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Caster effect on scrub radius

Caster effect on scrub radius

Caster effect on scrub radius

I'm a student and in the process of designing a offroad buggy. I've retrofitted one in the past but never worked with the front knuckles and hubs.

This might be a silly question, but does caster have an effect on scrub radius? I have the upper and lower ball joints lined up so I have 1" of scrub radius to give the steering a little bit of feel. I have 5 degrees of caster built in as well. This causes the caster line to fall a couple inches in front of the center of the wheel. Is this ok, or should I design it so the caster line is coincident with the center of the tire?

Any help would be greatly appreciated, thanks!

RE: Caster effect on scrub radius

The scrub radius is a front-view issue. The caster is a side-view issue. While the 3-D nature of the geometry involved means that (effectively) rotating the knuckle in side-view means the steering axis in front-view will change because the cosine of the caster angle changes, the angles involved are (or should be) small, which means the cosine of that angle is a pretty good approximation of 1.

You want the caster to be a positive angle and you want to have positive trail (the steering axis should intersect the ground ahead of the center of the tire contact patch). How much of either one is a darn good question, I know of nothing other than experimentation to pin this down.

I don't know the effect on "feel". I've driven vehicles with supposedly slightly negative scrub radius that have decent steering feel (front-wheel-drive ... means the scrub radius can't be too far from zero to avoid torque steering effects) and I've driven vehicles with supposedly positive scrub radius that have none (most trucks ... the overassisted power steering is to blame).

RE: Caster effect on scrub radius

Great! Thanks for your help. From all the chassis design books I've read, they say that having a 0 scrub radius gives you almost no feel. It's mainly because when you start to lose grip, you can't tell if there is more or less resistance feedback through the steering wheel. Which makes sense.

Thanks again for your help, this just popped into my head while going to sleep last night and I couldn't quite wrap my head around it.

RE: Caster effect on scrub radius

Someone recommended scrub at ten percent of the tire width but I haven't found any further verification of this. What are you using for king pin inclination? Seven to eight degrees is common.

RE: Caster effect on scrub radius

"From all the chassis design books I've read, they say that having a 0 scrub radius gives you almost no feel. It's mainly because when you start to lose grip, you can't tell if there is more or less resistance feedback through the steering wheel. Which makes sense."

No it does not make sense. When cornering your steering effort is governed by the total side view trail, pneumatic + mechanical.

Which book, and which page?


Greg Locock

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RE: Caster effect on scrub radius

Buggar - As the suspension is currently set (may change), the KPI is at 8.2degrees
GregLocock - To be honest I'm not sure exactly which books or exactly what page off the top of my head. I would have to dig through them and find it. As far as I understand, scrub radius does play in a large amount to steering effort. I could be wrong.

RE: Caster effect on scrub radius

Sure - scrub radius has an effect on steering effort. Two reasons come to mind:
1. Scrub radius is the lever arm that transfers longitudinal forces acting on the contact patch(es) back to the steering wheel. This is the source of "torque steer" in FWD vehicles and a similar effect during braking with unequal mu.
2. Weight jacking (inside front wheel pushes down and outside front lifts up) is a product of steering angle, castor angle and scrub radius. The effort required to thus "warp" the suspension is supplied by the driver through the steering wheel.

Not sure how scrub radius contributes to cornering feel. Logic suggests that "trail" is the lever arm that links cornering force and steering effort.

je suis charlie

RE: Caster effect on scrub radius

The car that I have right now is rear drive and with the stock front tires and wheels has near zero scrub radius. With the summer wheels (wider with about 10mm more offset) the scrub radius is a bit negative. It has good road feel for cornering grip (compared to other cars in this day and age) because the power steering provides minimal interference. Hard braking causes it to become darty under hard braking with the summer wheels on it, likely because the non-zero scrub radius is causing any slight inequality between left and right to affect the steering.

If (say) the left front brake does more than the right front brake for whatever reason (e.g. unequal slippery surface, or the driver dropping a wheel off the pavement into the adjacent gravel or mud), it is going to impose a yaw moment on the car as a whole which makes the car want to veer to the left. If the scrub radius is slightly negative, that same braking inequality will impose a torque around the steering axis that makes it want to go slightly right. If you are better with these calculations than I am, perhaps you could at least make some sort of estimate of how much negative scrub radius would be needed for these two effects to more or less compensate for each other. I'm just a glorified mechanic so I'd take a stab at it with a general understanding of the effects and fine-tune it with wheel spacers or wheels of different offset until it works right. (with the scrub radius about 10mm, the effect isn't bad enough to make me want to fix it - the effect isn't *that* strong)

Dual diagonal braking circuits in which one circuit fails cause an extreme case of this. IIRC Volkswagen was an early proponent of this (Golf Mk 1) and they made a big deal of their negative scrub radius steering back in the day.

You could likely say that ABS introduces its own set of reasons for doing this, since again it is no longer guaranteed that the left and right front brakes will be doing the same thing at the same time.

If you repeat this mental exercise but for acceleration rather than braking, that's why front wheel drive cars pretty much require a zero or slightly negative scrub radius.

Positive scrub radius would cause both a yaw moment AND a steering reaction in the same direction if you drop a wheel off the pavement. Drop a wheel off and the vehicle takes off into the ditch. I've never understood why it took until the 1970s to figure this out and I still don't understand why vehicles are built with a significant positive scrub radius today. But I'm just a backyard tinkerer. What do I know.

RE: Caster effect on scrub radius

Great stuff. What are you using for knuckles and hubs?
Off roading with a lot of scrub will break your fingers when you hit that "hidden treasure" won't it?

RE: Caster effect on scrub radius

BrianPeterson - Thanks so much for the insight, you bring up some really good points. I'll definitely be looking into some of the ideas that you bring up.

Buggar - Heavy Off-roading is pretty rough on the steering in general. As of now, the plan is to run electric power steering, and a steering stabilizer. Even if the power steering isn't needed (time will tell) I'll definitely run a stabilizer still. The tires are pretty large and the buggy is relatively heavy, so any abrupt motion will definitely yank the wheel right out of your hands.

RE: Caster effect on scrub radius

Caster has it's own pros and cons.
Increasing positive caster, from zero, will increase the difficulty of steering input, so unless you plan on putting power steering in your buggy, more caster means more work.
Some people don't mind that, that is fine, but note that if you are building an off-road buggy, rough terrain and turning the wheel back and forth over ruts, it might play you out.
While caster does offer high speed stability, it also creates jacking forces that "talk" with the diagonal spring from that corner of the vehicle.
If you are moving at a high rate of speed, and you are driving through ruts and turning the wheel left and right to compensate for a loss of traction, it will upset the feel of the vehicle.
While there are times the jacking forces from caster can be beneficial, usually on a "smoother" surface, ruts and bumps are not one of them.
I would personally set my caster as low as possible while still offering steering stability at higher speeds.

Also, keep in mind being in an off-road vehicle, there is a good chance you may get a flat tire at one point. In which case having a negative scrub radius may be beneficial to you in that event..
Same goes for a brake line being ripped off or any thing like that, you never know what happens.
Excessive scrub is also a bad thing, either way positive or negative. It all depends on what you are trying to accomplish.
There are many different ways to go about what you are doing, it all depends on what your personal preference is on the vehicle.

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