12 replies to this topic

[Canuck]

Was looking at pictures of the so-called super silhouette cars - stockish things with massive flares grafted on - and couldn't help wondering about the effect that filling those flares with rubber has on the suspension. Now perhaps I'm entirely out to lunch but I assume (based on the deeeep dish rims) that the location of the wheel flange relative to the centerline of the car is unchanged, so the additional tire width is all located further from hub.

How does putting all of that traction and weight that far out from the hub affect the handling characteristics of the car?

Image stolen from here

[Greg Locock]

WAG they are using 300mm tires on cars that originally had 200mm, probably can't squeeze the flange more than 25mm inboard, so they are running with 25mm more offset, and the outer flange is 75mm outboard of where it started.

Yes that will affect the handling!

The tires will have a huge effect as well, obviously.

[Canuck]

Okay - so it was a poorly worded question: What impact would having all of that wheel/tire offset like a big cantilever vs. the same 300mm tire in a more balanced geometry? What

[Greg Locock]

Okay - so it was a poorly worded question: What impact would having all of that wheel/tire offset like a big cantilever vs. the same 300mm tire in a more balanced geometry? What

more pull under braking and accelerating. It isn't really a bad thing. production cars used to run 100mm scrub radius quite happily, the current tendency for 13mm or so is probably a good move for the average joe, but for a racer I'm not convinced.

[Lee Nicolle]

Having raced that style of car extensivly the large offsets are not ideal but short of totally rebuilding the car from scratch is what happens. And to be quite honest the wheels were advailable wheras more central or offset in rims were not.Though I found that when I reduced the front scrub radius [near central] the car was easier to drive,, but no faster. Front suspensions though often prohibit reducing the scrub radius, and also it depends on the rules for the category.
The poor old diff, axles, or driveshafts really do not like that sort of offsets though, wheel bearings and axles have a fairly short life.

[Canuck]

The poor old diff, axles, or driveshafts really do not like that sort of offsets though, wheel bearings and axles have a fairly short life.

Interesting. I understand the load on the bearings being different at the hub but why the axles? Or is it eating them up right at the hub, so part and parcel of the wheel bearing failure? My guess was that behind the hub, in the rear at least, nothing would have changed except perhaps for the mass it's dealing with - which shouldn't impact the axles assuming the mass is reasonable.

In looking at my old BMW, there's very little room to go wider with the same scrub radius if you want to go around corners without rubbing the strut.

Okay - so it was a poorly worded question: What impact would having all of that wheel/tire offset like a big cantilever vs. the same 300mm tire in a more balanced geometry? What

I have no idea what that lone what was about.

[Greg Locock]

Interesting. I understand the load on the bearings being different at the hub but why the axles? Or is it eating them up right at the hub, so part and parcel of the wheel bearing failure? My guess was that behind the hub, in the rear at least, nothing would have changed except perhaps for the mass it's dealing with - which shouldn't impact the axles assuming the mass is reasonable.

Next time you get a reasonably powerful RWD hire car with IRS, get a paint pen and draw a stariht line down the halfshaft. Then go and play boy racer. Then look at your nice straight line. That's why axles only have a limited life. Also, in a live axle, the outboard end of the halfshaft does not have a perfect life - racers often camber and toe the wheel via various ungodly means (we used to touch the axle tube with a welding torch). As such it sees bending stresses that it wasn't designed for.

[gruntguru]

Perhaps Lee was referring to stub axles on front uprights. Increased offset would produce higher bending stresses in this component.

[Canuck]

Hmmm...an old client of mine owns a Chevy dealership. He tried to give me the keys to a Z06 once "for the weekend - see how you like". I declined out of fear for my wallet and general health but perhaps I need to revisit that...

A torch

[gruntguru]

A torch

A touch???

[Lee Nicolle]

Next time you get a reasonably powerful RWD hire car with IRS, get a paint pen and draw a stariht line down the halfshaft. Then go and play boy racer. Then look at your nice straight line. That's why axles only have a limited life. Also, in a live axle, the outboard end of the halfshaft does not have a perfect life - racers often camber and toe the wheel via various ungodly means (we used to touch the axle tube with a welding torch). As such it sees bending stresses that it wasn't designed for.

Whats wrong with getting camber with the oxy? Though a bit rough on the axle bearings,, and seals! Actually for small camber and toe changes it always worked fine. And with ball ended floaters it is not a problem at all. Like about 2 deg max.Or about 2mm of toe in.

The poor old axles dont like 6" out and 4"" in, or worse. They do flex making the car unstable. Though the modern way of 8" in and 2 out actually is not a great deal better and does the same thing.
Front stub axles dont like it either obviously.
As for white lines on axles it is a very good way of lifing components, though some still just chew the ends off immediatly outside the splines. Though I did once see some RX7 axles twisted a full 1/4 turn. Something that is part of basic preperation.
And would be very good on suspect hire car drivers on the Nostalgia thread.

[Lee Nicolle]

Perhaps Lee was referring to stub axles on front uprights. Increased offset would produce higher bending stresses in this component.

Does the same to the rear axle too. A good way to break the axle off on the shoulder where the bearing locates. That is where you crack test, regularly.

[RDV]

Even when having to run standard parts (hubs/uprights etc) the gain got by more rubber on the road was massive, offsetting any geo probs. And as far as bearing load went, if you draw a vertical load vs. lateral load and see the resultant through the axle, you could actually be better off as far as loads.