Wheel offsets

[tonto1]

Got a question about changing wheel offset or back spacing.
'41 Chevy business coupe with MII front end. Stock 15 X 6" Chevy wheels.
I don't like the wheels setting as far in as they do and wonder  what  the effect would be on the rest of the front end , ride height, steering geometry and so on,  if I put on wheels of the same size, but with ~2" less back spacing?

I'll appreciate any and all advice and comments

[jaybee]

Shouldn't affect ride height or toe.  Same is true of camber, although the wheel will have more "lever" on the rest of the suspension.  I once saw a big block 429 Mustang that had to do away with deep dish wheels because the rest of the structure wasn't strong enough to maintain camber, bending the shock towers and inner fenders that are so important to these cars.  in your case I wouldn't imagine it would be much of an issue but excessive positive offset has a seriously detrimental effect on wheel bearing life.  Scrub radius isn't much discussed, it's the distance between the center of the contact patch and where a line through the ball joints hits the ground.  This will be moved but again shouldn't be a big issue.  Not sure about Ackerman.  I can't figure out how it would change caster either, but all of these things are interconnected in complicated ways and you should have alignment checked afterward.

[tonto1]

Thanks for the reply.
One of the things I am concerned about is ride height. I don't want the front end any lower than it is and I was afraid the lever effect would possibly do that.

[tomslik]

Thanks for the reply.
One of the things I am concerned about is ride height. I don't want the front end any lower than it is and I was afraid the lever effect would possibly do that.

i put 14X7's on the frt of my 48, works fine.
you're not gonna change the ride heigth much if at all but don't go more than an inch on each side. and shoot for less toe-in  and less camber

[PeterR]

Got a question about changing wheel offset or back spacing.
'41 Chevy business coupe with MII front end. Stock 15 X 6" Chevy wheels.
I don't like the wheels setting as far in as they do and wonder  what  the effect would be on the rest of the front end , ride height, steering geometry and so on,  if I put on wheels of the same size, but with ~2" less back spacing?

I'll appreciate any and all advice and comments

Don't want to be Mr Misery here  --but after first-hand experience with a similar set up a few years ago I would not do it.

As others have indicated, the camber, caster and Ackermann will not be affected.   If you are running camber settings other than zero there is a minute height change which you would not notice.

The real implication is the change in scrub radius. During a turn the wheel swivels around the line that passes through both ball joints (or along the kingpin for older type front ends). Less back space means the tire contact point on the ground is further away from the steering axis resulting in significantly increased scrub radius.

As the tire contact point is further out from the steering axis, forces from braking and road irregularities have a longer lever arm which results in larger forces being fed back into the steering components.

I drove an Fseries which would often pull to one side or the other during braking and was heavy to steer at slow speeds with brakes applied, but this was just put down to poor steering of those vehicles. One night driving in light rain at 60mph I hit a pool of water about 30 feet long and 3" deep.   The drag of the water pulled the steering wheel through my hand; -just about smashed a thumb and sent the vehicle careering across the road.   At the time I thought it must have been caused by a flat tire, but it checked out OK and over time I forgot about it.  

About six months later quite by chance I was sitting on the ground dead ahead of a front wheel, and could see the centerline of the wheel was about 3" outside the steering axis -an instant explanation to the water incident.   Later investigation revealed the factory 16" rims had been replaced with 15" diameter wheels which would not clear the brake caliper so the wheels were built with less backspace without any thought to other implications.

[jaybee]

Peter, thanks for jumping in here, I knew one of the local suspension experts would pick it up.  Explaining what would happen with the scrub radius was above my head.  I do remember that the first VW Rabbits bragged about having a negative scrub radius so that a flat tire wouldn't try to tear the wheel away from the driver.

What about mounting a wider tire and wheel with slightly lower aspect ratio so the outside edge will be closer to the fender lip but scrub radius won't change much?

[enjenjo]

You can go up to about 1" positive offset with out affecting the scrub radius too badly. An easy way to do this in your case would be 1" billet wheel spacers. Good ones are about $80 a pair.

There will also be more load on the outer wheel bearing, but you just need to monitor it closer.

[tonto1]

Well, I'm glad you explained the effects of scrub radius. Being a dummy, I thought  it would mean a slight increase in tire wear, which I could live with.
Is a car with  power steering affected as much  as much as one without P/S.

[PeterR]

What about mounting a wider tire and wheel with slightly lower aspect ratio so the outside edge will be closer to the fender lip but scrub radius won't change much?

Is a car with  power steering affected as much  as much as one without P/S.

As enjenjo has indicated up to 1" scrub is OK, but I will bet a car of that vintage already has about 1/2", so there is not much room to manoeuvre.

Q1.
If you mount a wider wheel with the original C/L offset, then the outside face of the tire will only move outwards half the extra width and that probably will not achieve your aim.    If you retain the same backspace then the outside of the tire will move out the full amount of the additional width, but you will have increased the scrub by half that amount.

If you reduce the rolling diameter of the tire but keep the same rim offset that will increase the scrub.  

Q2.
Power steering helps a lot to tame down the beast.

[PeterR]

What about mounting a wider tire and wheel with slightly lower aspect ratio so the outside edge will be closer to the fender lip but scrub radius won't change much?

Is a car with  power steering affected as much  as much as one without P/S.

As enjenjo has indicated up to 1" scrub is OK, but I will bet a car of that vintage already has about 1/2", so there is not much room to manoeuvre.

Q1.
If you mount a wider wheel with the original C/L offset, then the outside face of the tire will only move outwards half the extra width and that probably will not achieve your aim.    If you retain the same backspace then the outside of the tire will move out the full amount of the additional width, but you will have increased the scrub by half that amount.

If you reduce the rolling diameter of the tire but keep the same rim offset that will increase the scrub.  

Q2.
Power steering helps a lot to tame down the beast.

[tonto1]

Thanks fo all the info.
Seems every time I ask a question, I learn something.
Maybe I'll just keep the wheels I have