King pin offset.

[mickrick]

Hello,
what would be considered an acceptible maximum for this?
I'm using a 22% rack, and trying to get 205/55 R13 CR500's on an 8" rim.
Without modifying the wing stay I can't get it less tha 49mm.
I obviously would like to get optimum center point, but don't think it's possible, even with a 6" rim. (Which is out of the question)

I don't have the car at home so I'll have to go back and take another look to see how far inboard I can bring the wheel, without interfereing with anything. i.e. wishbones or track rods with the wheel at full lock.
Looks like I'll definately have to modify the wing stays.

TIA.

[Shaun_E]

Loads of us sprinters are running 7.5"rims (215 width Kumho Ecsta V70a tyres) on the front with unmodified wingstays and using the same backdepth as a 6" rim (110mm). Doesn't seem to cause any problems.
If you modifiy the wingstay to bring it further inboard I think the most likely thing to happen is that the wing hits the side panel on full lock.

[mickrick]

Thanks for your speedy reply Shaun!
Do you know, I hadn't given the wing a thought!
Just goes to show, I was all deep into wheel calculations and missed an obvious blunder.
Forgot to mention my wingstays are the CSR ones, and they do seem slightly more inboard.
I think I can manage a back depth of 121mm (not including the bead)
Tire section is 230mm (Which is about the same as the wheel with the bead) and it gives me about 5mm at the closest part of the stay.
Nice to know that if I have to compromise it's not the end of the world.
I would like to loose 30mm of that KPO though....

Cheers,
Mick.

[6speedmanual]

Mick, to me 49mm ground offset sounds like a lot! I seem to remember the standard offset on 6" rims is in the region of 15-18mm if I remember correctly someting I read. Introducing more offset will not necessarily affect amount of grip available but will almost surely reduce the quality of the steering feel. This may make it harder to judge the limits on turning and also braking on uneven s7rfaces or with differing friction levels L/R.
Do you have a need for such enormously wide rims?
Peter

[mickrick]

6speedmanual said:

Mick, to me 49mm ground offset sounds like a lot! I seem to remember the standard offset on 6" rims is in the region of 15-18mm if I remember correctly someting I read. Introducing more offset will not necessarily affect amount of grip available but will almost surely reduce the quality of the steering feel. This may make it harder to judge the limits on turning and also braking on uneven s7rfaces or with differing friction levels L/R.
Do you have a need for such enormously wide rims?
Peter

Hello Peter, I've been giving this more thought, and doing a bit of ing.
I have to agree with what you say!
I have to get the wheel inboard some more. I'll draw it up again and see how close I can get it with a 7" rim. I don't want to go any narrower than that, as I have to package my brake set up.
From what I read, 10% of the tread width is acceptable, which means I have to loose that 30mm.
Hopefully some parts will turn up from Caterham tomorrow, so I can pop the rack in, and see how much room I have at full steering lock.
I was planning on having 8" front, and 10" rears, as I want to fit the 245's on the rear.
Maybe I can reduce that to 7" and 9" rims.
If all goes to plan, the car will be around 400hp, that's the reason for the bigger brakes, and more rubber.

[mickrick]

Hi EFA thanks for your reply.
I can't see your point though. I'm trying to get optimum center point with the steering axis of the upright intersecting as close as possible to the centerline of the tyre contact patch, and eliminate tyre scrub as much as possible.
Too much KPO will give me problems with tramlining, bump steer, and heavy steering apparently.

Cheers,
Mick.