Van

I think you're implying that the torque tube may have pressed down on this beam - but I don't think that is likely. First, the bend looks off-center - more towards the control arm mount on one side. Second, the rear of the car squats down on acceleration, but the transmission/torque tube shouldn't move realative to the chassis (at least not much - e.g. maybe 1/16" inch due to rubber flex in the mount).

mikey_audiogeek

Yep, that's what I'm implying. Agree with your comments, particularly the offset of the bend. However, it can be surprising how much a rubber mount can move. Anyway, just a thought.

Cheers,
Mike

TonyG

The cross member sits at the same location, and even if it didn't it's location would be irrelevant.

The transaxle torque is transferred to the engine through the torque tube. Which... is why they call it a torque tube btw.

The gear box doesn't move under acceleration because of this.

TonyG

gruhsy

How about "Moment of Force Tube" MFT

333pg333

Has anyone tried to phone Bruce?

chrenan

I've got his number if any of the other customers want it. At this point I'm done with this thing, I don't want to put any more effort into it. I'll just consider it an expensive lesson in being on the bleeding edge of product development.

333pg333

I would be hopeful that someone could contact Bruce and at least get his take on this. Does anyone live near him? The unfortunate aspect is that I don't believe he ever intended to rip anyone off with this part. I've always had trust in my dealings with Bruce. Clearly he's put a lot of time into it, but the problem now is that faced with a mass refund on monies that I assume are long since spent. I would say that those that purchased it (inc myself) have to assume risk on an unproven aftermarket part.

MC 968CS

In addition to the queries from Dubai, which I second, I suggest that this is not worth running with. There has clearly been no proper load analysis here, except by Porsche on the original item.

Just worried someone is going to get killed playing 'engineer', when they have no idea what they are doing. Madness.

Stay fast and safe and have more fun.
MC968CS
(non practicing engineer)

chrenan

Absolutely, I have no doubt at all that Bruce is a good guy and has the best intentions.

I agree, but I'll reserve the right to still be frustrated. I'll stomp my feet and pout for a while and then have to make my peace with the situation and move on.

mclarenno9

The real problem is you are taking a system designed around torsion bars and loading it completely different from how it was intended. There is a relatively large distance between the shock mount and the hub. This is why the effective rate when running just coilovers is something like 63%. This places both a vertical load (as Van described) and moment at the trailing arm to torsion bar carrier mount, which is bending the tube downwards. Yes, the same loading scenarios are present with just a shock, but are probably 10x less.

With decent relative measurements of the loading points and some assumptions of acceleration, bump, and cornering G's, you could perform a relatively simple static analysis of the system and size the tube accordingly.

I'm just guessing here, but say the tube is 1/8" thick... Increase the thickness to 0.25" and you would reduce stress (and deflection) by 40%. Even better, increase the O.D. of the tube.

333pg333

Dropped the original part around to the workshop today. It will be going back on.

Sure, there is no hard data in terms of this piece. Raising the pickup points in theory should allow the car to be lowered and not get into subterranean roll centres. My reason for buying this part was more to do with this than losing weight. The issue I had with the previous version of my car were mostly to do with it being too high and losing suspension travel. There is no way I'm going to let ride height dictate terms again yet I don't want to have the lowered car affected by poor roll centres. It is now sitting a lot lower than before. I only hope that by going back to the OEM part that we don't sacrifice too much in geometry.

mikey_audiogeek

Tony, I agree that the torque of the engine is coupled to the gearbox - but did you consider the torque reacted out through the wheels?

There is an overall moment arm from the drivetrain that is reacted out via the engine mounts (upwards force) and the trans mount (downward force).

For an engine putting out 400Nm, this torque would be approximately (400)x(1st gear ratio)x(final drive ratio) = about 4000Nm.

Reacted through the drivetrain, this would create an upwards force of about 170kgf (1700N) at the engine mounts, and a similar amount but downwards at the trans mount.

Cheers,
Mike

TonyG

It's the torque output at the wheel. And the fact that the exact same torque is also generated, in the opposite direction, which is transmitted through the torque tube, to the engine, then finally to the motor mounts.

Additionally, there is no torque causing a downward force on the transaxle. It's purely a rotational force. In other words, the transaxle doesn't drop under acceleration. Any downward force at the transaxle would rip the transaxle mount instantly.

TonyG

mikey_audiogeek

Hi Tony, I know you've got a lot on your plate at the moment.

So I've attached a free body diagram to show what I mean more clearly.



From Newton's 3rd law: Ra = - P

In this case:

P = reaction force from motor mounts due to applied axle torque Ma
Ra = reaction force from trans mount due to reaction force P

(of course, these forces are only about 170kgf, so equivalent to about a 3g loading)

With compliant mounts, this would result in the motor moving slightly upwards, and the trans moving slightly downwards.

Cheers,
Mike

zeusrotty

Just have the thing reinforced externally and move on? A few good solid pieces of steel in the right places will give it more than enough strength. And how dare any of you call yourselves racers without a single suggestion of racing tape or zip ties. I'm ashamed for you.