actually it's probably my second or third. This one stems from a close study of both Milliken and Milliken and Katz while in my favourite room of requirement. M&M reports a lot of results from the F1 ground effect era which indicates that the best downforce was actually developed with a ground clearance of 75mm (which just happens to be the minimum ground clearance for RGB) and there are some useful wind tunnel results from vehicles with underbody tunnels and no sealing sideskirts. So with careful consideration paid to the shape of the underside of the chassis, it looks like there might be the possibility of generating useful downforce for the car. Hmm.... I had been slightly upbraided by my solitary commenter (thanks Steve) for having a pushrod suspension system in the first picture produced out of SusProg. If I do manage to get significant downforce out of the car then having a pushrod system will make it easier to engineer in a third spring to deal with the extra aero-loading. I quite liked the idea of a pushrod suspension in any case as it makes it far easier to adjust the suspension in terms of ride height and damper adjustment as you can get at the adjusters simply by taking the bodywork off rather than having to get down on your hands and knees and contort your wrist around the suspension arms, driveshafts and brakes. I have to do that now with my daily driver and it's not a whole heap of fun.
I've also been thinking about the uprights as well, or more specifically the hubs that are going to bolt into them. I'm not a big fan of reinventing the wheel, and HiSpec Motorsports do very nice CNC machined (and you can't beat CNC bling) hubs based on patterns of classic Ford boxes of the 1970s and it would be churlish not to use something easily accessible just to be different. I know of at least one racer using these, so I might sidle up to him at some point in the future and ask to come round with my vernier callipers and see if I can come and measure them for dimensions. Once I have those dimensions, I can work on sorting out the other positions on the upright. The other thing I need to work out is how to avoid the dreaded REIB problem. I could make the suspension completely non-adjustable, but I reckon there are two main problems - firstly, I can't imagine anyone constructing the car is going to be so accurate that the designed geometry is actually going to be the real-life geometry and a bit of fiddling is bound to be necessary; and secondly, giving someone a non-adjustable racecar is bound to be a recipe for disaster. A lot of the RGB racers seems to be inveterate fiddlers.
Of course, I could just ignore the fact that I'm using rod ends in bending and make them nice and stout instead. As I'm not planning on winning a Formula SAE design competition and everyone else seems to manage fine with that sort of structure. I could get away with it, but every engineering bone in my system is yelling at me not to accept such compromises and spend 6 months developing a beautiful adjustment system. Knowing my luck such a system would weigh more than the rest of the car put together, although I do like the eccentric bolt system that's on my Mazda and it would limit the amount of adjustment options to what is sane.
Anyway, time to get back to the AP Racing catalogue and find a suitable set of callipers and discs for the virtual racer. Once I have both, and the dimensions of the hub, I can sort out a wheel offset and then, finally, the position of the suspension joints. Why do I need all this information? I want centre point steering on the car. This means that the line passing through the wishbone mounting points on the upright passes through the centre of the contact patch and there is no kickback through the steering when longitudinal forces are applied to the suspension. Kickback drops driver confidence and as I've stated before, an under-confident driver is a slow driver.
In the last post, I'd sort of produced a ballpark figure for the space inside the wheel rim of around 310mm. I know that the rim slopes inward at the front and I'd probably have more to play with than that provided the brake disk and calliper wasn't too deeply embedded. So, a quick search through the AP catalogue turned up the fact that 254mm discs aren't supported by 4-piston callipers, but 267mm discs are. Of course when you look at the outside radius of the callipers they need 312mm of space inside the rim. Close but scarily close to being a show stopper. There's plenty of options in the 2-piston calliper range and frankly this car is going to weigh very little. But I just have this horrible sneaking suspicion that if we do get sufficient downforce then 2-pot callipers may not be man enough for the task. I'm thinking here of braking down at the end of the Revett straight at Snetterton - it's not somewhere I'd like to run out of brakes. I'll wait until I can measure a 14" alloy (actually, come to think of it, there's five sitting in the garage ready for the kit car it looks like I'll never finish building because I'm better on a CAD system than I am with my hands.) before making a firm decision. Having seen the trouble others have had getting brakes and uprights to talk nicely to each other, I don't want to get this fundamental wrong.
Subscribe to:
Post Comments (Atom)
2 comments:
Is there any particular reason for using 14" wheels, the 13" ones are cheaper, lighter and more readily available.
Yes you get a bit more space to play with your brakes but with such a light target weight I can't see why the off the shelf kits such as the wilwood powerlites shouldn't be plenty big enough. The only area where the 13" wheels will be tight is over the handbrake mechanism on the powerlite handbrake calipers but even that doesn't seem to be a major problem in practice.
It was space for the brakes as the driver - I'll do the calculations and work out if I can get away with 13" wheels and the brakes that fit underneath. Nothing is set in stone, although having more space under the rim will help with the suspension kinematics as well.
Post a Comment