Steering Geometry with a Twist

[jimcroisdale]

Hi all,

I've had a gravity racer project on the go for quite some time - I posted about it here:

https://www.f1technical.net/forum/viewt ... 14&t=25410

I'm currently looking into the possibility of using what are essentially skateboard wheels, but the larger longboard variants, with 70mm diameter being a common size.

As all skateboard/longboard/street luge vehicles use traditions skateboard trucks to provide steering, there aren't any readily available parts to run skateboard wheels on "kart" style stub axles, which is kinds what I want to do.

Anyway, this got me thinking about steering geometry for wheels that are essentially solid, and for where the contact patch will only be a few mm front-to back, on a wheel that is 51mm wide.

To my thinking, this means that the front wheels must remain on the same horizontal plane during steering - so that all four of the wheels stay flat and in contact with the ground. Does that sound reasonable?

Assuming it does, I then began to wonder how i could build in some kind of self-centering stability when castor is out of the question, and the answer to me seemed to be mechanical trail, where the axle for the steering front wheels is mounted behind the kingpin. I've been dabbling with tinkerCAD (no laughing at the back):



Obviously there's no steering connections there as yet, but assuming accurate Ackerman angle, I'm not too worried about that at this stage. Whaddya all think?

Cheers,

Jim

[Greg Locock]

I suspect it will shimmy, like a shopping trolley. The jacking effect due to pure castor is symmetrical across the vehicle so won't change the corner weights and will help damp the shimmy out. I'd use 5 degrees of castor and 5 mm of trail as a first step.

[Greg Locock]

On the other hand the wheels are wide so if they are truly cylindrical (not barrel shaped) they will scrub which will help suppress the shimmy. Your solution is obviously much simpler than one with castor, so I'd say suck it and see.

[jimcroisdale]

To my reckoning (and correct me if I'm wrong) but caster will mean that when turning right, for example, the right front corner will jack up, and the left front corner will lower itself down?

They will still be on the same plane, so in theory the contact patched will be aligned and okay, but what of the rears?

A Sudden loss of grip from the right hand rear wheel as it is unloaded, leading to a potential spin?

[jimcroisdale]

Or am I dreaming if i think that my chassis will be so stiff that the wheels will be prised up from the road surface by steering geometry forces?

[Tommy Cookers]

I'm not really following the proposition of this thread, but afaik and fwiw ......
bicycle steering geometry is /was based on there being notionally zero rise or fall with steering deflection (testable by riding hands-off)
this of course obtains if the rolling radius is bisected by the steering axis
eg 73 deg head angle demands 45 mm offset, 71 deg demands 57 mm offset etc

[jimcroisdale]

Yes, I don't think I really explained my question very well.

Assume you have a car with no suspension and a 100% rigid chassis.

Assume the wheels are actually metal steamroller drums.

My point is this: All four wheels must always be on the same plane. If one wheel is "unweighted" during steering because of caster angle it would be tilted up onto the edge of the drum, and lose all contact area. The geometry must allow all four wheels to be perpendicular to the road at all times.

Given that the proposed vehicle has very similar properties to the above example, what methods of steering geometry can i employ to have some sort of self-centering action to the steering wheel.

[George-Jung]

Toe in?

[Tim.Wright]

then you'd need a vertical steering axis and a reasonable amount of caster trail. As Greg said, this geometry might not be super resistent to shimmy but this can be somewhat addressed by building friction into the steering column.

Any possibility of increasing the wheel size? Smaller wheels suffer more road surface induced drag.

If you are stuck with small wheels you will probably need to reduce the scrub radius a lot to reduce kick back from road surface irregularities.

[graham.reeds]

OT: I always wondered if a bobsleigh with roller blade wheels would make a good gravity racer.

[jimcroisdale]

then you'd need a vertical steering axis and a reasonable amount of caster trail. As Greg said, this geometry might not be super resistent to shimmy but this can be somewhat addressed by building friction into the steering column.

Any possibility of increasing the wheel size? Smaller wheels suffer more road surface induced drag.

If you are stuck with small wheels you will probably need to reduce the scrub radius a lot to reduce kick back from road surface irregularities.

I see what you mean now. Me putting the axle behind the kingpin position isn't really trail at all, is it?

Wheel size is max about 80mm - I'm using longboard wheels as they are cheap and I am poor...

How can I best reduce scrub radius? The kingpins would have to be almost horizontal to do it that way!

I'm thinking steering damper choose what I do...