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Watts linkage pivot height / De-Dion lowered chassis

se7ensport - 16/8/18 at 09:43 PM

Hello!

The evolution continues towards a sprint and hill climb car, as such I am reducing the rear ride height from 150mm to 105mm, with that the de-dion tube is now noticably above the centre pivot point of the Watts linkage; Do I need to raise the pivot point so that its in line with the centre of the de-dion tube?

Watts Linkage

All knowledge appreciated.

Sam_68 - 16/8/18 at 10:08 PM

Not necessarily - there should be plenty of motion left in the linear part of the Watt's linkage's range still (though by all means check that that's the case).

But what is the lowering doing to your front roll centre height?

Although there are ways of 'tuning out' any differences, the simple way to ensure that you don't fundamentally alter the chassis' understeer/oversteer balance is to make sure that your roll axis inclination doesn't change... so if you're looking at lowering the rear roll centre by 45mm, then unless the front roll centre lowers by a similar amount, you may find that you have to play around with the relative front and rear roll stiffnesses (spring and ARB rates) to recover the balance.

se7ensport - 16/8/18 at 10:16 PM

I've just had RJR (Cyana) make me new rockers and lower wishbones based around new uprights to accommodate a front ride height of 100mm. front end geo is now spot on.

Re the remaining rear motion, its got a good few inches before the pivot hits an extreme causing the de-dion to get pulled side to side, but I was unsure if if needed to be centered for other reasons. I was about to make a mock up out of technic lego...

Sam_68 - 16/8/18 at 10:28 PM

quote:
Originally posted by se7ensport
I've just had RJR (Cyana) make me new rockers and lower wishbones based around new uprights to accommodate a front ride height of 100mm. front end geo is now spot on.

But where does that put the front roll centre?

Have you changed (raised) the position of the front suspension pickups on the chassis, so that in effect you are lowering the chassis toward the ground whilst leaving the front suspension in the same place?

If so, then by leaving the rear roll centre (which is fixed by the chassis mount of the Watts link) in the same place, you are changing the slope of the roll axis (an imaginary line that connects the front and rear roll centres). This changes the diagonal weight transfer when you're cornering, hence changes the oversteer/understeer balance.

Not necessarily a problem, but expect to have to play around with the relative stiffness of the front and rear springs/ARB's, to bring things back into balance.

se7ensport - 16/8/18 at 11:02 PM

Chassis pivot points have remained the same to comply with MSA regs, but I’ve raised the upright connection point to the rocker which in turn lowers the roll centre.

What I don’t have is the new roll centre height... its been a gut feel rather than science approach!

Does changing the pivot point of the watts linkage actually impact the rear roll centre height? If so, how do I calculate it?

I will bring science in to it if I can.

Sam_68 - 17/8/18 at 06:31 AM

quote:
Originally posted by se7ensport
Does changing the pivot point of the watts linkage actually impact the rear roll centre height? If so, how do I calculate it?

The roll centre position on a Watts link is easy - it's at the pivot position of the centre rocking link (ie. where it attaches to the chassis).

As I implied in my first post, if you've lowered the chassis by 45mm. then you've lowered the rear roll centre (relative to the ground) by 45mm. also.

Without knowing exactly what RJR have done to the front geometry, it's difficult to advise. Personally, I would have been feeding the 'existing' and 'proposed' geometry into a computer analysis (I use a program called SusProg3D) to figure out what's going on, as the effects of such changes are not always obvious on a double wishbone system. Failing that; simply see how well it drives, and tune the handling balance with the springs and ARB's (and secondarily with the dampers), as you would normally.

Andy S - 17/8/18 at 06:41 PM

quote:
Originally posted by se7ensport

... but I’ve raised the upright connection point to the rocker which in turn lowers the roll centre.

Not clear with this, are you saying you have made the outer (upright) pivots further apart and the chassis ones remained the same - shorter swing axle length. Or made the outer ones (upright) closer together and the chassis pivots the same? Longer swing axle length.

se7ensport - 17/8/18 at 07:08 PM

I’ve increased the height at which the rocker arm joins the top of the upright while leaving the pivot point on the chassis in its original location.

The top of the upright is now above the centre line of the rocker pivot, it would be level without the extension.

Andy S - 17/8/18 at 08:52 PM

Still not much clearer but assuming making the upright taller - that raises the RCH not lowers it.

[Edited on 17/8/18 by Andy S]