Books and References Off Road Suspension

[Toolish]

I am looking to design an off road racing buggy from scratch.

I have been doing some research on the web, but I was wondering if anyone could suggest some books to help with sorting out handling and suspension concepts and geometry.

It will be double A arm all around, rear wheel drive, rear engined if that makes any difference.

I have had the following 2 books recommended, would you all agree

Chassis Engineering by Herb Adams and How to Make Your Car Handle by Fred Puhn

 

[GregLocock]

faq800-1327

...have you got any of Carroll Smith's books? - but I must admit off-road is a whole new ballgame in reality.

I'd be inclined to keep the design intent fairly simple - low cg, strength, travel, and keep the tires square to the ground, rather than worrying overmuch about frequency response and other rather theoretical things.



Cheers

Greg Locock

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

 

[JohnLear]

I'm no expert on off road buggies (or simply no expert), but wouldn't a trailing arm front suspension (similar to old VW Beetles) be a better choice than double wishbone? A lot of buggies use this sort of front suspension due to it's simplicity and excellent impact absorbing qualities. I suspect you might be able to buy whole front suspensions already sorted ready to bolt in.

 

[Toolish]

Do not have the Caroll Smith books, although have had them recommended too.

There are a lot of buggies out there with VW trailing arm type setups, but nearly all of the quick buggies are A arm front and trailing arm rear.

I can buy a complete bolt in front end, but where is the fun in that!

 

[JohnLear]

'Double wishbones' ought to have a better camber curve (if that's important with skinny front tyres on very loose surfaces?) and scope to place the GRC wherever will work best (wherever that might be for an off road vehicle, maybe higher than the rear GRC to lessen weight transfer away from the IR??).

Would those front ends have wishbones that are substantially angled backward so that the hub moves up and also backward as a twin trailing arm set up would? Just curious...

"I can buy a complete bolt in front end, but where is the fun in that!". I know where you're coming from...

 

[patprimmer]

In off road racing, bump behaviour is much more important than than race car type hanling.

Very long travel and the wheel moing back as it moves up helps a lot.

Toe camber and castor angles need to remain reasonably constant through the long travel. Some camber change on the rear helps to reduce CV joint binding to increase potentia travel.

It really is all about travel, shocker and strength. No matter how well is corners, it won't wi if you cnt negotiate bumps faster than your competitor.

On loose surfaces cornering grip is more about tyres and keeping the tyres on the ground than it is about geometry.

If you really need to hook into turns, use a touch of steering brake on the rear inside wheel and some throttle as required.

If the tyre guys advise lower pressure for traction, don't do it. Use quite high (40 odd PSI) to prevent tyre and rim damage from rock impacts. Having 4 straight wheels with air in the tyres beats a 3 wheeler anytime.

Regards
Pat
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[JohnLear]

Pat; "Toe camber and castor angles need to remain reasonably constant through the long travel."

I'd expect toe control (bump steer) would be an intractable issue with long travel VW style front ends? What about lateral scrub (wheel moving substantially outward with bump), I'd imagine a problem with upwardly angled wishbones (with high GRC?), but a + on the trailing arm score sheet...

 

[patprimmer]

Toe change or bump steer was the only real problem with VW style trailing arm suspension.

We minimised it by using the longest tie rods possible, with the inboard ends about 1" apart in the centre of the chassis. It was an issue, but everyone else also had the problem. We addressed it as best could be done at the time.

We used the GM J car steering box which was unique in that it had the inboard tie rods attach in the centre rather than the ends of the steering box. It came of a Holden Camira. The same car was the Vauxhall Cavalier and an Opel and Isuzu version was available. I don't recall their names.

We stopped off road racing before wishbone front ends came into vogue.



Regards
Pat
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[patprimmer]

This is the car we got the steering box from

http://en.wikipedia.org/wiki/GM_J_platform

We used a manual rack and power boosted via a torque multiplier in the steering shaft. The power steer took some working out with valve port position and pump line pressure.

Regards
Pat
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[JohnLear]

With a VW style front end, what about a single tie rod running from the Pitman arm (located as far to one side of the car as possible) to the the steering arm on the other side, then a drag link from that steering arm to the other? The tie rod end would have a shallower arc, and the drag link ends would also have relatively shallow arcs than two shorter tie rods...?

My wife had a Camira, not a fond memory. The steering suffered from severe kickback / pull / torque steer over bumpy or undulating roads, more than once ripping the wheel out of my hands. The Hillman Imp rack had a similar central tie rod location as the Camira, and was fitted to an old 'Nota' I once owned (Imp had swing axle front suspension, Nota had a somewhat similar suspension made from a divided Ford Prefect beam, the whole car being quite similar to a Lotus 6). The Imp rack wouldn't be suitable for off road use, just not robust enough. I think Citroen may have used a similar arrangement on the DS series...?