Scrub Radius

[rokker]

Scrub Radius - Ive been doing alot of reading of past threads on this board and any articles/other reading (online) I can find on the subject. I understand what scrub radius is (location or distance from the "kingpin inclination angle" to where it converges on the ground with the tire center line) but I havent been able to find real solid answers to what is best when choosing negative, positive or 0 scrub radius.

It seems to me that for a rear wheel drive vehicle positive scrub radius is preferred over none or negative scrub radius. Where as with a front wheel drive vehicle negative scrub radius is preferred.

Now lets say for our situation (not sure if it matters for a general answer) our rear wheel drive truck/prerunner that is for off-roading only (no street driving) has 33-35" tires, A-arm/J-arm design, moderate power and torque/v8, I dunno 400 and 350 respectively.

Can someone give me the facts and opinions on what is preffered and why is it preferred (positive, negative or 0 scrub radius)? Has anyone actually played with all 3 and noticed a real difference off-road? Will 1" either way (positive or negative) really make a difference as far as stress on steering components, toe-in or toe-out while breaking, stress on wheel bearings etc...

Thanks in advance for any information

[Blurredvision]

From what I have found out is that scrub radius has little effect on your handling compared to your other factors, such as camber, castor, ackerman, king pin inclination ect.....
The things that it changes are steering effort and the loads on your suspension components. From what i have read and experienced is that 0 scrub radius is the ideal. The larger the offset, the more strain it puts on your components. So too rectify this you upsize them and add more sprung weight, not such a good thing.
I am sure that there are other thoughts on this matter, but i hope this helps you.

[ACID_RAIN28]

with 0 scrub radius you can operate a vehicle with no power steering, and scrub radius plays a huge factor in the steering returning to center, while in motion.

[Scott_F]

When a wheel with positive scrub radius hits a bump, it wants to toe out. If you have front steering, that will put your tie rods in tension. If you had negative scrub radius, it would want to toe in, and your tie rods would be under compression. IMO a tie rod and heims are stronger under tension than compression. That's my understanding of the subject.

[Scott_F]

While studying my own design today, I had a minor revelation. If you design the scrub radius for a theoretical zero, then when you corner hard, the tire flexes inboard and creates a momentary negative scrub radius. The lower the air pressure, the greater the sidewall flex. Also, negative camber and sloping terrain can move the tire contact patch laterally. Hmm...