longitudinal forces from braking vs. bumps
longitudinal forces from braking vs. bumps
(OP)
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?





RE: longitudinal forces from braking vs. bumps
Which doesn't mean they're not here. There could be thousands of 'em in the potholes, and you wouldn't know.
Mike Halloran
Pembroke Pines, FL, USA
RE: longitudinal forces from braking vs. bumps
Why does it matter if the bump steer is set correctly and suitable castor is retained.
Link pin VW type front ends were used extensively on off road racers. The were set to give a lot of longitudinal compliance and gave no problems.
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RE: longitudinal forces from braking vs. bumps
Typical longitudinal recession rates are 300-1000 N/mm.
You can certainly design around the softer end of that, principally by making sure your inner tie rod is in the right place.
Cheers
Greg Locock
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RE: longitudinal forces from braking vs. bumps
Mike Halloran
Pembroke Pines, FL, USA
RE: longitudinal forces from braking vs. bumps
RE: longitudinal forces from braking vs. bumps
A 2D model of a wheel in a pothole will not give a very accurate estimate of the forces, but if you put all the right parameters in you can get within a factor of two or so.
Full 3D vehicle models with non linear tire models will get much closer than that, see for instance www.ftire.com, or ABAQUS Explicit. I've never done that, it's on the list.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: longitudinal forces from braking vs. bumps
To my knowledge the longitudinal compliance of the wheel centre is the important factor regarding impact harshness felt by the passenger. The braking forces act at the tyre-road patch. This could be designed stiffer longitudinally than the wheel centre point. Wasn't that the clever bit of the McLaren F1 front suspension?
Best regards
René le Grand
www.bourgest.com
RE: longitudinal forces from braking vs. bumps
RE: longitudinal forces from braking vs. bumps
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: longitudinal forces from braking vs. bumps
RE: longitudinal forces from braking vs. bumps
It's quite an interesting problem.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: longitudinal forces from braking vs. bumps
Best regards
René le Grand
www.bourgest.com
RE: longitudinal forces from braking vs. bumps
RE: longitudinal forces from braking vs. bumps
Yes that's the handwavy explanation, although I'd usually say RY for rotation. Lotus proposed that 'raft' design in 1987 and used a version of it on the SID car. I did the bushes.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: longitudinal forces from braking vs. bumps
Best regards
René le Grand
www.bourgest.com
RE: longitudinal forces from braking vs. bumps
We are supposed to be professional engineers here, so there is no need to dumb anything techie down. I'm actually rather suspicious that the handwavy explanation is misleading in the extreme, suspension engineers often think that bushes in the real world behave as they would like them to behave, rather than as lumps of fluid that vaguely behave like a solid. I think I can guess what the handwavy setup would have been, and I bet no one ever worked out the ACTUAL recession stiffness at the CP and WC. Because they'd have had to admit that the claims are untrue. Big clue:compliances are additive.
No I can't remember every single bush stiffness in all 6 DOFs from the last 25 years of working with them.
SID was a bit odd as it had a spine chassis, containing the powertrain, with a raft (separate, vertical, subframe), at each end for the suspension, that also suspended the body. So by choosing the appropriate bushes you could go anywhere from isolated body on rigid skid unit, to rigidly mounting the subframe in the car and suspending the powertrain softly.
The raft was originally sketched out for the M300 supercar that I suppose morphed into the Bugatti EB110, but they used some fancy sliding linkages that would not have made it into production, or even onto a working prototype.
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: longitudinal forces from braking vs. bumps
RE: longitudinal forces from braking vs. bumps
The problem is, it might work very well in reducing the longitudinal compliance at the contact patch compared with the wheel centre, but it sacrifices the main function of a suspension - to go up and down. It is not a mechanism.
Secondly, what is so great about reducing the compliance at the contact patch? Is stability of the castor angle so important?
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: longitudinal forces from braking vs. bumps
Cheers
Greg Locock
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: longitudinal forces from braking vs. bumps
They accoplished 970Nmm at the WC and 2380Nmm at the CP unfortunatelly the other measured cars are only giving one or the other
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
www.bourgest.com
RE: longitudinal forces from braking vs. bumps
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
SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.
RE: longitudinal forces from braking vs. bumps
Best regards
René le Grand
www.bourgest.com