Wisbone lengths

[Incorrigible]

Hi all

Just getting my head round some suspension ideas, Not sure if this should be in the suspension forum but it's quiet in there

I'm desinging a chassis for a special I'm hopefully going to build next year, and I've got quite a lot of it sorted even down to spring rates and roll centres

With my design there's quite a bit of flexibility regarding wishbone lengths. I could mount them quite a way inboard like an F1 car or other single seater. But it struck me that other cars that could do this don't, thinking sports cars (le Mans type sports cars) Ultima etc. The car will end up (hopefully) looking a bit like a radical XTR etc (general shape)

What's the advantage of the longer ones, is it purely to minimise the possible frontal area of the car or are there chassis dynamic issues. Should the front and rear ones be of similar lengths, are there issues if they aren't. Does this choice lead to a particular type of handling characteristic

Cheers

Ben

[dern]

Sorry if you already know of it but there's a special builds section on www.locostbuilders.co.uk/ and the site is very useful for discussions of this nature. I've read a few but it all went over my head so can contribute nothing bar the link I'm afraid.

Good luck,

Mark

[Incorrigible]

Thanks Mark

I have read a lot of that thanks, locosts are more constrained to the lengths of the wishbones if not the position on the chassis (as a lot of them weld the chassis up themselves)

Basically it means you have complete freedom over roll centre positions and therefore spring shock and roll bar types (all a compromised depending on what the car is for)

I have the option to put my pick up points a fair few inches inside that of a locost, just wondering if I should

[gentlefoot]

The only affect I can think of is that if the wishbone is longer, the angle that it will sit at will be less steep for the same amount it rises or lowers from one end to the other. Hope you get what I mean. This should have an affect on the roll centre.

I guess if you wanted to allow for suspension travel during extreme lateral forces, you would want longer wishbones. This would mean the front roll centre will move up and down less in roll and bump. Always a good thing.

My guess is F1 use short wishbones because they have no suspension travel at all almost. Spring rates of about 6000 lbs/in! This means the shorther wishbones have less affect on roll centre. Must be lighter too which is crucial when you think where that weight is.

[Incorrigible]

gentlefoot said:

The only affect I can think of is that if the wishbone is longer, the angle that it will sit at will be less steep for the same amount it rises or lowers from one end to the other. Hope you get what I mean. This should have an affect on the roll centre.

Yes, by by the same token you can change the position on the chassis to make it the same if required

gentlefoot said:

I guess if you wanted to allow for suspension travel during extreme lateral forces, you would want longer wishbones. This would mean the front roll centre will move up and down less in roll and bump. Always a good thing.

Yes, but I do wonder why the likes of ultima who by all accounts handle pretty bloody well choose very short wishbone lengths, maybe it's becasue the car is quite heavy and they use very stiff springs, maybe it's a compromise, I don't know

gentlefoot said:

My guess is F1 use short wishbones because they have no suspension travel at all almost. Spring rates of about 6000 lbs/in! This means the shorther wishbones have less affect on roll centre. Must be lighter too which is crucial when you think where that weight is.

I wasn't aware just how stiff the springs were maybe F1 was a bad example (I was only using that to put an instant picture in peoples minds) Would any set up with long wishbones need stiff springs ? I can't see why it should

Thanks for your input

[pdV6]

Incorrigible said:

Would any set up with long wishbones need stiff springs ? I can't see why it should Thanks for your input

More mechanical advantage (i.e. leverage) as the wheel travels up over a bump? Dunno - I'm not a mechanical engineer...

[pdV6]

Lightweight car with long wishbones:

http://dpcars.aprsworld.com/dp1/da.htm

[chassis 33]

F1 also use silly high spring rates, because they drive the shock/damper by bell cranks which give a leverage ratio. Bell cranks are used to keep the shocks out of the air flow.

I have some notes of suspension design etc and there was a reason why in inboard pivot is desirable. I think its along the lines of it keeps the roll centre more stable throughout the range of the suspension travel. The down side is that they need to be made from unobtainium to make them stiff and light.

I'll try and dig out the info and get back to you. Have you looked through any textbooks? eg Miliken and Miliken's huge book, basically a bible of suspension theory.

Regards
Iain

[Incorrigible]

chassis 33 said:

I'll try and dig out the info and get back to you. Have you looked through any textbooks? eg Miliken and Miliken's huge book, basically a bible of suspension theory.

I've tried to get a copy of Milikens but it seems to be about the same price as unobtanium

[Incorrigible]

pdV6 said:

Lightweight car with long wishbones: http://dpcars.aprsworld.com/dp1/da.htm

I'd seen one of those before, and there are annoyingly similar to one of my options (annoying that other people have more time and money to actually start their projects )

[Nick_F]

All other things being equal, the longer the wishbone the shallower the arc that the outer end moves through and the less lateral displacement of the wheel in bump.

For a given wheel bump travel a longer wishbone has to move through a shorter arc too, which is also good.

So the longer the better: but you have to fit a car in between them - relative to the track width the space available for wishbones will be less on something like an Ultima - where you need to fit the footwells for driver and passenger between them - and a Caterham - where you only have to fit the radiator between them.

You also have to make them strong enough to cope with road use - which tends to be a limiting factor, but if you look under the back of a Jag or a Honda - where space is less on an issue - you'll see that the wishbones have been made about as long as they possibly can be.

Carroll Smith talks good sense about double wishbone geometry in Tune To Win.