Scrub Radius

[silente]

Hi all!

i would like to ask in your experience how important is Scrub radius and why.

What i mean is its effect not only on Steer feedback but, more important, its influence on the power loss when the car go straight.

Is increasing scrub radius (for example making front track wider with spacers on hub) really a bad thing or its effect is only short?

How does it influence resistance during longitudinal accelerations and at costant speed?

Thx

[Goran Malmberg]

Originally Posted by silente

Hi all!
1
i would like to ask in your experience how important is Scrub radius and why.
2
What i mean is its effect not only on Steer feedback but, more important, its influence on the power loss when the car go straight.
3
Is increasing scrub radius (for example making front track wider with spacers on hub) really a bad thing or its effect is only short?
4
How does it influence resistance during longitudinal accelerations and at costant speed?

Thx

1
We must look at it in three ways. A, how it affect the driving, B, cornering capacity, C, the geometric effect.
As for A, scrub will create a force around SAI that in turn create steering wheel rotating forces if the grip alter in between the two front wheels. Might be heavy if breaking. Large scrub takes more static toe in. It also add to steering tilt if SAI is great.
B, no difference.
C, scrub distance in combination with camber compensation alter the motion ratio.
2
Power loss??
3
If all the above mentioned factors is taken in to concideration you can use pretty much scrub in favour to TW. I have been using up to 5 inches on shorter races without problems. But I should newer build a new car that way.
4
Is this something of the same Q as nr3?
Regards
Goran

[Goran Malmberg]

I see I made a misstake and wrote "steering tilt" i place of "steering lift"
Okej, the steering action do "tilt" the front of th car depending on SAI and caster angle.
Goran

[silente]

Hi Goran,

first of all thanks for your reply.

What i mean for power loss the effect on the power you have to accelerate the car dued to friction, rolling resistance, drag etc.

I though that, whith a big scrub radius, you would always have two forces acting on the contact patch which create a moment around king pin axis. This moment, if you use equal scrub radius and toe on left and right wheel, doesn't create a steering wheel rotation, but if you use a toe different from zero (for example a negative toe on the front), they can cause an increasing of this resistance. Am i wrong?

[Goran Malmberg]

Originally Posted by silente

Hi Goran,

first of all thanks for your reply.

What i mean for power loss the effect on the power you have to accelerate the car dued to friction, rolling resistance, drag etc.

I though that, whith a big scrub radius, you would always have two forces acting on the contact patch which create a moment around king pin axis. This moment, if you use equal scrub radius and toe on left and right wheel, doesn't create a steering wheel rotation, but if you use a toe different from zero (for example a negative toe on the front), they can cause an increasing of this resistance. Am i wrong?

As I pointed out under no1, we must use more static toe setting in the direction of toe in, the larger the scrub is. I cant tell how much as it is dependent on the ammount of flex in the steering system in question.
Even if the scrub is the same each side, the around SAI turning forces is still there but never reach the steering wheel since it is evened out over the steering rack. Provided both tires has equal grip.
It might be better to have the steeering rack ahead of the front axle as
it then is subject to pulling forces under braking.

Aside from to settings for turn in reasons, scrub is a key factor for using toe in settings. The idea is that the toe should be such that the wheels is pointing (or better expressed, rolling freely) in the direction of travel.

Big scrub can couse tremendous steering wheel forces if loosing brake grip on one wheel or if hitting a large bump or another racecar. I had a friend cutting up his hand in between fingers by the steering wheel spoke from such incident.
Goran

[Goran Malmberg]

Originally Posted by silente

Hi Goran,
I though that, whith a big scrub radius, you would always have two forces acting on the contact patch which create a moment around king pin axis. Am i wrong?

If not clearified by my previous answer I might try one more.
Scrub DO alwways create a force around SA. BUT, the only problem with this is mecanical strength to keep the wheel in the desired direction. NOT, the rolling resistance, provided the tire is KEEPT in the desired directions. Which includes the right amount of toe setting.
However, there MIGHT be som off using very big scrub distance as the toe only can be set for one load condition, but even so this is marginal and not of any influence of weight. Provided the mecanics knows what they are doing.
Regards
Goran