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Locost rear suspension.... Hypothetical question
alistairolsen - 5/5/10 at 02:39 PM

We all know the basic 4 link and panhard arrangement of the rear suspension, the panhard is fine in that its as long as possible, but has anyone experimented with mounting it lower, to lower the rear roll centre, and to what effect?

Also, I suspect the length of the parallel links was as arbitrary as the rest of uncle Ron's design. Can anyone who's into suspension tell me if there is any real advantage in making them longer?

Finally, rubber bushes work, as do rose joints, but i cant help thinking stiff poly bushes will massively add to the roll stiffness as they wont deform as they should when one side is in compression and the other in droop.....

Anyone proffer any advice or experience?

Cheers!

MikeRJ - 5/5/10 at 03:08 PM

Longer trailing arms are certainly better, the standard Locost design has them too short which causes roll steer. Trouble is there isn't much scope to increase their lengths unless you don't mind them protruding through the front of the rear arches like the Sylva Striker.

Poly bushes shouldn't significantly increase the roll stiffness; they don't get moved a huge amount and the deflection is shared between both bushes on each arm.

coozer - 5/5/10 at 03:12 PM

How about a longer bottom arm as the Striker has and an A frame connecting the middle of the axle to the chassis. Does away with top trailing arms that are difficult to make any longer and no need for a panhard rod.

The Striker has shorter top arms that run backwards.

MikeRJ - 5/5/10 at 03:21 PM

quote:
Originally posted by coozer
How about a longer bottom arm as the Striker has and an A frame connecting the middle of the axle to the chassis. Does away with top trailing arms that are difficult to make any longer and no need for a panhard rod.


Just like the Caterham. Trouble with this is it puts a lot of torsional force on the axle housing itself, and English axle is such a flimsy thing to start with I'm not sure how long it would last.

boggle - 5/5/10 at 03:44 PM

the landrover defender has the same setup on the rear....

tomgregory2000 - 5/5/10 at 04:03 PM

quote:
Originally posted by boggle
the landrover defender has the same setup on the rear....


but the defenders a frame is made from girders(sp) i dont want to have to remove mine agen

alistairolsen - 5/5/10 at 04:06 PM

Its not for a locost, so I could very possibly make them longer.....

Question is should I? and how much longer?

How is the roll steer caused? Ive seen it done with unequal length arms on oval racers so it pulls the axle forward on one side as it rolls, but cant see how it happens with 4 arms of the same length, unless it pulls the axle forward so far the panhard rod becomes responsible for pulling it over to one side?

I'm not keen on the A frame arrangement, and I'm also not sure how one would attach it to the diff housing as its cast. It would be easier to implement a satchell link to the axle tubes on either side of the casing.

blakep82 - 5/5/10 at 07:32 PM

as long as possible really. the longer they are, the less of an arc the end is likely to create as it moves up and down. the idea is to keep the axle as central as possible obviously, with little sideways movement.

so at rest on normal loading the panhard rod should be horizontal.

if you have space and are designing the back end yourself, watts link would be a better idea i think

Sam_68 - 5/5/10 at 07:55 PM

quote:
Originally posted by alistairolsen How is the roll steer caused? ...can't see how it happens with 4 arms of the same length.

There can be lots of conflicting arcs. Remember that the roll centre is usually below the 'neutral axis' of the trailing arms.

Also remember that cars do not actually rotate around their geometric roll axis when they roll. Due to different roll centre heights and roll resistances front and rear, there is usually a combination of roll and squat (rear wheel drive, typically) or dive (front wheel drive, typically), so the two sides of the axle are not rising and falling by equal amounts when the car rolls.

And on top of roll steer, there's the obvious bump-steer you get when just one end of the axle is deflected by a bump.

You can do all sorts of clever stuff with trailing arm linkages (think of them as wishbones turned through 90 degrees and you'll realise the possibilities), but the Locost's short, parallel, equal length trailing arms are about as good an example of how not to do it as you're likely to encounter.

kb58 - 6/5/10 at 01:25 AM

A four-bar setup only works when two or more of the link bushings are compliant. Unless all four links are Exactly the same length, as the suspension compresses on one side in a turn, the parallelogram effect causes that side of the axle to rotate (because the links are never identical.) Meanwhile, the other side of the axle that's in droop is being controlled by its own two links, and will resist like crazy the rotating being forced upon it by the links on the other side.

I saw this first hand when my brother "improved" his Mazda by swapping in stiff bushings into the four-bar links. We knew something wasn't right when he tried jacking up one rear tire, and it only got about 1" off the ground before the other tire came off the ground. In effect, he'd converted his setup into using the axle tube as a huge anti-roll bar... lesson learned.

As far as lowering the Panhard rod, it's usually a problem of finding what to attach the structure to. The idea typically doesn't ge to far and is given up.

alistairolsen - 6/5/10 at 08:09 AM

I still don't understand why, given 4 links of identical length and therefore perfect parallelograms, extending the arms should improve the system at all?

Obviously the panhard wants to be as long as possible as its end describes an arc which will shift the axle from side to side......

Compliant bushings it is!

Incidentally, I recently spent a good portion of my life reading the project thread for Kimini and Ive started on the Midlana one. Some inspiring stuff, you have a lot to be proud of!

MikeRJ - 6/5/10 at 10:19 AM

quote:
Originally posted by alistairolsen
I still don't understand why, given 4 links of identical length and therefore perfect parallelograms, extending the arms should improve the system at all?


Because the trailing arms also describe an arc exactly the same as the panhard rod, except they are much shorter so the effect is more pronounced.

In roll the angles of the left and right trailing arms will rarely be equal and opposite, and certainly this will never be the case in single wheel bump. The shorter the arms, the more the axle is moved longitudinally over the suspension range, so longer arms give you less roll and bump steer.

alistairolsen - 6/5/10 at 12:54 PM

Found this with some pretty pics, I get it now!

http://www.afcoracing.com/tech_pages/4link.shtml