Roll centres with swing arms

Hi, question about roll centres with swing arm suspensions as used in formula vee. Obviously the camber changes substantially with heave of the rear, and most vees run a "zero roll resistance" setup without rear arbs and with camber limiting devices which essentially stop the rear wheels drooping to negative camber. My question is with such a high roll centre being the limiting factor, does this negate the need for a rear arb, or is there still a benefit in running an arb

I may be missing something in your query, but my thoughts.

You may want to review the rear suspension characteristics, especially the dreaded "tuck under", and why a rear ARB is generally not a good idea as it ties the rear sides together to encourage droop.

What is usually used to provide/enhance the lower rear roll resistance is a "Z-bar", which works in the opposite way to a conventional ARB. So, rather than further complicate the rear suspension with an additional ARB, with it's linkages, I'd suggest looking at introducing adjustment into a "Z-bar" instead, if it isn't already present.

If this is a VW based racecar, or special, you may also wish to consider the front suspension options, too.

Hi Gord and thanks for the reply.

Yes the class rules dictate that the rear suspension and geometry is from a type 1 style beetle, and all the negatives that come with it. I'll have a look at the z bar setup you mentioned.

How would a rear arb encourage droop? Is this because the loaded tyre is now linked with the unloaded one, essentially pulling it down? This is the reason that a camber limiting device is used, otherwise the tuck under occurs especially during corner entry.

You might need to check the rules, but if legal a "Z-bar" should be the way to go, especially if it's easily adjustable be sliding/rebolting the links' on the arms for adjustment. They're commonly used on swing axles and it works the opposite to an ARB in that as one wheel is compresed it pushes down the other wheel. It's called a "Z-bar" because one link's arm goes forward and the other backwards, rather than beins symetrical, with the centre part of the bar mounted diagonally from front to back, looking like a "Z" from above.

Yes, with an ARB as the inner drops it pulls down on the outer, increasing the 'jacking' force. This is worst when backing off/braking as you surmise, especially as the side loading at the tyre/track interface is applying the 'jacking' force too.

The "camber limiting", because of the suspension design, mechanisms work to limit the travel in droop, although it's been a few decades since I had anything to do with these. They may be commonly used because they appear to be much easier to fit to the road vehicles, but a race car - Formula "V'? - shouldn't really have that problem.