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4 link bars
oadamo - 19/6/07 at 10:56 AM

on the de dion axle can you have the top 2 bars shorter then the bottom. and for the fixing points what would be the best adjustment for them. to move the top bars up and down or or the bottom
adam

Peteff - 19/6/07 at 11:28 AM

If they are different lengths they will rotate the de dion slightly, I don't know what the effect of that will be, probably nothing if the wheels are perfectly straight and the bushes on everything are flexible enough. I don't see why you would want to move the bars (trailing arms?) up or down.

[Edited on 19/6/07 by Peteff]

JonBowden - 19/6/07 at 11:40 AM

Simple answer is no, they need to be the same length.

Bob C - 19/6/07 at 12:24 PM

and parallel!!!

MikeRJ - 19/6/07 at 12:47 PM

quote:
Originally posted by JonBowden
Simple answer is no, they need to be the same length.


Jeremy Phillips got away with different length arms on the Striker, though they are watts linkages rather than trailing arms.

Differing trailing lengths will have several effects:

1) They will impose a twisting force on the axle as the car rolls, or with one side in bump. This would act as a crude anti-roll bar, but the forces on the axle locating points can be very high, so care would be needed to ensure everything is strong enough to resist bending and the consequent fatigue cracks. Certainly you would not be able to use a fully rose jointed setup in this kind of application.

2) They will introduce even more roll steer, which is already present due to the short trailing arms. The Jeremy Phillips design eliminates this problem.

3) The entire axle will rotate as the suspension is loaded, which will introduce unwanted angle changes to the UJ on the diff. The Jeremy Phillips design also eliminates this problem.

All in all, not a good idea unless you know exactly what you are trying to achieve, and have worked out the effects of your geometry changes.

kb58 - 19/6/07 at 01:12 PM

Exactly, all the above.

The implication of de dion, while unsaid, is that it's so much simpler to build (mechanically) that it requires no thought and anyone can do it. Yes, it's simpler, but anyone thinking they can turn the brain off is in for a rude surprise. De Dion doesn't get the builder out of the responsibility of knowing what's going on, and it's good to have these discussions.

3GEComponents - 19/6/07 at 02:37 PM

Although if you look at a Caterham, the A-frame is much shorter than the trailing arms. Rescued attachment Picture 016.jpg
Rescued attachment Picture 016.jpg

britishtrident - 19/6/07 at 05:53 PM

Caterham is a 4 link
Locost/Westie is 5 link

4 link gives perfect restraint in all the required axis.
In a 5 link set up is over restrained, it has one link too many unless it is dimensionally perfect it can put very large loads into the chassis

Peteff - 19/6/07 at 07:16 PM

The diff is fastened to the chassis, it doesn't move with the de dion axle. The CV joints on the drive shafts will cope with any movement you throw at them as long as it's not too far out of their operational range.

kb58 - 20/6/07 at 05:47 PM

The issue is that if the links aren't parallel to the ground at normal ride height, it's very easy to end up with toe-out in bump. What I was referring to above is that I only hear about toe-out in bump being associated with IRS. It's just as easy to have the same problem with trailing links.

The links also need to be the same length. As said above, what happens is that when cornering, one side goes into bump while the other side goes into droop. The problem is one side's trying to rotate the axle housing clockwise, while the othersides trying to rotate it counterclockwise. The end result is binding, and a rear suspension that is far stiffer than intended, resulting in oversteer and spins. [As noted, this is only true for live axle, not de dion, though the comments above about toe still apply.]

[Edited on 6/21/07 by kb58]

MikeRJ - 20/6/07 at 10:09 PM

quote:
Originally posted by Peteff
The diff is fastened to the chassis, it doesn't move with the de dion axle.


Well spotted at the back I was of course referring to the live axle which the OP doesn't have

oadamo - 22/6/07 at 09:25 PM

iam asking because ive read on a few sites that if you move the top two bars down you get anti squat so you get more power down. but i want to make it so i can adjust it so i can play with it so suit my driveing. foot on floor red line lol
adam

kb58 - 23/6/07 at 01:30 AM

quote:
Originally posted by oadamo
iam asking because ive read on a few sites that if you move the top two bars down you get anti squat so you get more power down. but i want to make it so i can adjust it so i can play with it so suit my driveing. foot on floor red line lol
adam


Check out Carroll Smith's books; he has a good graphical illustration of how to do it. Just be aware you don't get something for nothing. What you get in anti-squat you pay for in potential binding.

Anyhow, to have anti-squat at the rear means you have to aim both trailing links at the CG of the car (for 100% anti-squat).

CGILL - 23/6/07 at 08:07 AM

The anti squat works from the torque reaction of the axle, so with a live axle when the front of the diff wants to lift (gear climbing the pinion) the top arm tries to go parrallel (furthest distance) and so pushes the axle down to the ground.
With a dedion the torque reaction isn't there.. so no point, this is where dedion is great, it doesn't try to rotate the axle, and it doesn't try to lift one end of the axle as the diff is fixed, team this up with inboard brakes, it removes torque reaction from braking.
I take it the original poster is wanting to set up a satchel link, where the angle top arms (when viewed from above) locate the axle laterally aswell, best off with 4 parallel links and a panhard/watts/mumford for lateral location. I'd only use shorter top arms on a live axle and if I'd made the bottom arms longer than they are.