inadequate flex coupling (guibo)

My engine upgrade has overloaded the flex coupling on the back of the transmission. After 5000 miles it is damaged and causes vibration under even moderate acceleration. Is there a better, stronger, tougher version available to tolerate the higher power of the S50 engine? I imagine that the E-36 coupling will not likely fit.



Bob in Lake Havasu

I would just replace it with what is standard for the car. Could be the rubber was going south due to age and this just capped the deal.

Wish it were that simple. It was a new one about 5000 miles ago. The new motor has almost 240 hp so second gear is just too much fun. I will have to dial it back a little so this will not be a bi-annual exercise. If there were a polyurathane version it might last longer.



Bob in Lake Havasu

I have discovered part of the problem with the flex disk. The cabrio has a smaller version than the sedan and coupe. Probably need an expensive custom drive shaft to put a regular Universal Joint there instead of the rubber disk. I have saved the old one for a pattern to see if I can make something to replace it. I am thinking of some thin sheet steel on each side of it to add torsional stiffness without adding longitudinal stiffness.

Failure modes may be more detrimental when it goes bad though.

Bob in Lake Havasu

Make sure it's not in backwards.

Check this thread. [www.r3vlimited.com]

I looked all over the new one and the old one for an arrow to indicate which way it would be installed. I think the main thing is that it is a 110 mm diameter and not the larger one from a sedan. If there is one made of sterner stuff I would try it to have a longer life. Having to baby the thing to keep from having to change it again is no a lot of fun either. I was enjoying the higher power available. Now I am remembering the 4 hours of work to change it.

Bob in Lake Havasu

Beside what was said above, You may have installed a defective part, who failed prematurely.
It seldom happens, but is possible, and in that case you are worrying over nothing.
I had a V-belt snapping after only 3000km (previous endured 100000km), and an engine mount changed under warranty after few months of use.

If you think the sedan part is stronger and is possible to fit, there may the best way, and your car will still be "stock".
There have been BMW with that kind of power around for decades, and it's not a very frequent issue.

Despite the power increase, the flex coupling mostly "feels" torque and rotation, and I think there was not so much of an increase there, just more spirited driving (which takes a toll on the drive train).
Good luck!

...shouldn't this thread go into "technical"?

I debated about putting it in technical when I first posted but decided it could fit here too.

In the line of car talk though, A local museum is hosting a BMW 100 year anniversary event on May 15th. A local chapter of the BMW CCA is asking for folks to bring their cars and have a nice gathering of the mark. It should be good fun looking at all the nicest BMW from all over the Puget Sound area.

Bob in Lake Havasu

A local friend has found a "flex disk" replacement that should do the job. It is made of aluminum with some inserts of polyurethane with sleeves for the bolts.
It is made by Revshift.com. Not much flex to it though. I hope my engine is aligned well enough to the drive line. The website recommends the alignment be very good or it might rumble or vibrate.
I do worry about how much flex is needed torsionally to protect the drive train parts from crankshaft vibration. It may overload the transmission or drive shaft.

Bob in Lake Havasu

The reason for the existence of that part is to avoid transmission vibration and noise.
If the part is not flexible, any vibration will not be filtered.
Any experiences form people using that modification?

My friend put one on his car last week and says it does not seem to be causing any issues.
I want to put it on my engine modded car but am a bit concerned about not having some compliance in the drive shaft to absorb crankshaft vibrations that might damage the transmission.

Bob in Lake Havasu

Have any of you in this esteemed group used one of these to replace the guibo? [www.revshift.com]

Bob in Lake Havasu

So I ordered up one of the revshift disks. The instructions were pretty adamant that the alignment of the drive shaft and transmission had to be very close. So I started in to replace the damaged one. finally got the old one out and began trying to check how well aligned the two were. They were way out of alignment. There must have been two or three degrees of misalignment from the transmission to the drive shaft. Probably about a sixteeth of an inch in the 78 mm across the spyders. The standard bushing is only good for about 0.020 inches in that distance. No wonder my disk was not lasting very long.

I have moved the trans mount down a bit and the engine up a bit and have cut the misalignment about in half. I hope it will be enough. I am going back in with a standard disc and keeping the revshift version until I can figure out how to get the alignment correct.

Bob in Lake Havasu

Quote
Bob in Everett
So I ordered up one of the revshift disks. The instructions were pretty adamant that the alignment of the drive shaft and transmission had to be very close. So I started in to replace the damaged one. finally got the old one out and began trying to check how well aligned the two were. They were way out of alignment. There must have been two or three degrees of misalignment from the transmission to the drive shaft. Probably about a sixteeth of an inch in the 78 mm across the spyders. The standard bushing is only good for about 0.020 inches in that distance. No wonder my disk was not lasting very long.

I have moved the trans mount down a bit and the engine up a bit and have cut the misalignment about in half. I hope it will be enough. I am going back in with a standard disc and keeping the revshift version until I can figure out how to get the alignment correct.

I will give an update on the solution: The drive shaft alignment required fabrication of a new rear transmission mount with more adjustment. I managed to get within about .025 inches across the spyder. After 10,000 miles it is still going well with the stock disk. So far, so good.

Bob in Lake Havasu

Another update...after 40,000 more miles, it is still going. It is showing some cracks though so have procured a new one. Might get installed this week. With the modified exhaust the new engine required, it takes a while to disassemble and reassemble. A plus is that the new rear seal on the transmission I put in last time is not leaking yet. So, do not have to do that over.

Bob in Lake Havasu