What sort of numbers for scrub and castor?

[rcx194]

I've been reading through about suspension geometry, and advantages/disadvantages of big/small scrub, castor, kingpin angle, anti's etc. I've got Kingpin and anti-squat/dive sorted, but with tyre scrub what would be considered a lot of tyre scrub? Little is probably zero scrub. Same goes for castor, what sort of range is it usually in?

This is for an off road racer, 4x4. The Carroll Smith books and off road builders have said that off road these things have less effect (but camber curves and squat are still important important). But I still need to set my castor and scrub radius to something!

 

[GregLocock]

In the olden days cars with manual steering had +100 mm of scrub. When the Golf came along they went to -20mm. So long as it stays one side of zero or the other I'm not convinced it is important.

Castor is much trickier. One rule of thumb is that under full pitching it should still be positive. That sets a lower limit of 3 or 4 degrees in your case. 12 is probably too much.

Cheers

Greg Locock


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[rcx194]

Thanks Greg! What do you mean by "stays one side of zero"?

 

[GregLocock]

As your suspension moves the scrub radius will change.In my opinion its sign should stay the same whatever jounce+steer position the suspension is in.

Cheers

Greg Locock


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[gruntguru]

Remember that scrub + castor => jacking which has an important effect on understeer/oversteer.
Also helps to have some jacking if you have a rear spool, to unload inside rear and enable turn-in.

je suis charlie

 

[rcx194]

With this rule of thumb, will it be enough to ensure the steering self centres?

One rule of thumb is that under full pitching it should still be positive. That sets a lower limit of 3 or 4 degrees in your case. 12 is probably too much.

I also got this from the Milliken textbook:

Front wheel drive cars MUST minimise spindle and have negative scrub, which leads to kingpin upto 16 deg

Will a four wheel drive vehicle need to follow this advice from Milliken as well?

My rig will have a lot of suspension travel, something like 14" depending on what my CVs allow. The tyres will be very big as well, 35" diameter. From what I'm being told it's important for the wheels to stay flat to the ground as the wheels bump over obstacles because traction is most important.

 

[BrianPetersen]

Four wheel drive includes front wheel drive. (The front wheels are driven.) So, yes, if you want it to be well behaved.

 

[GregLocock]

No and no. I think Milliken is wrong, the original Mini had positive scrub, and had better handling and steering feel than most modern cars. It also had lower ultimate limits, but that is not germane. So I think you'll get away with positive scrub. As to self centering on 4wd, I assume you'll have a rack, you may need to fit a centering spring arrangement. It is possible to calculate the centering, but you would need to know an awful lot more about your suspension and steering than most race teams do.

One OEM who used to pride themselves on their steering and handling has gone into production with an AWD that didn't self centre in various (realistic) circumstances.

Cheers

Greg Locock


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[rcx194]

We've been looking at castor in terms of degrees, but is it more relevent to look at it in terms of horizontal distance on the road surface between the tyre contact patch and the wheel centre? I will have huge 35" wheels, so even 3 degrees of castor equates to 23mm of castor at the ground.

As I'm putting my design together it looks like various constraints are limiting my options down anyway. E.g. I need the CV centre to be on the kingpin axis to minimise the extension on the driveaxles, that fact is already constraining things.

 

[BrianPetersen]

Yes, that is "trail", and the effects of caster versus trail are linked. If you are using a traditional 4x4 solid front axle, you are stuck with the steering axis passing through the universal joint. If you are using an independent design, you are probably going to find that the "plunge" of the halfshaft associated with steering motions is pretty small compared to that associated with suspension movement. Most front-drive designs don't have the steering axis passing through the CV joint.

 

[rcx194]

Brian, I'm going independent. I know most designs don't have the steering axis pass through the CV joint, but I've been told that this is needed off road as I'm using production CVs which only allow 50mm extension, which otherwise won't be enough to allow for big suspension travel.

I have to say my progress is very slow. I designed my last buggy with solid axles and that was much easier, just had to set my anti-squat and check my prop shaft angles were ok. Independent is a whole load more work!

 

[BUGGAR]

What are you using for your spindles/uprights? Any chance you will make your own?

 

[BrianPetersen]

Once in a while it's worth doing a fact-check on those "I've been told ..." situations. This is one of those situations.

Work out how much CV plunge you are going to get if the steering axis is offset from the CV axis. Put some numbers to it. Having the steering axis offset from the CV joint in side view will have significant effects. Having the steering axis offset from the CV joint in front view shouldn't be unless the separation distance is excessive ... which it shouldn't be. The practical need for having some "trail" combined with the practical need to have some "caster" usually works out with the steering axis in side view coming very close to the wheel centerline, so you really only need to look at the front-view situation.

If it is an upper-and-lower-A-arm setup, careful selection of the A-arm lengths (i.e. comparable to the length of the CV shaft between joints and at comparable angles) minimizes the CV-joint plunge. Again, it doesn't have to be exact, that's what the plunge is there for. "Somewhere near" should be good enough.

 

[rcx194]

What sort of range would the spindle length lie in normally? I.e. what would be considered a short spingle length and what would be considered long?

If anyone is interested, the Milliken textbook actually has a step by step guide to design independent suspension geometry. It gets a little complicated at the end where you need to draw in the instant centre axis and then project those back into 2D for your swing arm mounting points. But with Solidworks 3D sketches you could probably do this much easier, as the Milliken procedure does the whole thing on paper using 2D drawings.

 

[rcx194]

Brian, thanks for that explanation. I'll run those numbers.

BUGGAR, I'm using the spindle from a Mercedes Vito. Reason is that I bought a job lot of parts from a company that abandoned their off road project, and that's what they gave me. The parts were already customised to match up together, included narrow diffs with lockers (narrow to allow for long suspension arms), wheels with the Mercedes PCD, driveaxles all matched up to work together. There's a pair of custom uprights in that job lot, but I don't have to use them, they are set to zero scrub radius and zero castor, which the company said was because they were going to use full hydrostatic steering.

 

[GregLocock]

In the usual nomenclature a short spindle is one that is entirely contained within the wheel, ie the vertical separation of the ball joints is a bit less then the wheel diameter. A long spindle has the LBJ inside the wheel, but the UBJ is above the tire. This gives you a bit more room to fiddle with geometry. there's also a semi long spindle where the UBJ is alongside the tire, I've never worked on one.

Cheers

Greg Locock


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[BUGGAR]

Of course you can change the scrub easily with different wheel offsets or spacers. Have you played around with one of those suspension analysis programs? On some of them, you can replace the steering arms with your half-shafts and get the plunge.

 

[rcx194]

Yup I've got suspension analyzing software and started using it.