FredR

Bertrand,

Many years ago when I was running the GTS stock wheels I ran with a 235 section up front and it was no problem whatsoever but then it was sat on the correct GTS ET65 wheels. That is not to say your issue is for sure the offset but it could well be a contributing factor.

For purposes of elimination I would try running with your specified tyre pressure but hot rather than cold just to test whether the issue modulates at all- I suspect it will not make any noticeable difference but who knows? If nothing else it helps eliminate a potential variable.

I have done several things that have potential to induce tramling but to date this has not happened and I put this down mostly to the fact that my front wheels have ET68. Indeed given the current [somewhat delapidated?] state of my front suspension bushes and upper ball joints I am puzzled [but pleased] as to why I have not noticed such issues.

Regards

Fred

dr bob

Jim's analysis reminds me that "running" toe changes based on offset and actual scrub radius. With the original 68mm offset, wear in the steering and suspension causes more toe in. As we move to neutral at ~55mm offset in front, changes in load/pressure/resistance will cause toe to move in or out depending on where on the tire the load is applied. Since the loads are not the same on both tires at the same time, the toe changes will "pull" the steering side to side within the wear limits of the steering bits. Read: Tramlining.

Conclusion: If you have less than factory offsets, some additional toe-in to your alignment settings may stabilize the steering a bit. It may help a little but is not a full answer obviously.

Bertrand Daoust

Thank you very much Jim.

I'll take a look at all this closely.

Is it better to do this with the car on jack stand or on the ground - if this is even possible?

Bertrand Daoust

Thanks Dr.

I'll take this into considaration on my next allignment.
Just did one last spring...

StratfordShark

That's funny Bertrand - I wrote that article and was just about to look round for a copy of it so I could post to this thread, but you beat me to it!

I bet you find a more elegant method than mine of torching the old bushes! In my defence I had no air tools then - this time round I would use air chisel and it would go much faster. Even after burning out the bushes, it was hard work by hand tool pushing in the peened ends of the bush so the steel sleeve could come out the rack.

Adamant1971

It's way easier just using a hole saw. I spent more time charging cordless drill batteries than changing them.

https://rennlist.com/forums/928-forum/825519-question-re-steering-rack-mount-bushings.html

StratfordShark

I wanted to do it that way first, but my local diy store did not have saws the correct size. Sounds much better!

Bertrand Daoust

I tried the original wheels this morning and... BINGO!

I started with only the front ones and the difference was instantly noticeable.

I then removed the back ones, adjusted the ride height to where it was before with the D90's, and put back the D90's on and went for a ride.

The car is now way more stable at any speed. No more tramlining. I didn't touch anything else.
All the original suspension parts like the tie rods and the steering rack and bushings are still in place.
Of course, they will need to be looked at in the future, but right now, everything will stay in place.

Now I got back the car that was so nice to drive. It wasn't like that this Summer.
Also, beside the tramlining, I like the smoother ride with the 16's. Day and night.

One last thing, smoother ride equal way less, if not any, "Airbag Malfunction" warnings that I had many times on every ride this year with the C3 wheels...

Thanks every ones for the good advices.

dr bob

I think we need to invent a lower balljoint with about a 20mm offset pin going into the spindle. So the ball could sit about 10mm outside the current location, relative to the spindle. That would effectively take care of the those who want to run 55mm offset wheels. Might make the support part 20mm or so longer too, so camber could be adjusted correctly.




















(no, it wouldn't really work... Caster would be way wonky)

Adamant1971

Good news. I felt the same when I ditched my C3's.

Adk46

I'm reading this with interest since I've agreed to buy Otto's C3s. They have 265/35ZR18 and 225/40ZR18 tires. He says he's not had a tramlining problem. Otto and I will be putting his wheels on my car tomorrow. I guess I'd better confirm that they're OK before making a commitment. How much driving around is necessary to make a judgment?

There's a highway near me that's worn down enough to cause tramlining on my manholes, and even for my Cayman wth stock wheels and tires. So, it must be "normal" to some degree. Otherwise, I would not know what tramlining was.

jcorenman

Interesting! What is the ET (offset) for the front C3 wheels?

BIMMERMIKE

Front offset is either 57 or 58
I'm running C3's with 235/40 and 275/35(21mm spacer) with no tramlining

Bertrand Daoust

Mine are 57.

Never thought that 8 mm would make such a difference...

Curt,
Many people ride there car with those wheels and are satisfied with them.
I was not. But that's me!

See you at frenzy my friend.

FredR

Bertrand,

Whereas I am not at all surprised by your conclusions it may be prudent to ensure you eliminate other possibilities such as a wheel not being as straight as it should be and possible tyre defects.

Tramlining will invariably happen to some extent on just about any modern high speed vehicle and the older the vehicle the more likely it is going to be felt. Then of course there is the driver factor on top of all this but at the end of the day all that matters is your perception of what is acceptable and what is not.

As with all things mechanical the difference between what excites certain conditions can be very marginal indeed. At least you now have a marker and if changing the front wheels made the difference between being acceptable and otherwise that boils down to one of four variables in play- wheel size/condition, tyre pressure, tyre type/condition or ET. ET would be my nominee but I would want to eliminate to my satisfaction the other possibilities in so much as I could before discarding/selling on the presumed offenders.

You are definitely making progress here.

Rgds

Fred