scrub radius poll

[theTTshark]

Have you physically tested it?

We did physical testing by changing the construction of some 3 piece wheels from 50mm scrub to 100mm scrub and found it had very little impact on steering input force required. (It did also adjust vehicle track at the same time though so wasn't a completely isolated test).

Our Adams simulation also supported this - that there would be little increase in steering force.

Based on this we ran 70mm of scrub radius.
You mechanical and pneumatic trail will have a significantly greater inpact of steering force and

I should have clarified more, but wanted people to think about it a little. Think about times when only one wheel experiences a bump or one corner locks up while the other isn't. That's where it can really screw with your steering forces. So not as big of a factor as long as everything is going normally, but we know how often race cars are in that position...

Hey Trent: Say "Hello" to Dave Rogers from Terri & me down there in Huntersville. Been a while...

Will do Bill.

[J-rob68]

There is some good info in here in regards to using the scrub to find the MX and using that to calculate steering force at the end of the system. Things you need to determine and validate are what steering forces are acceptable? How does steering force change as grip changes?
Will braking force, especially trail braking change this force with large scrub radii? TTC data and matlab simulation are great places to start.

One thing that I don't think has been mentioned is that scrub radius can also contribute substantially to the amount of energy that you are applying to the tire. AKA: It can and will affect the heat up rate and magnitude of your tire temperatures.

Its really not a simple matter of how much scrub do you want. There's a lot that plays into it but mostly steering force, tire temp and steering linearity.

[Claude Rouelle]

Jason

"One thing that I don't think has been mentioned is that scrub radius can also contribute substantially to the amount of energy that you are applying to the tire. AKA: It can and will affect the heat up rate and magnitude of your tire temperatures. "

Can you explain this?

Claude

[Claude Rouelle]

Jason, Let me refine / add to my question... do you have in mind both the front and rear suspension? Claude

[J-rob68]

Claude,

In context to tire temperatures I was strictly referring to the front suspension geometry, or any end that has significant steering rates (in the case of some silly rear wheel steer cars).

As with any given scrub radius you are dragging the contact patch around the steering axis if the value is other than zero. If you have a large scrub radius and have a very rapid or continuously changing steering angle, such as any typical FSAE track demands you will be heating the tire up quite significantly.

This could be beneficial in events such as autocross, or it could be harmful in events such as endurance depending on how you design and use the system. I'm simply stating that its something to be aware of.

[Claude Rouelle]

Jason.

We are on the same wave length qualitatively: the scrub radius has an influence on the energy that is "stored" in the tire and therefore the tire temperature and the tire temperature rate of change.
Unless you do not steer.

However
1. The scrub radius is not the only parameter that influence the tire temperature and the tire temperature speed.
Track changes (mostly a factor of the roll center height) and damper setting for example are very important.
By experience the damper is by far the number 1 setup parameter that influence the tire temperature. But by experience only and that come to my second point.
2. It is possible to calculate the energy going in each tire from lateral and longitudinal slip (by simulation with a decent tire model of even better from real track data especially if you have a slip angle sensor)
and learn something about the tire thermal properties thanks to this energy calculation and measurement from IR temperature sensors (from the outside - thread- or in the inside - we use IR sensors attached to the rim and "looking" at the carcass)
However I never saw any study that comparatively quantifies the effect of, for example, the car kinematics, stiffness, damping, pressure on tire temperature...

The tire is still a black hole

[BillCobb]

That's why you need one of these. The Ford Firestone tire debacle produced mountains of tire temp quantitation by figmosity. That's me with glasses. The horns have been removed surgically.

[J-rob68]

This type of procedure could be done in the ttc testing if wheel spacers where provided no?