141 replies to this topic

[Adax]

I could not hear the audio of the vid on my work computer, but by the title, I have rebuilt one transmission that had a twisted counter shaft I could get it apart but it was a real PITA. it still worked fine but not much fun to rebuild. It could just be the result of the extra stresses we put on the box that are above its design limits. 5th gear pulls on that part of the shaft so it must be something to do with the stresses in 5th. What did you say on the video. I will read it before i go to bed to put me to sleep.

I've suspected a good bump draft on the back straight does it.

[Danny Steyn]

Saul

we have two recent transmissions with twisted countershafts. This is something new for us, in your opinion, what is causing this?

is it related to

• downshifting?
• upshifting?
• bump drafting?
• anything else?

[Jim Drago]

Danny
We just had one ourselves, spoke with Mark and he put up the video after our conversation. IMO, I think it is contact that is causing, in our case bouncing off wall with rear wheel.

[Danny Steyn]

Jim

 

no contact in my case, other than strong assists from the rear :-) 

 

One tranny only lasted one day at Sebring, and then locked in 4th gear. Thought it might have been a hard bump draft from the rear. Was expecting it to be a bent shift fork, but it was this twisted countershaft that we see above.

 

But could be turn 7 hairpin. Have to leave the 3-2 downshift late for that turn and if you dont match revs correctly its possible to get wheel hop there as the rear wheels are so un-weighted (like at Barber Turn 6) But I have raced at Sebring and Barber for years without ever experiencing a twisted countershaft, so very keen to figure this out

[SaulSpeedwell]

 

Saul

we have two recent transmissions with twisted countershafts. This is something new for us, in your opinion, what is causing this?

is it related to

• downshifting?
• upshifting?
• bump drafting?
• anything else?

 

 

If we look at the powerflow diagram, we know that the countershaft twisting has to involve 5th gear.  In any other gear, the splined (and weakest) part of the shaft is "along for the ride" and only carrying its own inertia.  That would suggest downshifts are not a culprit unless the 5th synchro clutch has a problem and is not "releasing" when you come out of 5th.

 

NOTE:  When you hit the brakes with the clutch engaged - you ARE "braking" the spinning engine and transmission.  The ring and pinion and transmission gears "feel" that torque.

 

Questions for those experiencing this issue:

What years of transmissions do you notice it in?

Is it happening at tracks with high 5th gear usage?

Do you frequently "blow up" the 5th gear parts or stick the shifter when you shift into 5th?

Do you "pounce" on the brakes in 5th without having the clutch disengaged?

Do you happen to be at a track where you would be raging the curbing in FIFTH???

 

More later ... I'm trying to do some math to help us guess what "mode" is causing this ...

 

[SaulSpeedwell]

 

One tranny only lasted one day at Sebring, and then locked in 4th gear. Thought it might have been a hard bump draft from the rear. Was expecting it to be a bent shift fork, but it was this twisted countershaft that we see above.

 

 

Danny, When you say locked in gear, you mean the trans was stuck in 4th? But where was the shifter stuck?  And what were you doing before/when it stuck?

 

Of the twisted shafts I've seen, they would not CAUSE a stuck trans, and I don't think the shaft could be an EFFECT of a stuck trans either. 

 

More later ...

[Adax]

Danny, check with your transmission builder. According to Tom you and Skip have been twisting shafts for years.

[SaulSpeedwell]

Math update:

The rotating inertia of the parts "behind" 5th gear is approximately double that of the parts "in front of" 5th gear.

The engine is capable of applying ~100 ft-lbs of torque "through" 5th gear.

The two rear Hoosiers are capable of "resisting" momentary spikes of ~1150 ft-lbs "through" the ring gear, or ~270 ft-lbs "through" 5th gear.

 

Stir this all into a math salad, and it suggests to me that scenarios that "backdrive" the transmission have the highest torque potentials - such as crashes, going off-road and coming back onto pavement, wheel to wheels, braking that could lock the wheel, braking with wheel-hop.

 

I think the heavier/taller/stickier AND ROTATIONALLY STIFFER Hoosiers are a likely contributor - if your car has a super grabby unsprung clutch and +40% pressure plate, and you run Hoosiers, you are forcing something else to be the "fuse" - 3rd gear, the countershaft, etc.

 

NOTE:  With very little "springiness" and damping in this system, the shock loads and spikes are going to be MANY TIMES worse than a Miata with an organic spring hub clutch and little 185/70-14 M+S tires on little 11 lb daisy wheels.

 

Driving habits make a huge difference, and the clutch matters.  The unsprung 3-puck clutch is not a great laptime reducer - midpackers and below should consider running something gentler to avoid wear and tear on the car.

[SaulSpeedwell]

Seriously, nobody has any answers to the questions above?  Not even Boemler, Dewhurst, Karl, Daniels, Bernard, Scheifler, Holland, or Garza?  (Haha, joke for old-timers ...)

 

I have yet to personally see any of the stripped 3rd gears, and only one slightly twisted shaft on a 99+ trans - both were salvageable. 

[Danny Steyn]

Hi Mark

sorry I missed your questions above. I just need to clarify with my tranny builder that it is in fact the counter shaft that I am twisting and not the main drive shafts!!!

But in any case, I had two transmissions go in one weekend at Sebring, and there we get into 5th gear twice per lap and sometimes 3 x per lap if we get a great run out of T5 and a draft to T7,

At the entry to T17 we are in 5th gear and do some pretty aggressive braking (about 600 - 700psi), but we are slightly off balance but we are bouncing under braking.

On those occasions when I get to 5th gear going in to T7, yes there I am getting to the brakes really hard in 5th gear.

No raging on curbs in 5th gear at all.

No 5th gear parts blowing up as afar as I know.

One of the things to really watch out for is when there is someone closing really fast on your tail, to get your shift completed before he gives you the massive bump. At tracks like Daytona and Homestead we have seen several cars stuck in 4th with bent shift forks when they were bumped hard in mid-shift.

[Alberto]

 

NOTE:  With very little "springiness" and damping in this system, the shock loads and spikes are going to be MANY TIMES worse than a Miata with an organic spring hub clutch and little 185/70-14 M+S tires on little 11 lb daisy wheels.

 

Driving habits make a huge difference, and the clutch matters.  The unsprung 3-puck clutch is not a great laptime reducer - midpackers and below should consider running something gentler to avoid wear and tear on the car.

 

Thanks for posting that. I had considered 'upgrading' to a puck clutch.  I guess I'll just keep my ACT HD disk and pressure plate.  I got it super cheap when I built the car and it does the job.

[SaulSpeedwell]

Hi Mark

sorry I missed your questions above. I just need to clarify with my tranny builder that it is in fact the counter shaft that I am twisting and not the main drive shafts!!!

But in any case, I had two transmissions go in one weekend at Sebring, and there we get into 5th gear twice per lap and sometimes 3 x per lap if we get a great run out of T5 and a draft to T7,

At the entry to T17 we are in 5th gear and do some pretty aggressive braking (about 600 - 700psi), but we are slightly off balance but we are bouncing under braking.

On those occasions when I get to 5th gear going in to T7, yes there I am getting to the brakes really hard in 5th gear.

No raging on curbs in 5th gear at all.

No 5th gear parts blowing up as afar as I know.

One of the things to really watch out for is when there is someone closing really fast on your tail, to get your shift completed before he gives you the massive bump. At tracks like Daytona and Homestead we have seen several cars stuck in 4th with bent shift forks when they were bumped hard in mid-shift.

 

OK, that helps, Danny - that is a LOT of 5th gear stuff going on that I imagine most of our racers don't experience.  Unless I'm crazy or missing some transient jamming/locking "mode" of the trans, I think you HAVE to be in, or going into, or MAYBE coming out of 5th gear, in order to twist a countershaft.

 

If I had to guess, it is the braking/bouncing WHILE in 5th with the clutch engaged that is causing the issue ... but I know how it can be to try to change habits or setups when you are successful and fast.

[Michael Novak]

Mark,

 

I  have ripped the teeth off of 3rd at least 4-5 times in the past couple of years. One time at Waterford I would swear I heard each tooth come off one by one...

[SaulSpeedwell]

Mark,

 

I  have ripped the teeth off of 3rd at least 4-5 times in the past couple of years. One time at Waterford I would swear I heard each tooth come off one by one...

 

Year of gearbox?  Sprung or unsprung clutch?  RA-1s or something newer and stickier?

 

I really think this problem can be "fixed", its just a matter of how exotic and expensive we want to get with the gears, versus backing off on the clutch setups.

[Michael Novak]

Year of gearbox?  Sprung or unsprung clutch?  RA-1s or something newer and stickier?

 

I really think this problem can be "fixed", its just a matter of how exotic and expensive we want to get with the gears, versus backing off on the clutch setups.

99+   Have had it happen in my 99 and my 02.  SM6.  Sprung (ACT) and unsprung (3 puck).   Most problems have been with the unsprung..  Going to go back to the ACT setup. I race all over but I have actually broke most at Waterford. The second to third gear up shift has been 2/3 of the failures.

[Bench Racer]

Year of gearbox?  Sprung or unsprung clutch?  RA-1s or something newer and stickier?
 
I really think this problem can be "fixed",.

Agree 100% and when the root cause is found, the ROOT CAUSE should be eliminated, not a fix in between. If it's the sticky tire that anybody can feel like superman with, the tires need to go.

Who had transmission gear issues with the RA1 or 888?

For info, I'll touch bases with Jesse Prather if prod guys using the OEM transmission are having transmission gear issues. We didn't with Hoosiers and an F prod car when using the OEM transmission and the car was/is a Majors front runner.  Syncro rings, yes.

 

Brian Linn, I see you lurking. You have any issues with your F car with the OEM transmission. 

[wheel]

I'm not sure if this applies, but in ASedan, we found that the replacement gears for the T10 and T5 are all coming from China and have questionable metallurgy.  It is not that they are all crap, but some batches of gears just disintegrate during their first race.  

[davew]

This is not anything new or abnormal. I have been seeing this for years. Probably 100 times. But only on race boxes, never on a virgin core. Sometimes I can pry the reverse gear off the splined shaft. Sometimes I throw the box in the scrap pile.

 

Saul, I have a theory on how to keep this from happening. But I do not have the engineering expertise that you have. Call me and we can discuss

 

Dave

[davew]

I am going to take a few minutes and put this into some sense.

 

If you are breaking 1-2-3 you are abusing the down shift. Trans gear teeth are cut at an angle like this /l. The force under normal driving would be right to left <-. Under decel, the wheels drive the engine in a left to right direction. ->/l. The angle cut teeth act as a ramp, which tries to spread the shafts. This puts all the force on the tip of the gear tooth instead of on the root. The tips gain leverage and breaks the tooth. Once chip number one enters the gear mesh, all the others follow suit.

 

gears 1-2-3 are individual gears and can break individually. 3rd being the most common as we downshift into 3rd a lot.

 

4th gear really is not a gear, but a direct locking of the input to the output shafts. If you loose only 4th, it is usually a broken syncro.

 

When you loose all gears except 4th, you have broken the input shaft gear that drops power from the input shaft to the lower cluster. Since 4th gear doesn't use the lower cluster it still works.

 

5th gear fails rarely. Every 5th gear failure I have seen has been preceeded by a broken pinion damper. Syncros can break and notches on the hub clutch can get worn off by poor shifting technique.

 

When you look at the gear stack, the rear most gear is 5th and Reverse being next. The gear Saul is prying on is actually the reverse gear. Between that reverse gear and the 5th gear is a spacer. It is under this spacer that the cluster shaft twists. 5th gear always comes off the shaft easy, the rev gear only comes off part way and stops.

 

The twisted shaft in itself does not cause a problem. But it does make the box virtually impossible to disassemble.

 

Some drivers go through gearboxes like crazy. Why do you think Brian got so fast at changing gearboxes? If I got 2 weekends out of a trans for Voytek, I was happy. After years, I finally convinced him to shift properly. The gear box currently in the car ran  a Regional, the Runoffs, a Regional, two 1-hour enduros, the ARRC including the enduro, and now NOLA.

 

I'm not saying things don't break. With the abuse we put on these cars it is amazing they do what they do. But some drivers never break a trans. Tom Brown has had 1 trans failure in the last few years, and that was a 5th gear caused by a pinion flange. If you are consistently going through gearboxes, work on your technique. It will pay off in the long run.

 

Dave

[SaulSpeedwell]

Thanks for chiming in, Dave!  I haven't had a customer strip a gear yet, except 2nd in a 240+ rwhp autocross car with a zillion runs on it.  I honestly don't think I'm doing anything better or different, I think I've been lucky to have some gentle drivers running at gentle tracks.

 

One the ones you've seen, do the teeth indicate failure from coast-side impact or drive-side impact? 

 

Drago and I were joking about coming up with a "Thumbtack Covered Shift Knob" and how it would solve all SM transmission problems.  I never lost a transmission in my own cars, but I am wired with (probably too much) "mechanical sympathy" ....