offset versus scrub radius Options

[r_towle]

Does it make a difference what the offset is on a wheel with regards to scrub radius?

It does not seem that it would, given the stock suspension, no custom struts etc.

Aside from that, what is the reason (aside from cost and availability) for using large spacers versus getting wheels that mount up with no spacer, yet are wide for flared cars?

rich

[J P Stein]

The following sketch does not represent all of the aspects of SR vs its effects....but the best I got.
It does show the method of measuring SR and one fella's attempt to fix it thru camber changes

An increase in SR does indeed effect the car & handling, basically one increases the leverarm of the axle against rotation....leading to increased steering effort....that's the easy part to figure out.

The increase also puts the CL of the tire/wheel farther fom the center of rotation.This gets this center "out ahead"of the rotation when turning....not a problem for minor (say 5-10 deg) steering inputs. Major inputs , such as found at AX, are another story. These will cause the tire to "shuffle" sideways rather than roll. ....been down this road with 10 inch wide front wheels...giving me about 2.5+ inches of SR.....resulting in an increable PUSH.

Fixed it with 8 inch wide wheels but with only a .5 inch loss in tire width.

[Matt Romanowski]

The following sketch does not represent all of the aspects of SR vs its effects....but the best I got.
It does show the method of measuring SR and one fella's attempt to fix it thru camber changes

An increase in SR does indeed effect the car & handling, basically one increases the leverarm of the axle against rotation....leading to increased steering effort....that's the easy part to figure out.

The increase also puts the CL of the tire/wheel farther fom the center of rotation.This gets this center "out ahead"of the rotation when turning....not a problem for minor (say 5-10 deg) steering inputs. Major inputs , such as found at AX, are another story. These will cause the tire to "shuffle" sideways rather than roll. ....been down this road with 10 inch wide front wheels...giving me about 2.5+ inches of SR.....resulting in an increable PUSH.

Fixed it with 8 inch wide wheels but with only a .5 inch loss in tire width.

That diagram has nothing to do with camber. The guy is changing the strut to spindle angle, which changes the scrub radius. Changing just the camber of the wheel has no affect on scrub radius.

[J P Stein]

That diagram has nothing to do with camber. The guy is changing the strut to spindle angle, which changes the scrub radius. Changing just the camber of the wheel has no affect on scrub radius.

Gee, I left out some context on the sketch. That will almost always get a rise from some expurt.

The author(?) of the sketch changed his strut angle to increase the *obtainable* negative camber on his 911. The mod had the effect on SR also....two good reasons to do the mod. The downside is the slug (or extra seam) in the A arm...I'm agin' it..do I have to explain the term "slug" + the whys & wherefores?

I too modded my A arms but only to increase strength in the area I'd seen them break.

[Matt Romanowski]

That diagram has nothing to do with camber. The guy is changing the strut to spindle angle, which changes the scrub radius. Changing just the camber of the wheel has no affect on scrub radius.

Gee, I left out some context on the sketch. That will almost always get a rise from some expurt.

The author(?) of the sketch changed his strut angle to increase the *obtainable* negative camber on his 911. The mod had the effect on SR also....two good reasons to do the mod. The downside is the slug (or extra seam) in the A arm...I'm agin' it..do I have to explain the term "slug" + the whys & wherefores?

I too modded my A arms but only to increase strength in the area I'd seen them break.

If we want to be rude about it, look at the drawing again, expurt. He increased the kingpin inclination angle, which would reduce the amount of camber he could get provided he didn't move his top strut mount inward.

We could say he is making the strut to spindle angle more obtuse.

Edit: And to help you out, I did notice he lengthened the A-arm. Without the exact numbers of what was done, you don't know the changes that were made. Most likely he ended up in the same boat he was in before. If you want to do these things right, you have to run the numbers through a suspension design program to find out what you are really getting.

Also note, real race teams will change roll centers for different tracks, but don't mess with scrub radius. It's defined by the uprights which are the design of is rarely changed.