longitudinal forces from braking vs. bumps

[formula94lt1]

There are a few suspension tricks I have seen utilized that control longitudinal compliance for the sake of ride quality without giving up very much, sometimes extremely little, in the way of overall wheel control. Some examples are the Acura NSX and its compliance pivot, the Lotus Elan M100 and its suspension raft, the McLaren F1 and its ground plane shear center subframes. I am curious though, in allowing longitudinal motion over bumps to decrease the shock of it that the passengers will feel, this obviously would allow for longitudinal motion in the wheel under braking. I would assume that braking force, especially at the front wheels, would be far greater than the force imposed by pavement junctions and lane dividers, even a pothole. Or is it?

 

[Bourgest]

Just scrolled over the article I have :An presentation at ImechE by S.J.Randle back in 1993.

They accoplished 970Nmm at the WC and 2380Nmm at the CP unfortunatelly the other measured cars are only giving one or the other ;-) These where measured values 2.5:1 but they actually calculated a 4.7:1 ratio, which thus was not acieved.

I don't know the SID cars/raft suspension. Anyone got a picture?

Greg, I don't understand your comment on the "to go up and down" part. The part we are talking about was designed in at the subframe mounting points to the chassis. The suspension is mounted to this subframe in a "normal" way even using Glacier DX metal-polymer bushings. So I can't se why the suspension wouldn't go up and down.

//René

Best regards
René le Grand

http://www.bourgest.com

 

[GregLocock]

Ah OK, I was assuming they'd use the arm to body bushes, not bother with subframe bushes. Using subframe bushes makes a lot more sense.

Interesting numbers - so did Steve identify /why/ they went to this extraordinary effort? 2380 is a pretty good achievement. My guess would be that all the other compliances in the system (and of course the radial stiffness of the bushes themselves) prevent the bushes from acting as pure linear bearings.

Vehicle Dynamics magazine ran an article on SID a couple of years back, but I don't think it had any drawings.

I've just found my notes on the first pass rates for M300, which used the same concept as SID's soft setting, with the subframes (rafts) mounted relatively stiffly into the body, and the powertrain spine flexibly mounted into the subframes. Target wheel center recession rate was 200-300, as it was supposed to be a refined car.

The subframe bushes were 1000-5000 N/mm, but the suspension guys thought they'd give too much subframe steer. The powertrain to subframe bushes were 90 at the front and 250 at the rear, it was mid engined. M300 never got built.

However SID was almost certainly built with stiff bushes, where the powertrain was more or less rigidly mounted to the subframes, to control subframe steer a bit, and give the active suspension a better foundation.

We also designed and built a show car for GM (the Corvette Indy) that used almost the same concept, I seem to have won the argument for soft bushes on that one, but it was still very noisy.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Bourgest]

This is what I googled...

Best regards
René le Grand
www.bourgest.com