Hello, I've been trying to do some calculations and there are so many variables so I thought I'd ask here. I have 2 sets of wheels either can be used for track use, both with a 195/50 fitted Set 1: Front 15x6 et13 Rear 15x7 et13 Set 2: Front 15x6 et 25 Rear 15x6 et 18 Let pretend All suspension and Geo set up will be the same for each set up. Now I understand have a slightly wider track on the front might benefit handling on a FWD but for this purpose, this is how I will be running them, with the lower et wheels at the rear. My thoughts are Set 1 will have a worse scrub radius and with them being staggered and low offset which may result in alot of understeer - which leads me to think set 2 will be best; but then again Set 2 are an inch narrower so will this reduce the contact patch even with the same size tyres? and there fore will I forfeit mechanical grip? Set 2 will have a better scrub radius but will have a narrower track... So as you can see I'm going round in circles, all opinions and comments welcome, lets discuss
I'd be putting the 15x7 on the front instead of the back, car may handle worse but it will be faster and grip more
Yes I know but thats not really what I was after... 'Now I understand have a slightly wider track on the front might benefit handling on a FWD but for this purpose, this is how I will be running them, with the lower et wheels at the rear.'
I think your own conclusions are pretty much spot on. Et13 on a 6 & 7" rim will give terrible scrub radius issues with 195/50 R15 Set 2 is pretty much spot on, most track orientated 195/50R15's will be designed for a 6" wide rim and being Et25 means you can use an eccentric topmount to alter your KPI without negatively affecting scrub radius.
Brilliant Mike for your comments, I've just ordered Some Solid top plates so will see how I get on, Although these are just camber adjustment unfortunately
Thats not strictly true, camber is directly related to KPI. Camber is set by adjusting the relationship between the hub and the strut, 'camber' top mounts then allow you to alter the KPI and therefore, as a result, alter the camber, if you see what I mean...
Yes I do! that makes sense, so in relative terms its better to run more negative camber on the front which will give you more positive KPI, but then this will throw out the scrub radius..? no.. Swings and roundabouts..? lol
Here are my thoughts. First Geometry. There is a trade off with increasing KPI. The static camber set will drop off faster with steering angle. Think of KPI as camber for only when the wheels point straight ahead. Any steering angle will see the outside wheel start to loose camber. Adding caster is like adding camber as if the wheels were 90 degrees to the chassis. The wheels can't physically be in this plane but the caster serves to maintain the camber that you had when the wheels were pointing straight. Basic rule is that is if castor is 50% of your KPI, then you will maintain your camber through your steering arc. Obviously this doesn't account for body roll but your camber should be set to suite the slip angle of your tyre choice. Around 3 degrees for the usual R tyres. So basically a car with less KPI and less static camber will actually have a better camber curve through steering angle than a car with more KPI and more static camber. Best solution? Don't change the KPI and add some castor with your top mounts. So what wheels? Reality is fit the highest offset wheel you can and try to keep camber adjustments to the lower shock bolt method as this does not alter KPI. I would suggest et35-38 wheels. Your limiting factor will be how much camber can you add before you touch the shocks with a high offset wheel like that. Gurds
Not really, no Like Gurds says don't mess with KPI too much, 'camber' plates are a little of a misnomer, a better name would be KPI plates as in reality you are adjusting your KPI, camber can still be set independently at the hub/strut interface... There's no subtitute for testing when it comes to geometry
This is brilliant info, thanks guys! I've got a track day booked for the 21st Feb so I'll do some testing and report back
I bought some PMC ones which used to be the Dibbed versions, I've got some pictures in my build thread
