supertwin21suspension

Super Twin21 Baja

I want the suspension on this vehicle to be mind blowing. Real Class-1 desert racers have upwards of 2 feet of wheel travel all round, and often have 3 shock units per wheel. If my car is going to be about 1:6 scale, I need about 4" of travel. Working through some dimensions by eye and using CAD, I have come up with suspension arms 225mm long, using some industrial springs 205mm long unloaded. I can get over 4" travel out of this if necessary. The arms will be machined 1/4" alu, or possibly pocketed-out 10mm alu if I can get hold of the material and a milling machine which can handle work of that size. One of the big differences between a real desert buggy and the 1:10 and 1:8 models is that their suspension arms hang down a long way from horizontal, giving extra up-travel. On an RC model you will most likely see the arms pretty much level with the horizontal. My car is not going to be of the 'slammed' generation. It will be jacked up high, a bit of body roll is not going to kill anyone. With the laid down engines it should not roll too badly.

The springs will run over an air damper, which will have a Teflon piston in it. The air dampers' pistons have one-way valves which double the amount of damping they provide on the up-stroke. This will help to absorb big hits. There will also be separate oil damper units which will run next to (rather than the conventional inside) the springs. The oil dampers have less travel than the springs and air dampers so will be angled-in to give little damping at the down-travel limit, but much more when the suspension compresses. If I get it to work out I could upgrade these to long-travel magnetodynamic dampers in the future. With the proper springs and dampers, it will not be necessary to have more than one of each per wheel. The recent crop of fast monster trucks with 8 shocks only have them, in my opinion, because they use shocks which are designed for smaller, lighter work.

Below is a graphical simulation of the damper performance of this twin-damper system. It is the result of a numerical calculation taking into account shock geometry for both damper units, with separate damping coefficients. This model uses coefficients which express damper force as linear with damper compression speed. Maybe a square relationship would be slightly more accurate. The calculations would then be quite slow on my computer though...

Why build it?

Chassis

Suspension

SUSPENSION

UPDATE!

I have got my new milling machine, a Chester Conquest. Well known worldwide under many different brand names. I have made some parts now, with the main difference being that I have changed the air damper to oil, because I found it gave very little damping! So I will be losing the smaller Hyper-7 shocks and running a more conventional looking setup. The damper graph looks similar with this new setup, but I have more damping all round. I have kept the 1-way valve concept in the shock.

The new shock unit attached to the wishbone. Almost 30 hours of machine time sitting there... and only another 3 of these left to do *gulp*. Outboard hinge pin is precision ground 3mm tungsten carbide shaft held with M3 grub screw.

A close-up of the seal assembly. It is a double o-ring type, held together with those 2 M2 screws, which also allow you do preload the o-rings to the required amount to generate max sealing with the least friction. That is a silver steel piston rod, and the nicely plated spring is from Ondrives.com.

The wishbone is 183mm long, about 7". This is the first one which took about 15 hours to machine. I was getting used to my new machine and the first one is bound to take longer, I expect the next one to take about half as long. I have blocked the unused shock mount locations with button-head screws to stop them corroding up or getting filled with cow shite. This has been a sore point with me before...

This rather stupid looking picture of me serves as a comparison of the sizes of (L) the super-twin shock and (R) a shock from the Hyper-7 1:8 buggy. The Hyper shocks are brilliant, just not quite big enough for this application!

UPDATE - 28/11/04

Front suspension is now fully articulating on both sides, have not made any more shocks yet though. I have done a lot of work on the rear end, and it is at pretty much the same stage as the front now. I have the rear shock tower made, which is a big lump. It's turned out pretty nice, because it is huge, and very stiff, but also very light. Here's a pic before I bolted it down to the chassis. Rear wishbone in the background.