Rear Roll Centre...Thoughts

[djm411]

I have been doing some reading on roll centres or if your reading an American book roll centers, and have become somewhat confused about the benefits of lowering the roll centre on my de-dion. So here is my understanding:

Irrespective of the roll centre height you are going to get the same amount of lateral load transfer for a give lateral acceleration, aka cornering speed, as this is dependent on Centre of gravity (CG) height not roll centre height.

The cg on my alfetta sedan is about half way up the door.

the amount of roll generated by the car is related to the lateral acceleration acting through the CG and the distance between the CG and Roll centre height, i.e. the centre of the watts linkage.

So if you lower the roll centre, you move it further away from the CG and create a greater roll moment, and more work is needed to be done by the springs or sway bars to resist this roll.

From my readings the only thing that would discourage someone from RAISING the roll centre would be Jacking forces and dodgy camber curves, both are products of independent suspension systems.

So I'm confused as to the reasons for lowing a rear roll centre, please show me the light!!!

P.s. if my spell checker underlines CENTRE one more time!!!!

[Duk]

A lower rear roll centre aids corner exit traction. A higher rear roll centre gives better initial turn in.
The obvious disadvantage is that the rear roll couple length changes with braking (roll couple length gets longer as the rear lifts) and acceleration (roll couple length gets shorter as the rear squats) with the Alfa set up Watt's linkage.

[djm411]

A lower rear roll centre aids corner exit traction. A higher rear roll centre gives better initial turn in.

Duk, can you please explain how this works?

[Duk]

Duk, can you please explain how this works?

Not really, so: http://alfagtv6.com/phpBB3/viewtopic.php?f=10&t=3370&st=0&sk=t&sd=a&start=15

Also (same guy as the stuff I quoted on GTV6.com) http://www.toymods.org.au/forums/showthread.php?t=45257&highlight=rear+roll+centre

[aggie57]

Paul is on the right track - it's more about sorting out the differences between the front and rear roll centres on these cars than the absolute roll centre vs. CG.

Essentially what you are doing by lowering the rear roll centre is moving it more in line with the front.  The next step is to raise the front by something like an extended upright or some of Vin Sharp's knuckle risers.

The net effect of those two changes should be to reduce the the propensity of the car to "kneel" through corners.  The downside is a reduction in response at turn-in, giving the sensation of increased understeer but by balancing the car better in my experience you actually end up with less mid-corner understeer and faster exit speeds.

But (again in my experience) once you start making changes like that you'll either need to or want to make some other changes as well, such as fiddling with roll stiffness front and rear, and adding some negative and toe in at the rear. 

There are plenty of threads about this both here and on the alfabb. 

[hammer]

If you go back through Jim Nielsen's posts from a few years ago there are some interesting pics and explanations of the watts link and roll centre set up on the Guilietta that Jim built and I now own. It has some quite radical changes and it handles like a dream.

I'm not suggesting you follow Jim's lead, just interesting reading.

Cheers,

Brent