Uprights and wheels

I've been doing a little playing around with SusProg in order to look at the fundamental effects of locating the instant centre along the vehicle centreline. I've revised my estimate of track after looking at the track values of someone else's car - I'm now looking at a track width of around 650mm. Anyway, I've modelled two suspensions - one with a fairly conventional wishbone setup and one with sharply inclined wishbones so that the instant centre is along the centreline.

Look at the two photos:


The upper one is the conventional. The pictures aren't to scale (blame finger trouble on my part), although the grids are identical at 40mm spacing. The big black dot is the ground on the centreline and we're looking at 40mm of bump travel here. The obvious difference is that there is a lot of camber change in the 'centreline' suspension - this is to be expected, as using such suspension gives a very fixed virtual swing arm length (approximately half the track width), while the normal suspension has a significantly longer swing arm and less camber change in bump. The question is whether the absence of camber change in roll makes up for the change in bump. The short answer is probably, but only at the rear. At the front the camber change that would result in the wheel acting like a gyroscope and kicking back through the steering. That would be something that could decrease driver confidence and is therefore a bad thing as far as my design principles go.

I'll keep tweaking the values for a while and see if I can get to a better centre point solution. I'm pretty limited in where I can put the bottom wishbone joint as it can't be lower than the 75mm limit mandated in the rules. As I'm planning to design my own uprights, I've got a fair bit of flexibility in where I put the roll centre so I can lower it from where it currently is to prevent jacking of the vehicle under cornering.

I'm getting to the stage where I should really stop fiddling with SusProg and work out what shape the uprights are. Key to this is going to be being able to fit a suitable set of brakes within the wheel envelope. This might sound easy, but I know at least one racer who had all sorts of problems trying to get a set of brakes to fit. So first job is to find a suitable wheel and get some measurements off it. I'm limited to 6" wide wheels and luckily, I've got a dented 16" alloy wheel sitting in the shed (before any of you comment, I know that the A048R spec tyres don't come in 16" sizes, but I wanted a quick and dirty check on what I had here before I pester one of you for a measuring sessions). A quick bit of steel tapery and I get a 360mm envelope within which to work. If I assume that there's no change in wheel web thickness, each 1" reduction in rim size means 25mm less envelop to play with and I doubt there's many 6"x15" wheels out there, so I should think that a 14" wheel rim is likely to be the norm. Certainly the above-mentioned racer is running 14" wheels. So, I think the maximum envelop I have to work with is going to be 310mm. Now all I need is some brake callipers to fit underneath.

One quick glance at the Wilwood catalogue and their Powerlite callipers will do the trick. The trouble is that in order to pass scrutineering the car needs a working handbrake and the 'add-on' callipers I've seen probably won't fit comfortably inside the wheel. So, the obvious choice is HiSpec Motorsport, who do specific callipers with a mechanical handbrake that doesn't impinge on the outside of the calliper. Their version of the Powerlite looks very similar, although their mounting diagrams leave a little to be desired. I'll assume a 254mm disc for now.