IRS alignment setting for wide rear tires

[Sandeep]

:raiseroof

Yessss ... The dark side !



I was reading that the 911 RSR's had OK handling but it was the width of the rear tires that gave them the amazing grip in the corners .. I LOVE IT !

Sandeep

[boygenius]

LOOKS AWESOME!!!!!!!! :righton:

[Bruce2]

You can adjust camber on VW trailing arms. Just loosen the bolts securing the trailing arm to the spring plate and allow the rear of the trailing arm to rise relative to the spring plate. If it won't go, file the holes into slots.

When I ran 225/50s I noticed the right tire was wearing out faster than the left. I measured the camber and found -1.5º on the left and -3º on the right. Eventually the left tire wore out completely even across the tread. When I replaced both tires, the replacements wore out the same.

[Panelfantastic]

Anybody have any tech info on what would be a good camber setting for a street car??

[Sandeep]

Bruce2,

Thanks for this information ! I will try your method first.

I'm going to try -1 deg front, -1.5 to -2 rear camber first, not sure about toe yet and see how that performs first. Still researching though. I'm looking to keep tire wear at a minimum but still agressive enough to have fun on the track.

Sandeep

[Bruce2]

Panel,
-1.5º on the rear produced perfectly even tread wear on my car. My tires had a contact patch of 8.5" wide. Sandeep's should be wider, so he may see more wear on the inside than I did at that same camber setting.

[Panelfantastic]

Bruce, I think he and I are looking at about 11"s at the contact patch.
The fun part is gonna be trying to figure out how to set up the rear suspension with all this new adjustment I have. Do I set it soft and enjoy better ride and better squat when I accelerate? Do I set it firm (like the front is factory ) and enjoy the great handling and bone jarring ride? I can't wait to play!

[miller]

Well panel from what ive read a stiff suspension isnt always teh way to go. It seems the purpose of springs to make them handle good is to control body roll and control camber curves (the less the suspension moves the less the camber changes). So I think a moderate suspension and anti-roll bars would be a good setup. It will also allow your car to take bumps better letting the tires to more work on the streets. eh thats just my 2 cents

[petevw]

how are you measuring this? big protractor?

pete

[Bruce2]

I think in theroy with a wider contact you will be able to run with less neg camber to ensure even wear. Or run the -1.5-2º and just accept the increased wear. It won't be too bad. If you drive more agressively you can go with even more neg camber and have even wear. But the local constabulary will get you if you do. I doubt you would drive harder than me. I get the inside front off the ground all the time. A couple of times I've even had the inside rear off.

Pete, I measure the camber with a machinists protractor. They come in what's called a Machinists combination set. A good one made by Mitutoyo will set you back at least $150. You get the 12" steel ruler that you can slide into a square, a 45º, or the protractor. The protractor has .5º marks on it and you butt the machined edge onto the machined face for the center cap of the wheel. Then rotate the protractor until the bubble indicates level. Take a reading then flip it around and do it again to eliminate the error of the protractor.

The low tech way is a string with a weight. Make some kind of stand to hold the string steady. Measure from the top of the rim to the string then measure from the bottom of the rim to the string. Then measure the diameter at which you did the first two measurements. Apply simple high school trigometery to get the angle.

[Panelfantastic]

...we are low-tech all the way around here! Strings, plumb-bobs, levels, framing squares, whatever it takes! I can actually get pretty darn close... and the alignment shop ain't too far .

...oh and Bruce, I wish I could still remember high school much less simple trig !

[Humble]

Sandeep, you mentioned before that you planned on racing solo 1 in the future. Here's a bit of knowledge I picked up from Jack Van Wettering that might help also.

Off hand, I can't remember what camber settings he used, but one thing I do remember was the toe-in settings. I mentioned a problem with bad oversteer, both on and off the throttle, and push at neutral throttle. I'm sure it had something to do with the porsche shocks i was using, way to stiff. Jack said to set a 1/8th inch toe-out on the front wheels to help turn in, and 1/8th inch toe-in on the rear to help prevent oversteer. Now with the 1/8th inch toe-out in the front, the steering will feel a little more "squirrely" and follow ruts more easily. Something to think about for daily driving.

[Sandeep]

Thanks for all the great suggestions folks !

I measured my camber with a framer's bubble level (24" long), a small ruler and simple trig All measurements are on the rim, not the actual tire.

I read on a Porsche site that 930's running 11" wide rears were using between 0 and -1 deg camber for tire wear (street use). I realize that they have much more weight in the rear but this may be a good starting point. I think I'll be measuring the tire wear every couple of weeks just to make sure things are acceptable.

I took a close look at the trailing arm mounts by the frame horn last night. Things are a little tight to get a camber box fabrication going but I am looking closer at it. The problem is that the inner area where the pivot bolt actually bolts into looks like it is inside the framehorn by about 1/2".



pic from www.blindchickenracing.com

If you could get a weld-in irs pivot, it looks like there is room to weld this new pivot ontop of the factory pivot. This would move the trailing mount upwards by about 2 inches .. non adjustable but a move none the less. Not sure if this is too much movement but it would be a start. If it didn't work, you would still have the lower stock pivot intact. Just a thought.

Sandeep

[boygenius]

Hmmmm. I just had an idea. What about using off center pivot bushings. You could tap the trailing arm to bolt on a retaining ring that would hold the notched outer bushing metal ring. The same way the 944s use the eccentric bolts to control suspension alignment. That way it would be reasonably priced and it would be a bolt on modification. If you guys think this is a good idea I will make some drawings up. If it will even work, I'm not a suspension wizard...

[Sandeep]

Great idea !

I think I understand what you are saying ... make an eccentric bolt to fit the stock inner trailing arm mount ?

This would also give the ability to change the wheelbase .. but I realize this would not be the intent.

How much vertical offset do you think you could get ? Please post some drawings

Sandeep