Scrub Radius

[demuire]

Something I found that I thought explains scrub radius relatively well...

 

Scrub radius

[irokin]

Interesting. I'm still not exactly clear as to how to measure it. But the celica has full castor wound on...so hopefully I have a fairly good scrub radius...

[demuire]

Hmm.

 

Irokin: Measuring it is pretty easy. You'll need to draw 1 line, and make 1 point.

 

First line from the pivot point of the top of your strut (where the strut top bearing is) to the pivot point at the bottom of your strut (where the ball joint is). Extend this line until it touches the ground.

 

Now make a point where the middle of your tyre is, on the ground.

 

If the line crosses exactly where the point is, then you have zero scrub radius. If it is "inside" the point, you have positive, and if it is "outside", then it is negative.

 

Now, what to do with this? I don't really know :P

 

As far as I can understand it, there are 2 main things about scrub radius:

 

First of all, zero scrub radius is bad, because it causes "squirm". What this is, is when you turn the steering wheel, the tyre will pivot around an axis in the middle of your tyre. This will cause the part of the tyre on one side of this axis to try to twist in the opposite direction as the part of the tyre on the other side. This is bad. So from my understanding of this, having scrub radius that puts the line on either side of the tyre (whether neg or pos) is good.

 

Secondly, scrub radius affects the "levalage" the tyre has on the stub axle. What does this mean? Well when you're driving, there are forces acting on your wheel all the time. Forces to slow the wheel down when you're braking, forces when you hit a bump etc, and in a FWD car forces when you accelerate. The larger the scrub radius, the more levalage the wheel has to turn the wheel, so the more force it will transfer back to the steering wheel, and to the rest of the car. This is why FWD cars typically have negative scrub radius, to minimise torque steer.

 

In fact, I can't really seem to find any benefit in having positive scrub radius, apart from maybe amplifying the effect of castor? Not sure.

[irokin]

Ahhh thanks for that...makes heaps more sense to me now...

 

 

That would mean I have about 3cm positive scrub...

 

I think you can modify it with spacer plates...but that would only allow for more positive...right???

[demuire]

Yeah... I think spacer plates and running more offset wheels etc is good for increasing your track and in that sense making your car handle better, but it will make your scrub radius worse...

[demuire]

Fred Puhn's "How To Make Your Car Handle" doesn't appear to say much about scrub radius (or at least I can't find it), but it does say this:

 

Adding positive offset to wheels can make a car corner faster because the resulting wider track reduces weight transfer.  However, there are some side effects.  On the front of the car, increased positive offset increases the distance between the steering pivots and the center of the tire.  This makes the car steer harder.  It also increases the loads on the steering system which increases the steering-linkage deflection, and this may require extra toe-in to compensate.  All these things are undesirable, so if possible keep the front offset near stock

 

It then later says this:

 

Changing offset afects wheel-bearing loads.  On some cars the wheel bearings are so marginal in strength that an increase in positive offset causes them to fail prematurely.

 

And, later:

 

A road car is designed with wheel offset to minimise loads on the bearings in straight-line driving, but in corners the wheel bearings get higher loads.  In most cases a positive-offset wheel will increase the loads in straight-line driving, but reduce the loads in a corner.  This is shown in Figure 35.  The best bet for street driving is to keep the offset as close to stock as possible.  If necessary go ahead with small changes in offset and don't worry about bearings.  It only becomes an intolerable problem with a large change in wheel offset, say more than 2 inches.