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I've just fitted new wheels - 7's at ET55 on the front compared to 7.5 ET65 Cup 1s before. 225/45 Cup 2 Connect tyres.
The front wheel is visually very close to the front of the fender. I suspect it was this close before but disguised by the difference in the ET figures between the wheels.
The front had an 18mm spacer with the 7.5 wheel and I've left it on for the time being.
Visually it would probably look better further back in the wheel arch, but my understanding is that more caster is better for handling on these cars.
I could remove the spacer and that would probably improve the visuals from the side but it doesn't look like it's sticking out too far? I’ve not driven it yet.
The rear has a 15mm spacer and the wheel is 9 ET55 with 255/40 tyres.
Thoughts?
964 w/ 7ET55 has a scrub radius of 0, w/ 18 mm spacer that goes to +18.
for ABS to work correctly scrub radius needs to be 0 or negative
Factory recommendation for fitting wider front is to trim the flange where the fender and bumper cap come together
964 w/ 7ET55 has a scrub radius of 0, w/ 18 mm spacer that goes to +18.
for ABS to work correctly scrub radius needs to be 0 or negative
Factory recommendation for fitting wider front is to trim the flange where the fender and bumper cap come together
Thanks Bill.
Are you saying that no spacers should be used or the ABS will not work correctly?
The front had a 15mm spacer but I've removed it for the time being but was thinking of moving the front wheel back out just a small amount. The rear has an 18mm spacer on it - I was going to use the 15mm spacer that I removed from the front but the back 18mm one is on there pretty good (nuts look a bit rusty and are on there tight).
It’s a personal taste but I really dislike sunken wheels, so my vote would be to space out the wheels.
I agree.
I've been giving this some thought and doing a bit of research.
@Bill Verburg - your comment about scrub radius got me doing some reading. With the 7J ET55 wheels on the front (with 225 tyres, which I think has no relevance) and 15mm spacers I did notice a significant squirm at speed braking into a bend - this only happened once so it might have been the road surface. Without any spacers the steering felt a bit sharper and easier.
It's interesting to note that a 15mm spacer was used with the 7.5J ET65 Cup 1 wheels that were originally on the front - I guess that spacer cancelled out the offset difference and kept the scrub radius at zero?
Assuming the spacer size roughly equates to the amount of scrub radius, how much is too much?
Front is 7J and back is 9J, 225 tyres up front and 255 in the rear. All wheels are ET55. At the moment I have no spacer on the front and 18mm on the back. I'll change those for the 15mm that were previously on the front I think, although the bolts on the rear spacer seem a bit stuck with rust on them, so I guess I need to buy an impact wrench.
With no spacer up front and 15mm at the back I believe the front track will no longer be marginally wider than the back. Should the same size spacers be used on all wheels to maintain the difference between front and rear track? Or is it ok to have 15mm at the back and maybe 5mm up front?
Sorry to bang on about this. The more I read about it and the more I think about it... you know how it goes. I do think the front looks too tucked with no spacer though.
I've been giving this some thought and doing a bit of research.
@Bill Verburg - your comment about scrub radius got me doing some reading. With the 7J ET55 wheels on the front (with 225 tyres, which I think has no relevance) and 15mm spacers I did notice a significant squirm at speed braking into a bend - this only happened once so it might have been the road surface. Without any spacers the steering felt a bit sharper and easier.
It's interesting to note that a 15mm spacer was used with the 7.5J ET65 Cup 1 wheels that were originally on the front - I guess that spacer cancelled out the offset difference and kept the scrub radius at zero?
Assuming the spacer size roughly equates to the amount of scrub radius, how much is too much?
Front is 7J and back is 9J, 225 tyres up front and 255 in the rear. All wheels are ET55. At the moment I have no spacer on the front and 18mm on the back. I'll change those for the 15mm that were previously on the front I think, although the bolts on the rear spacer seem a bit stuck with rust on them, so I guess I need to buy an impact wrench.
With no spacer up front and 15mm at the back I believe the front track will no longer be marginally wider than the back. Should the same size spacers be used on all wheels to maintain the difference between front and rear track? Or is it ok to have 15mm at the back and maybe 5mm up front?
Sorry to bang on about this. The more I read about it and the more I think about it... you know how it goes. I do think the front looks too tucked with no spacer though.
For the self correcting nature of ABS to work the scrub radius must be 0 or negative, positive scrub radius cause the car to dart to the side the more positive the stronger the nature of the dart
std. for a 964 is 7ET55 this imparts a s/r of 0, a spacer makes the scrub radius + by the thickness of the spacer
pre 964 911s have a s/r of +52.4 when a 7T23.3 wheel is used, this imparts a huge amount of feedback into the steering wheel which is what makes them feel like living things when driving, they do not have ABS so the self correcting feedback loop doesn't matter, but even so every effort needs to be made to not increase s/r any more.
If you care more about looks than driving by all means space them out, drivers will use 8 or 8.5 and tires to fill the wheel wells out
For the self correcting nature of ABS to work the scrub radius must be 0 or negative, positive scrub radius cause the car to dart to the side the more positive the stronger the nature of the dart
std. for a 964 is 7ET55 this imparts a s/r of 0, a spacer makes the scrub radius + by the thickness of the spacer
pre 964 911s have a s/r of +52.4 when a 7T23.3 wheel is used, this imparts a huge amount of feedback into the steering wheel which is what makes them feel like living things when driving, they do not have ABS so the self correcting feedback loop doesn't matter, but even so every effort needs to be made to not increase s/r any more.
If you care more about looks than driving by all means space them out, drivers will use 8 or 8.5 and tires to fill the wheel wells out
Well, I care about looks and driving.
It's primarily for road use and not a track tool, so looks do count and a reasonable compromise has to be reached.
The 8 in 17" have an ET of 52 and an online calculator seems to indicate that these would extend 16mm further out than the 7 ET55, so that wouldn't work.
If I am going to use spacers then is it better to use the same size spacers on all wheels to maintain the front/rear track difference or does it not really matter too much for the size spacers I am looking at (probably 15mm rear and 5mm up front, maybe 10mm all round if it's a better option)?
It's primarily for road use and not a track tool, so looks do count and a reasonable compromise has to be reached.
The 8 in 17" have an ET of 52 and an online calculator seems to indicate that these would extend 16mm further out than the 7 ET55, so that wouldn't work.
If I am going to use spacers then is it better to use the same size spacers on all wheels to maintain the front/rear track difference or does it not really matter too much for the size spacers I am looking at (probably 15mm rear and 5mm up front, maybe 10mm all round if it's a better option)?
964 Cup used 8 ET52 w/ 235/40 x17 tires, fits fine even on a lowered car
up to 8.5 ET48 w/ 235/40 also fits fine
I run a 17x8 +56.7 with a 15mm spacer in the front and 17x9 +44 with 15mm spacer. 215/45 front 255/40 rear on a C4 Now at most I do Autocross but not in any competitive fashion. Its' mostly street and spirited drives on the backroads
Thanks, interesting information. I'm not sure 235 would be good on the front of a C4 with 255 in the rear?
It depnds on what you want from the car.
The original 964 Cups wanted less understeer so the spec'd 8ET52 235/40 & 9.5 E47 255/40 x17 for the rear
as the series progressed and spread some went to235/40 Y 265/35 or 245/35 & 275/30 x18, to complement what ever suspension they were running
currently its common for 8.5 Et48 235/40 & 10ET51 275/35 x18
in 17 235/40 & 275/35 on the same spec wheels only in 17
It depnds on what you want from the car.
The original 964 Cups wanted less understeer so the spec'd 8ET52 235/40 & 9.5 E47 255/40 x17 for the rear
as the series progressed and spread some went to235/40 Y 265/35 or 245/35 & 275/30 x18, to complement what ever suspension they were running
currently its common for 8.5 Et48 235/40 & 10ET51 275/35 x18
in 17 235/40 & 275/35 on the same spec wheels only in 17
Those would be on C2s though. You think Cup racing car setups are transferable and useful for a C4 that will primarily be used on the road? I wonder how much those setups might require on some aspects of the suspension being beefed up and presumably those cars weighing less? I've only just bought these wheels so I'm not really inclined to buy more just yet.
I think the basic question I want to answer at the moment is how much of a spacer would be the limit for not introducing significant issues due to scrub radius, with the goal being to move the front wheels out a bit so they don't look quite so tucked.
My secondary question is that I think the rear track might end up wider than the front and I wonder how important that is. I suspect it's not going to be a significant enough of a change to make much difference for mostly fast road driving with the possibility of the occasional fun track outing, but it's yet another thing I don't know enough about.
I run a 17x8 +56.7 with a 15mm spacer in the front and 17x9 +44 with 15mm spacer. 215/45 front 255/40 rear on a C4 Now at most I do Autocross but not in any competitive fashion. Its' mostly street and spirited drives on the backroads
I'm guessing all of your wheels are going to be a bit further out than mine - particularly the rear. Maybe the front is similar - slightly wider with slightly more offset
Do you ever get any rubbing on the front or rear?
Have you had the fenders rolled?
You'll presumably have a bit of positive scrub radius - I guess you don't notice any issues.
Your wheels certainly seem to fit well based on the pictures I've seen.
I'm guessing all of your wheels are going to be a bit further out than mine - particularly the rear. Maybe the front is similar - slightly wider with slightly more offset
Do you ever get any rubbing on the front or rear?
Have you had the fenders rolled?
You'll presumably have a bit of positive scrub radius - I guess you don't notice any issues.
Your wheels certainly seem to fit well based on the pictures I've seen.
I haven't really noticed any issues. Hope the pics helps a bit even though not the best angles
The front fenders feel like they've been rolled by the previous owner. As for the rear fender it's such a tiny sliver I've heard other people just shave it off rather than roll.
Regarding rubbing, I only hear it in the rear when there is a dip in the road. That's mostly b/c the sidewall of my tires are too square I'm sure an alignment adjustment would fix that but i'm ready for new tires so i'll just pay closer attention to my choices.
Last edited by 964Luftballoon; 03-31-2022 at 06:21 PM.
03-27-2022, 12:36 PM
