TonyG

Update...

The car is out of the engine shop and back at Vision for a further sorting and finishing of a few things that we didn't get time to do last time.

But what's real interesting is that the car was on the dyno today. And I was under the car while it was on the dyno inspecting for oil leaks or any problems visually while the car was under full load.

And I witnessed the most amazing thing...... The torque tube actually bows under load. I really couldn't believe my eyes when I actually watched the trans slightly rotate relative to the 997 suspension cross brace. WOW.

I told Dwain (owner of Vision Motorsports) about this and he confirmed what I thought I saw..... Yes... the torque tube actually flexes under load.

Amazing!

Umm.... that will be the last time I'm under a 944 making big power on a dyno at full load. EVER.

What was I thinking?

TonyG

V2Rocket

Guess that's the result of 500+ ftlbs trying to make it to the wheels through a few small mounts...
Time for a beefed-up torque tube?

333pg333

Interesting. Did it bow in a constant direction or rotate?
Wouldn't be standing under there at all!

TonyG

What I saw was the transaxle very slightly counter rotate to the wheel rotation.

The transaxle tail must have rotated, relative to the 997 cross member, at the about 1/2"-3/4"(visually observed... see avatar).

I took some cell phone video. It's not great, but you can still see it if you look closely. I'll have to edit it to cut everything else out and post it.

TonyG

333pg333

Have you braced your gearbox to the chassis? Simple mod worth doing.

TonyG

The brace you have is a lateral brace (left-to-right).

No I don't have one. And I don't need one because the trans is mounted rigidly to a custom mount which is part of the sub frame. It can't move left or right.

But what I see is the trans rotating, counter to the axle rotation. Not left to right movement.

This can only occur if either the engine is moving upward, or the torque tube is flexing.

TonyG

333pg333

So the trans is moving up and down (fore to aft)? Seems like it's floating out back but I'm just using your Avatar as reference. I'll go back and have a bit more of a look for a larger pic.

TonyG

No. The trans is rotating very slightly under full load, in the opposite direction of wheel travel.

TonyG

URG8RB8

I am with Patrick's thinking on this. The problem being the output shafts are perpendicular to the torque tube/input shaft. The way you are describing the rotation would indicate a fore to aft rotation opposite of the output shafts rotation. At least that is how I am picturing it. Is the transaxle body/housing rotating opposite of the input shaft rotation direction? ie... Looking straight at the back of the transaxle CW or CCW?

TonyG

No.

The transaxle is rotating in the opposite direction to the axles.

What makes this odd is that you would think that this would not be possible because the transaxle is connected rigidly to the engine via the steel torque tube.

But apparently the torque tube, under high torque conditions, actually flexes albiet a small amount... it's enough to observe with the eye.

TonyG

Chris White

Good old basic physics. For every action there is an equal and opposite reaction. The torque from the engine is being transmitted by the shaft and the main thing stopping the whole assembly (engine/trans) from spinning is the engine mounts. So the torque output from the engine is traveling down the drive shaft to the transaxle and an equal and opposite force is required to keep the whole assembly from spinning.

If you want to remove the stress on the torque tube shell you would need to stabilize the transaxle - the best way is two cross braces similar to Particks but further from the center line of the transaxle and going to opposite sides.

Here is a crude sketch up from Patrick's pic. (you don't need the original brace anymore - just the upper and lower - also if needed that can run to the same side of the car but it would be better to run them top opposite sides. The longer you can make the triangle mount the better it will work.

V2Rocket

Chris,
Ideally wouldn't the braces to counteract this phenomenon be mounted vertically?
From the tail of the trans up to Tony's cage X-brace perhaps? Then the torque bending the torque tube would be directed through the whole cage/car instead.

The OD of the torque tube should just allow for a 3.5ID steel tube to slip over it and weld into place, if you wanted to try to stiffen it.

URG8RB8

Looking at Chris's mock up picture, again I only see this helpful for CW or CCW rotation looking straight from the rear, which I thought Tony was originally describing. Apparently, this is not what Tony is experiencing, or I am totally lost. Can't wait for the video.

Big E

Sounds like the transaxle is rotating in the car's pitch plane, and if it's opposite direction of the wheel spin, then the rear of the transaxle is moving down, and the front is moving up, with the inner cv joints being the axis or rotation.

Interesting stuff, would be better to observe via a remote camera than by eye at 3 feet I think!

JustinL

Tony, what gear did you observe this in when on the dyno? To me it seems like the torque at the transmission is equal and opposite the torque at the wheels. That would suggest that the torque tube will bend most in low gears. The suggestion to grab the back of the transmission and brace it to the chassis with a vertical strut makes a lot of sense to me. You could also weld some angle iron down the length of the bottom of the torque tube to try to get some stiffness out of it that way.