloce Raptor

Using compression braking to slow the car?

Seriously????????

noturavgm

Mmmm, not sure I fully agree. Higher gears with a lesser rpm differential; agree. However the given example was 1st @ 40mph. I looked on my drive home today and there is about a 1200rpm differential in my car from 2nd to 1st. I would feel bad about myself just ramming the shift lever into 1st and asking, what are probably the second most overworked synchros in the whole box, to spin the layshaft up 1200 rpm in a fraction of a second. Personal opinion: no reason to downshift into 1st at over parking lot/traffic crawl speed. Could a new P car gearbox take that abuse? Probably. Would I do it to a 10+ y/o gearbox? Nooope.


Dog box is a whole other animal

ProCoach

Yes. I don't know anyone that would select first gear in a street car (or a racified street car, what I assume our man Kibort is driving) anywhere on any track at racing speed...

1200 rpm is not a big spread. Sounds more like a top gear drop. In following proper procedure, you would BRAKE until the revs were lower, by some margin, than 1000 rpm less than redline in the higher gear, then blip and d/s at the very end of the braking zone.

There are too many BASIC skill executions being mixed and mashed here, it's hard to keep up!

ProCoach

Now, c'mon! I did that when I sneaked out in my grandmother's VW Beetle. When I was 12. 'Cause it sounded cool...

noturavgm

Of course. I am just thinking that 1) I don't ever put my car into 1st beyond basically traffic speed, and 2) If I ever had to downshift even into 2nd at track pace (haven't been on a tight enough circuit to necessitate that yet), I'd probably double-clutch that particular shift. Self preservation instinct for a 1999 gearbox ;-)

ProCoach

Again, I have spent the last thirty years of my life, 154 days so far this year, at the race track. The slowest corners at the major North American tracks that I have been to this year are Namerow at Le Circuit Mont Tremblant, Turn 11 at Mazda Raceway Laguna Seca, Turn 11 at Circuit of the Americas, just to name a few... The slowest corner at Watkins Glen is twenty-four miles per hour faster than those, the slowest corner at Lime Rock thirty-two miles per hour faster.

There is NO reason for anyone with street car gearbox ratios to engage first gear ANYWHERE in North America, except leaving the pits or returning to the paddock. Second gear? Well, that could be a usable gear at the three corners I've noted above, but braking takes a higher priority ALWAYS than whatever convoluted heel-and-toe, double-declutch, "big blip" rev matching most people decide is necessary. If you can't get it done, leave it in third...

My .02, YMMV.

noturavgm

"Any day at the race track is better than a day at the office"... wait ;-)

Sounds like an excellent profession.

ProCoach

It's fun. What has been illuminating is for years having studied what the fastest guys do to go fast...

Those techniques and executions that make them fast are remarkably similar among a broad range of drivers and across a wide range of cars.

It can be learned... Which gives us ALL hope!

Veloce Raptor

45 pages

333pg333

That's the Aussie Peter Brock, not the US version. Both legends in their own backyards!

mark kibort

again, always a post with no value.

Pretty funny. this is a discussion. the debate and suggestions are all good... even your partial quotes are amazingly effective in making key points stick. (for those that are receptive to learning and maybe helping out to try and solve an issue)

You come on the scene, blasted by your own bros here, about not knowing the math. you try and quote a smart brake guy, but miss the punch line about the power dissipation. (where your quoted expert's last sentence says, "more importantly" that the speed is what is more weighted to influence the KE.
you conveniently miss this. classic VR. are you here to solve the issue or just prove your right, and God like to promote your own, "ask the coach " threads. .

another comes on to blast my response about going lighter, not being a solution, and then he actually strengthens the case AGAIN by replicating my math, using simple numbers, and shows that 10% loss in weight would have to have a 5% greater speed to be an absolute wash in heat dissipation. Amazing that my approximation of 4% increase in speed is any less valid than anyone elses here. thats the problem with this discussion. the physics are the physics. the data has to be in the ball park and mine is.

Then on other threads, someone says they get 6mph faster for a 20F degree temp change and you guys praise and join in like you see that all the time.
(again, 20 degrees can be around 5-10hp, and my 200lb loss is more like 20hp just to show how more believable that might be)
There are so many factors, none of which you even consider in your arguments or when you start to try and make fun of folks with different opinions.

HERE IS THE DEAL:

I have a braking problem. it happens in a narrow section of track, using small rotors, but dealing with big energy. (and we are not just talking 130 to 50mph, we are talking this range of speed change over a fixed distance and/or time)
this points to a "heat bucket " issue. happens no where else. not many other cars have this issue, and no one even looked at the 911 vs 928 weight balance to see how you can have double the weight under moderate braking, in the rear and 20% less weight in the front, greatly reducing the force capability at the limit of the 911 front tires. this is well in range to solve the problem, if i could mount my engine in the back... And, the problem happens, even with a cool down hot lap, using no brakes as fast as i can go.

Someone also mentioned they see downhill and same speeds at a track near them, and quotes aero factors are not being considered as well. i provide slope information of this section of track being a 10% downhill grade.... steep enough to keep a chevy tahoe at 80mph with no decel.
But, i also mention that even if they have the same speed, and downhill grade, it means nothing unless there is a time or distance that is similar.. and the car is the same weight and of course , tires.

Others talk of lighter cars, and then miss the point again, of the KE being much more weighted by speed, than weight for the same vehicle speed.

The net net here is that i have a problem. the discussion has ruled out several ideas. the easiest is to just mount the larger rotors and call it a day , with 20% safety margin to solve the issues. ducting , and bias also were talked about. ducts are in my hand now, and i have the expensive temp paint to do some testing.

so, let me get the ducts in. put the temp paint on key parts., install the ducts and also put on some 2 piece rotors. Im also going to buy a set of the XP20s. dont see how that can hurt either.

mark kibort

uhhhh, yeah.... ever feel the decel of the engine braking, without even using the brakes? Ive run laguna faster that you EVER have, by not even touching the brakes! why???? engine compression braking is a huge rear brake bias assist!