Caster change as suspension compresses/cycles on 3-link suspension

[rcx194]

I'm building a 4x4 buggy. I've "mocked up" the front suspension using cheap thin tube for now just to check it all cycles correctly and parts aren't colliding with each other. The buggy uses a ladder chassis and solid beam axles from a donor car. I've gone for long travel 3-link suspension at the front, plus panhard bar, which gives 32cm travel at each wheel. I've aimed to keep the anti-dive within 25-75% throughout the suspension cycle and other suspension geometry variables such as Ackerman stock with what the donor car had.

The caster on the donor car was +2 degrees, and I thought to set it same with the new suspension links. I've found that the caster increases by 4 degree when the suspension goes from ride height to fully compressed. As I haven't finished the top axle bracket I can still tweak things to keep the caster more stable (ie a smaller increase of caster).

What should I be aiming for in terms of caster change as the suspension compresses or droops?

 

[BrianPetersen]

If you want much antidive, you're going to get caster change. You can get some antidive if the wheel centerline moves forward with suspension compression (as opposed to rotating the steering axis - and brake mountings! - in caster with suspension compression), but that has bad side-effects, too.

If your suspension works the way I think it does, make one end of your upper link adjustable-height. Give it a couple of alternative holes that the link could attach to if you don't like the results. That way you can fiddle with the trade-off between front-end dive under braking, and caster change.

But, there are vehicles with way more caster change with suspension movement than what you're discussing. None that I can think of, have sports-car handling. But then, nothing that I can think of that has a solid front axle, has sports-car handling.

 

[BrianPetersen]

I spy Suzuki markings on the hubs. Jimny?

 

[rcx194]

Hi Brian. The buggy is based on the Suzuki SJ413 as it's very light, strong for what it is, and the gearing can be modified for bigger wheels and for low range crawling. It has a Vitara engine which is one of the lightest in it's class and fits beautifully. Unfortunately I had to go for Jimny axles as the SJ axles have steering arms at the front of the hubs and meant there was no space for the panhard (unless I lengthen the chassis).

How can three link suspension be set up for zero anti dive? Maybe if the links are parallel?

 

[BrianPetersen]

Yep, parallel upper and lower links will mean zero antidive and zero caster change with axle position ... in a simplistic first-order view of the situation.

Bear in mind that if you have no antidive front or rear (parallel links front and rear) and you slam on the brakes, the very act of bottoming the front suspension and topping the rear suspension thus causing the entire vehicle to go into a nose-down angle ... will result in loss of caster because the whole vehicle is going into that nose-down position and your parallel-links geometry will make the front suspension follow it!

Unless you have a lot more know-how and analysis tools than I have, you'll probably want to experiment to find the happy medium between the amount of brake dive and the amount that it mucks with the steering. Zero antidive is probably bad. 100% antidive is probably also bad. Somewhere between is probably just right. Where ... above my pay grade.

 

[rcx194]

Brian, I have gone for ~50% anti dive.
From what I've read, how much anti-dive is exactly needed is not above anyone's pay grade, as all factors are a compromise, so it's a creative process to decide on the right blend, rather than thinking there's a magic number (says the man who has zero experience and does this as a hobby).

 

[BrianPetersen]

I think we are on the same line of thought: get the design in the ballpark but then make it adjustable for what's left! Especially on the front suspension.