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Curious if anyone has actually measured scrub radius on their 944.
I’m working on a suspension project and this is relevant info to know.
i did the quick test of chalking the tire and turning it and it appears to be about +15mm positive scrub radius on my early offset car, with stock size 7”ET23 wheels and 215/60R15 tires.
i would assume the late offset cars would be around -14mm due to the 29mm offset change.
Great topic Rocket. Just last week I started researching the effects of moving wheel offset has on scrub radius. I would be happy to measure mine for you but I'm not sure how accurate it would be. How are you measuring yours? How do you account for tire pressure? I was wondering if its better to measure with the wheel off using a plumb blob on the face of the hub.
Its interesting how much scrub radius can influence the feel of the car, especially turn in. I was running 7" early offset wheels on my S2 and later had them widened (back spacing increased) to 9" which puts them at about zero scrub radius and the increase in responsiveness/turn in was very noticeable
Last edited by walfreyydo; 08-21-2024 at 09:47 AM.
Its interesting how much scrub radius can influence the feel of the car, especially turn in. I was running 7" early offset wheels on my S2 and later had them widened (back spacing increased) to 9" which puts them at about zero scrub radius and the increase in responsiveness/turn in was very noticeable
About $250 per wheel. I had early offset turbo 16" phone dials and had the 7/8s converted to a 9" square set. On a late offset car this corrects/reduces the positive scrub radius issues running early offset wheels improving the turning feel. Based on my calculations it brought scrub radius back to close to zero.
The widening was done mainly for performance reasons (reduce understeer/increased front grip), but a side benefit was scrub radius was corrected as well. On an early offset car, such as the original 86 turbo they came on, the widening would likely degrade the feel due to decreased or possibly negative scrub radius that increased backspacing would bring. Also to note I am running a manual rack so turning effort while cornering is much more able to be felt versus a car running power steering and I think scrub radius has a large effect on that.
Last edited by walfreyydo; 08-21-2024 at 12:29 PM.
Very unscientific— rubbed a bunch of chalk on the tire and ground to make it dusty.
roll the car so the tire is over the dust spot, turn the steering wheel left and right a full turn each way.
Roll car forward off the dust spot and study the swirl patterns on the tire tread and ground and measure.
There is a hockey stick lined up across the two sides of the tire buldge and lined up best I could with the 9inch mark. Center looks to be at about 5.25in coming in from the hockey stick side. Is this how you measured or did you come in from the other side? These tires are 215/60/15 Definity hp800 if that helps, not sure about the treadwidth or
Its interesting how much scrub radius can influence the feel of the car, especially turn in. I was running 7" early offset wheels on my S2 and later had them widened (back spacing increased) to 9" which puts them at about zero scrub radius and the increase in responsiveness/turn in was very noticeable
I can't find it now but there was a picture I saw the other day that showed if you widen the rear trackwidth using rim offest it will make turn radius smaller and turn in quicker. Opposite effect for narrowing rear trackwidth with rim offest. I was originally measuring how much space I had in the rear wheelwell. Then I thought if I could go wider the same amount inside and outside it would keep the stock feel but hopefully with with more grip.
There is a hockey stick lined up across the two sides of the tire buldge and lined up best I could with the 9inch mark. Center looks to be at about 5.25in coming in from the hockey stick side. Is this how you measured or did you come in from the other side? These tires are 215/60/15 Definity hp800 if that helps, not sure about the treadwidth or
my tires have a thin groove directly in the center so i just measured off that. Again very unscientific. If your tire is 9” across then the center is 4.5, 5.25 is 0.75” or 19mm but negative. Is your car 87+?
This should give a good understanding for those wondering.
To measure it youd have to find the actual pivot point - usually where the angle of the strut hits the ground.. Then measure the centerpoint of the radius left by your tire and measure the distance between which is your scrub radius. You can see how lower offset wheels will push more positive scrub radiuses, and how increased backspacing can help reduce it since the centerline of the wheel can be shifted forward or backward depending.
Last edited by walfreyydo; 08-21-2024 at 05:41 PM.
Yep- this is what we’re trying to approximate with the chalk swirl marks. It’s pretty clear to see the pivot point of the tire not being in the center when using the chalk dust.
How are you measuring/locating the pivot point of the strut? You need to measure that point against the center point of your tire swirl, if I understand correctly
The steering axis is an imaginary line from the strut mount through the lower ball joint. The end of that line shows up as the center of the swirl marks on the tire.
you don’t need to actually measure the strut to see it.
08-17-2024, 04:43 PM
Early vs late scrub radius
