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Old 10-18-2017, 01:13 PM   #1
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camber impact on toe

hi. looking to change my camber at the track (ie, run a modest camber on the street, and crank the strut tops inboard at the track). anyone have any quantitative info on how that will impact toe? all i can determine so far is ...

increasing negative camber increases negative toe.

on a 944 (only info i could find online) one degree of camber causes 20' of toe change per wheel (say 2 mm for a 2' wheel?).

so, if oem is +5'/+0.5 mm per side, and at the track i want -10'/-1 mm per side, then i am looking at a 15'/1.5 mm toe change per side = 0.75 degree chamber change without throwing things out of whack.

just not sure if 944 data works for our cars (or if my math is out of whack); any help?

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Old 10-18-2017, 06:24 PM   #2
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TRK,

I imagine this will depend on suspension geometry design and set up for each vehicle model corner considered. For the rear of the Boxster there is significant toe change on bump or compression or lower ride heights. Once your ride height and camber is set toe is put into spec IIRC.

So if the linkage geometry in question changes toe with camber at rest, you could be putting toe out of the acceptable range with your track side camber change....without a proper setup for toe after
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Old 10-19-2017, 12:13 AM   #3
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Since the tie rods in our macpherson strut equipped cars are located behind/aft of the axis between the ball joints and the strut mounts, and vertically a little bit higher than the ball joint (since both the ball joints and tie rod end are designed to fit inside the round wheel barrels, the tie rod end is higher to work from packaging perspective), trigonometry tells us that moving the upper strut mount inward would also require the tie rod end to move a fraction of that distance inwards in order to maintain the same toe setting. If tie rod were left at same position, moving the upper strut mount inward would cause toe-in.

It would be the opposite effect, however, if the way you accomplished changing the camber was to move the lower ball joints outward (e.g., using adjustable a-arms).

If would also be the opposite effect if your suspension design had the steering rack in front of the front axles. Not sure how many cars have setups like that, but I'm sure there are a few.

I remember a fellow autocrosser would adjust his toe before and after each event, on site... he had a homemade, I think substantially wooden, trammel bar he set up across the tires/rims, and adjusted the tie rod end at only one side of the car (for speeding up making the adjustment vs adjusting at both sides). I don't think he was adjusting camber, though! I think he was dialing in some additional toe-out for the event, and then reverting to conservative, tire-wear friendly toe settings for the street trip back home.
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Old 10-19-2017, 06:56 AM   #4
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I wonder, what if you took it to an alignment shop, and set front camber at minimum (strut tops all the way outboard) and set your toe the way you wanted. Then, move the strut tops all the inboard (max neg camber, gaining ~-1.0) and then re-set the toe, while counting the number of turns it takes on the toe rods each side. Then you could reproduce these amount of turns when you hit the track?
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Old 10-19-2017, 07:39 AM   #5
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Quote:
Originally Posted by steved0x View Post
I wonder, what if you took it to an alignment shop, and set front camber at minimum (strut tops all the way outboard) and set your toe the way you wanted. Then, move the strut tops all the inboard (max neg camber, gaining ~-1.0) and then re-set the toe, while counting the number of turns it takes on the toe rods each side. Then you could reproduce these amount of turns when you hit the track?
yeah, what i am going to do. have a camber gauge. going to get a toe gauge. going to find max camber for track and set my toe, then back off on the camber to something streetable and see what happens to my toe - whether it stays within oem range or not. if not i will have to set toe at the track as well. something for this winter - i will report back on findings.
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Old 10-21-2017, 09:55 AM   #6
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It seems amazing to me that no one has measured and published the 986 suspension curves. I looked everywhere and came up empty.
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Old 10-21-2017, 10:57 AM   #7
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Quote:
Originally Posted by jakeru View Post
Since the tie rods in our macpherson strut equipped cars are located behind/aft of the axis between the ball joints and the strut mounts, and vertically a little bit higher than the ball joint (since both the ball joints and tie rod end are designed to fit inside the round wheel barrels, the tie rod end is higher to work from packaging perspective), trigonometry tells us that moving the upper strut mount inward would also require the tie rod end to move a fraction of that distance inwards in order to maintain the same toe setting. If tie rod were left at same position, moving the upper strut mount inward would cause toe-in.

It would be the opposite effect, however, if the way you accomplished changing the camber was to move the lower ball joints outward (e.g., using adjustable a-arms).

If would also be the opposite effect if your suspension design had the steering rack in front of the front axles. Not sure how many cars have setups like that, but I'm sure there are a few.
In fact, our Boxsters DO have the tie rods in front of the axle on the front suspension. If you increase negative camber, you'll increase toe-out.
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Old 10-21-2017, 04:13 PM   #8
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In fact, our Boxsters DO have the tie rods in front of the axle on the front suspension. If you increase negative camber, you'll increase toe-out.
Thanks for the comment/correction, Racer! I just checked and sure enough, at the front of the car, you are indeed right! Tie rod ends in front of the steering axis.

At the rear of the vehicle, the situation is opposite, with the equivalent of the tie-rods (adjustable, control toe, but just not connected to any moving steering rack) being located behind the rear "steering axis".

The more I get to know this car, the more I continue to be impressed and fascinated by the "unique" mid-engine design (which seems to affect many different systems).

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