Modifying my front suspension and tires- effect on steering.

[Rebeca_Lynn]

On our project we are using +2" wider suspension set up along with wider tires and wheel package most likely with less back space or more offset from the normal. The tires overall height is taller than the original size also.
The car is a GT40 midengine setup, total weight of the car is around 2400-2750.

What effect on the steering will this have?

Will this make the steering stiffer? We looked at a power steering unit for the car, but have been to we most likely will not need it. The power steering unit is speed sensitive and adjustable. Our biggest concern is at lower speeds around town.

[Matt@BOS]

On our project we are using +2" wider suspension set up along with wider tires and wheel package most likely with less back space or more offset from the normal. The tires overall height is taller than the original size also.
The car is a GT40 midengine setup, total weight of the car is around 2400-2750.

What effect on the steering will this have?

Will this make the steering stiffer? We looked at a power steering unit for the car, but have been to we most likely will not need it. The power steering unit is speed sensitive and adjustable. Our biggest concern is at lower speeds around town.

How big of a wheel and tire combo are you talking about? Are you widening the front as well? I'm having a set of 18x8s with 4" backspace made for the front of a mkI, and to my knowledge, that is about as big as you can go without modification. I know the mkII front provides more clearance for upper wheel travel, but I am curious if it is wider as well.

Matt

[Bryce]

you will affect scrub radius and bumpsteer by widening the suspension and changing the tires from what the suspension was designed for.

You should probably run it through a suspenson program and understand what changes are being made.

More scrub will make the steering "stiffer" a wider stickier tire will make the steering stiffer.

[exwestracer]

So you are pushing the hub out further into the wheel, but the overall track (outside to outside of tires) will stay the same? If that is the case, it sounds like you will end up with less scrub (for the same SAI angle). Bumpsteer is a total unknown; but if the rack was correctly positioned before and you've only changed the control arms, it should be correctable. With the longer arms you should see less camber gain per inch of travel, but since we don't have any tire or wheel numbers it's hard to say if that will be good or bad.

[MichaelLeeHearn]

The suspension set-up is a package from the company we are getting the car from. They modify both the control arms and hub to get the added width. We are adding overall width to the wheel and tire package looking at running either 19 x 9.5" or 10" with 285/40ZR19 Michelin Pilot for the front which are 28" dia. For the rear we are planning on running 19 x 12" or 12.5" with 345/30ZR19 Michelin Pilots fro the rear that are 27.2" dia. Compared to the factory size RCR offers of 15 x 8 or 17 x 8" front and 15 x 10" or 17 x 10" rear. On reason for going with the tire and wheel package we are going with is the same size tires are offered in R6 series by Hoosier so we do not need to worry about that issue down the road. Also we are running a larger brake set-up that we need the larger wheel size to fit.

We are still working on the offset for the wheel as we are not running RCR's centerlock system they normally use as they use a classic 5 pin set up. Instead we are planning on going with Forgline's centerlock system and wheels. We will have to see how much of a diferance this will make with the set-up also.

I hope this helps with what is needed for answering our concerns.

Our main concern is of we should just get the rack modified or go with a power steering set up to ensure we are not having to muscle the care when we are driving on the street.

69MSA - Yes we are having to widen the body front and rear some to fit our tire and wheel package, overall only about 3-4" front and rear. So about 1-1/2" wider than a Gulf model would be. The standard wheels and tires with the +2" kit would fit a regular Gulf model.

[Matt@BOS]

Without knowing any specifics about how the upper and lower control arms are modified, it is going to be hard for anyone to tell you how things are changing. I'm interested to see the changes you're making and glad you're sharing the process with us, but wouldn't a simple call or email to Fran net you the most knowledgeable and specific answer? (I'm just assuming he is making the control arms for you, not some third party).

By the way, am I correct in believing that you are going 1-1/2" wider per side than the Gulf cars, which are already 4" wider than the earlier MKI cars. If you are really building the car 8" wider overall, back to front.

Matt

[exwestracer]

Michael,
The wheel offset is really what matters when it comes to scrub radius and steering effort (and "kickback"). If you don't go crazy aggressive with the static caster, you shouldn't see too much difference. Did you ask the supplier about any changes to the steering when running the modified front suspension? Seems like they should know if they are selling it...?

[MichaelLeeHearn]

Yes the rack is modifed to for the updated suspension.

Yes between 6" to 8" wider overall front and rear. Will depend on if we can tuck the wheels in more or less with out effecting steering and other things.

We already know of another build that has done this and does not look out of place on his build.

[David Pozzi]

Can you find out the expected scrub radius? Ideal would be around 1".
I'm guessing you could get to 2" or 2.5" before steering effort got too high.

[Rebeca_Lynn]

Ok, so we will skip the power steering unit for now. If we needit, it is only adding a few wires, 2 ujoints and a mounting block. Just a fun area of the car to work with at any time, the foot box / under dash area.

[David Pozzi]

A good plan. Keep your steering ratio around two turns lock to lock.