Anti-dive if side view IC is ahead of vehicle

[damienmavis]

I'm designing a small, one seat, motorcycle engine powered offroad vehicle. Double wishbone front and rear w/ about 20in. total suspension travel. The front lower wishbone outer pivot will be connected at spindle height to the front upright (for increased ground clearance and packaging). I'd like the wheels to travel rearward a bit as they move up for a more compliant ride. This forces the lower wishbone to be tilted upward in front because the wheel path, (in side view) is governed only by the lower arm, in this scenario. For packaging reasons it will be dificult to tilt the upper arm even more than the lower to get some anti-dive. But due to the low spring rate and light damping anti-dive is important. For packaging reasons the side view instant center ends up infront of the vehicle. How do you compute % anti-dive with this setup?

Second question I can see why it might be dificult to get 100% anti-dive without seriously affecting the ride but 100% anti-squat seems possible are there any drawbacks to achieving it?

Thanks

Damien

 

[BillyShope]

There's no problem with the IC ahead of the front wheels. Perhaps you were concerned in that it would be above the line, rather than below. Don't be. While the line can be extended to infinity in either direction, the effects are mirrored (mirror in plane parallel to YZ plane) at the tire patch. This is easier to explain at the rear of the car. Suppose you have parallel links, at the rear, which are also parallel to the ground. (Link lines and ground line...not that the ground line matters other than to give you an idea where the IC is relative to the 100% line...meet at infinity...and we'll assume that the meeting occurs at the front.) We know this means a ton of squat during acceleration. Now, suppose you moved one of the link pivot points a very small amount and the IC is now half a mile or so from infinity at the rear. Common sense dictates that, though the IC is now far above the line, the rear of the car wouldn't suddenly shoot upwards. No, the effects are mirrored at the tire patch.

As for 100% anti-squat at the rear: Go for it! There are those who contend that such a setup tends to "lock up" during braking, but I've yet to have anyone provide a mechanism for such a response. It's like switching channels on the idiot box: There are braking geometry considerations and then there are acceleration geometry considerations.

 

[Fabrico]

Unlearn the pavement. Although the attachment points of your suspension are important, they will have less to do with dive and squat than the effects of long travel suspension. You're in luck because without lots of dive and squat, you won't stop or start fast in the dirt anyway.