34 replies to this topic

[SaulSpeedwell]

Hello all,

 

I have received a lot of inquiries about "stripped" 3rd gear in SM gearboxes - reportedly happening with multiple drivers, multiple years of cars, mutliple rebuilders and years of transmission. 

 

I have not had one of my personal or customer gearboxes experience this and it generally isn't something we commonly see in GLDiv/CENDiv that I am aware of, but I do have some thoughts on the topic:

 

1.  The solid-center 3-puck (similar to ClutchNet 3BK10451), and 4-puck, discs favored by many is a worst case scenario for drivetrain shock loading.  These discs do not need a +40% pressure plate, which only makes things even worse.   

 

If you are having repeat 3rd gear failures, you should consider running something like a spring-centered clutch with 6 pucks (e.g. ClutchNet 6BS10451).  I ran this setup for years in the 1.6 with 1.6 diff.

 

The tiny weight difference that close to the center of the crankshaft is not going to keep you from pole or a win.

 

2.  If your driving style is to pounce on the brakes without having the clutch RELEASED, you are shocking the bejesus out of the drivetrain.  An SM has 120 ft-lb of engine torque but many times that in brake torque.   

 

3.  If you don't revmatch on downshifts, you are obviously adding to the problem - but I doubt this is enough to cause the failures.

 

4.  I have no idea if fluid could help or hurt, because I don't have any personal experience with the problem.  I don't love MTL and MT-90 for these trannies, mainly because they seem to cause a lot of wear on the forks.  Both are also a little thinner than Mazda specs, although oil viscosity rating methods are about as consistent and meaningful as ranking jalapeno hotness.  MT-90 is closer to the specs, last I looked.

 

I try to get my guys to run Motorcraft Full Synthetic Manual Transmission Fluid Part# XT-M5-QS, but many of them still run MTL and MT-90.

 

If someone has failed parts they can send me, we might be able to make some sense of it.  If nothing else, we might see if the failure started on the drive side (upshifts) or the coast side (brakes and downshifts).

 

Mr Hindsight says:  We should have spec'd friendlier clutches and smaller tires

[SaulSpeedwell]

P.S.  I have an ultra-abusive 240 RWHP blown Miata autocross customer on fatter tires than we run.  After dozens of autocrosses, and thus hundreds of drag-style launches, a few trackdays, and 3 years of street hooning, all on a 100K+ mile core, he eventually ripped 2nd gear out of it. 

 

What he DOES NOT HAVE is an unsprung puck clutch - he has a very streetable full-face clutch with a sprung center and a stronger pressure plate.

[Michael Novak]

Mark,I have a few I can send. I think Waterford hills where I run often is much of the problem. I have a sprung ACT, and some of the time I would say I am terribly abusive also. We had been thinking it was two corners where you are on the brakes hard and the car than skips over a few bumps sometimes almost locking up.Mike Novak

[Jim Drago]

Mark

We had one customer who repeatedly ripped all the teeth off third gear. We determined at least his failure was  downshifts.  He was trying to downshift as fast as humanly possible, like a sequential shifter.  I have had only one trans failure in all my years of Sm racing on a built box. That was totally on me and lost third gear in the same way described above. I It was at hallet where you have a fast 4-3 down shift in high rpm revs.  I think repeated hard hits into third on the downshift were the culprit, then once I hit in the same spot twice and it sounded like a bomb going off  

Jim

[Jim Drago]

Hey, been looking forward to a new catchy screen name for the last 2 years

[MPR22]

In the southwest division it seems to a be a new phenomenon, I can almost trace the dates of the rebuilds and their failures. We have not seen a problem with the old street trans. 100% rebuilds with new gears. Several of us have are now using sprung cluthches and stock pressure plates to help the situation. I'm not a good case study as I am a ham fisted transmission abusing SOB. Chris Haldeman just lost a third and I believe he can send you pictures of several of the grenaded gears.

[HoneyBadger - BrianW]

The failures that Chris and I both have had, have been under load in third gear. I wouldn't doubt that high speed downshifts into third and heavy breaking while still in gear (both of which I do) contribute to stress. But the actual failure typically occurs about 3 or so seconds after up shifting into third. 

[Ron Alan]

Hey, been looking forward to a new catchy screen name for the last 2 years

 

Elvis

 

No need to explain

[Bench Racer]

Hey, been looking forward to a new catchy screen name for the last 2 years

 

And I don't mean Fred or Son.

[Ron Alan]

+ DOG would be catchy....assuming you also substituted yard

 

Man that clip brings back memories!

[FTodaro]

Mark, I have only seen one box with a detonated 3rd gear, and I think I already shared those pictures with you. I just wonder it this uptick in failures is connected to running Hoosiers vs. toyos?

On the oil, I agree that MTL is to thin, I have switched to MTL 85 but have not used it long enough to record Snyder results.

[SaulSpeedwell]

In the southwest division it seems to a be a new phenomenon, I can almost trace the dates of the rebuilds and their failures. We have not seen a problem with the old street trans. 100% rebuilds with new gears. Several of us have are now using sprung cluthches and stock pressure plates to help the situation. I'm not a good case study as I am a ham fisted transmission abusing SOB. Chris Haldeman just lost a third and I believe he can send you pictures of several of the grenaded gears.

 

OK, wait a minute - this is interesting.  What do you mean "old street trans"?  And are you sure your rebuilders are using new gears? 

 

Less than 5% of the transmissions I build get a single new gear, and nearly all of those get a used gear from a bad core.  I can't think of a single box that has gotten a new 3rd gear.  I don't want to speak for other builders, but I would be very surprised to hear they are putting in new gears routinely or preventively as a matter of course-  it just isn't cost-effective, and we've had a lot of lessons in "New Parts Ain't The Best Parts" for these cars.

 

Could "new gears" be the problem causing a "new" phenomenon in SOWDiv???

[chris haldeman]

2 of the recent failures were new trans from Mazda. I should have 6 sets I can ship too you. 1 of the drivers having trouble down here is notoriously hard on things but has lost a third gear every single weekend this year. I have raced for 2 years on a stock 150k mile street trans with out failure. Then changed too rebuilt trans and increased holding power pressure plate and have lost third gear every other event since. Including 2 dnf's that really sucked. I like you am blaming the pressure plate and am installing a sprung disk in the future.

[chris haldeman]

With all that in mind the failures also started at the same time we went too Hoosiers.

[Jim Drago]

FWIW..
I have always used a stock Mazda pressure plate as I never felt we needed extra clamping force with all the power we are putting down I do use the three puck unsprung disc though.

Also agree with Mark, if we need to replace "gears" we start with a different trans. That is main reason I like building street transmissions rather than raced ones.
Jim

[davew]

I probably do as many SM gear boxes as anyone. I have not seen any correlation between sprung and unsprung hubs. If you do not know how to downshift, you will break these gears. I have never purchased a new gear. Always use the existing gear unless some weird things have happened. Then I use a salvaged gear from the pile.

 

I have never purchased a brand new gear box from Mazda. But the people I know who have, have told be they do not have very good reliability. My theory is that an old box has been work hardened over 100,000 miles of use.

 

I have had customers who go through a gearbox ever other weekend, once they learned how to down shift properly their boxes last 2 years.

 

At every trans fluid change, inspect the magnet for the "tang of death". If it appears, change the box before she blows up.

 

Early down shifts are the kiss of death. Brake-wait-wait-wait-downshift/turn in. The guys that brake-downshift-wait-wait-wait-turn in are the worst offenders. Even worse than those that do not throttle-blip on down shift.

 

Transmission teeth are not an equal-lateral triangle. They are more of a right-angle triangle shaped like this /l. Under normal use the force is put on the verticle side. When using the engine to brake (as described above) all the force is put on the angled side of the tooth. The angle acts like a ramp and the teeth start to walk up the ramp. This bows the shaft (not good) and puts the contact force on the tip of the tooth, rather than the root. Then the teeth break.

 

Early downshifts are also very hard on engines. Brake pads are easier to change and a lot cheaper than transmissions or engines. BRAKE_WAIT_DOWNSHIFT

 

I use and recomend MTL. I have not seen any excessive wear when compared to other fluids. I also agree that gear breakage is not at all related to fluid. It is either a drive controlled issue or just plane old age on the parts

 

Dave

[MPR22]

Will call trans guy and ask about the gears to confirm.  I often hear what I think I am going to hear and not what is said. 

[FTodaro]

This may sound strange but for me i think gear breakage and transmission issues is also related to blood pressure. over the last few years I have been trying to be calm in the car. When I get excited, i start banging the gears and shifting like a monkey. This season i have brought my blood pressure down considerably  and the repair bills have gone down too. Taking your time on the down shifts, when you can, is a big help.

 

I have to think that people going through a gear box a weekend are "excited" as i define it.

[chris haldeman]

I wish it was that easy for me frank!! Last one I lost I had a 6 second lead and was in cruise control

[Danny Steyn]

Early down shifts are the kiss of death. Brake-wait-wait-wait-downshift/turn in............Transmission teeth are not an equal-lateral triangle. They are more of a right-angle triangle shaped like this /l. Under normal use the force is put on the verticle side. When using the engine to brake (as described above) all the force is put on the angled side of the tooth. The angle acts like a ramp and the teeth start to walk up the ramp. This bows the shaft (not good) and puts the contact force on the tip of the tooth, rather than the root. Then the teeth break.

 

 

Dave

 

Hi I am Danny, I was a third gear addict, but I have been cured. I routinely consumed more than my fair share of 3rd gears as my tranny builder Joe Varble will attest too. However, once Joe explained how using the gearbox with any compression-load during downshifting, (even when perfectly blip matching the RPM),  places significant forces on the weak, reverse side of the gear, well that cured me, and I now downshift way way way later than I ever did,  And while I still consider myself prone to relapse at any time, I am coming up to my 12 month (40 race) anniversary real soon and I suspect Joe will be handing out my well deserved NO-3rd-gear button