206er wrote:most bike frames using air shocks have what, a ~2.5:1 leverage ratio? not sure what leverage ratio you will be going fo rwith cantilever suspension but I'd assume its close.
Offroad terrain produces more hard hits to bottom the suspension, and generally a lot more harsh shock movement than a FSAE car will see on nice smooth tarmac. seems to me that the main suspension movement will be a lot less harsh of hits than a mtb will see, and mainly from braking and cornering forces.
the car will weigh ~500lbs/4=125 pounds per corner. so the air shock will be running at a lot less than maximum psi, and under less harsh conditions than normal, so durability is *probably* not going to be an issue.
so I disagree that the vehicle will be putting a higher load on the shock than the typical mountain bike. the coil vs air battle more seems to be an issue of weight, tunability, and cyclic load affecting spring rate as you mentioned. but for an autocross of like 30 seconds, I would doubt that excessive heat is going to come into play. now for something like the baja cars, coil suspension is going to be far superior.
93forestpearl, what are the autocross times, and what is your opinion of the suspension hits the car will take? what is the weight bias of the car?
BAC5.2, what kind of weight savings could actually be seen by using air shocks over coils? I dont really keep track of the weight of air shocks. both my bikes have progressive 5th elements, hell Im not even sure what they weigh.
Yea, the car only puts 125lbs on each corner, but what about weight shift and transfer? What happens to that 125lbs when you get full on the brakes, trying to haul a 500lb car to a stop?
A coilover will be a better option, and well worth the weight penalty, IMO.
My downhill bike had a leverage ratio of about 2.75:1.
The swinger will give you great flexability. You can tune for almost any circumstance, and instead of having a linear spring rate, you can have a progressive rate.
Why is that good? Lower static spring rates (lower rating on the coil). Why is THAT good? GREAT bump sensitivity. What does THAT mean when the going gets smooth? INCREDIBLE body control.
Picture this. You set everything up based on the curb weight of the vehicle. Hop into the car, and adjust preload (sag). At this point, you can push down on the car, and it becomes more difficult to compress the suspension, the further down you push. Linear spring rate. You'll hit the bumpstops eventually.
That's fine, and you can increase spring rate to stiffen up the ride and keep the car off the bumpstops.
But you increase the spring rate, and you lose sag. Losing sag is not a good thing. Why? If you are riding at full extension of the shocks, and you nail the gas, what is the front end going to do? No compliance, means the front end is going to want to lift. Sag gives you the buffer to control the lift of the front end (by using rebound control).
So fix it? Well, you could add more suspension travel, and let a softer spring deal with progression.
But why would you do that? This just expands the range to cover the problems you are experiencing. You just now have to move a further distance to get things to happen.
OR, you could have a helper. Keep your spring rate low, but utilize the volume adjustment feature of the Swinger.
Add some volume to the chamber, and your well on your way.
Now, you compress the assembly, and it becomes exponentially more difficult to compress. Helping prevent bottoming out. So now, you can keep your spring rate low, and have an exponentially stiffening shock, that still allows you to sag a proper amount, retaining the buffer to control lift and extension during cornering (helping prevent the "peeing on the apex" syndrome common among cars).
So you have that motion taken care of.
That covers preload, rebound, and volume adjustments of the swinger. You've got 3 adjustments left.
You have the pedaling threshold adjustment. That is, you can adjust initial load required to begin compressing the shock. Kind of neat, you could tune lots of little things with this, but it's likely that you'd want to leave this chamber empty and not use this function. It wouldn't really help anything, with the loads your experiencing anyways.
You have the most important adjustments next.
High and low speed compression.
NOW, you can tune for low-frequency movement (accelerating and braking forces would constitute low-speed movement), and high-frequency motion (bumps in the road, and such).
Kinda neat!
The Swinger Air only has 4 adjustments, none of which are compression related. Fox makes air shocks that have compression adjustments, but Fox rear shocks tend to be less reliable. Like I said, I've serviced a TON of FLOAT rear shocks.
There's a TON of possibility for tuning with the coilover shock, and you don't really need to worry about cyclic heat loading, like you would with an air shock.
As for Megasquirt, you could probably build the unit for around $200 or $300 or so.
And why is your motor going to taper off at 10k+? Gixxers don't make peak power until well above that.
Oh, and yes, motorcycle transmissions ARE dogboxes. No synchros, just clunk it into gear, and your good to go. Downshifting, a clutch should be used, but for upshifting just lift throttle and bang it into gear. It should be durable enough for a few races if you really beat on it. You might want to consider stock piling a few Gixxer trannies if you can. They can usually be found for cheap in the spring and summer. That's when people wreck most often.
