Monoshock wheel rates and roll stiffness 1

[tristancliffe]

Hi

I am going to be racing a Dallara F3 car next year with a monoshock front suspension, using a belleville washer stack to supply some roll stiffness. I am hoping to calculate various things on the car to determine starting points for stack stiffnesses and spring rates etc.

1. Because the wheels are not independent, how does one go about calculating wheel rates for each corner? Do you assume the whole car is in pure bump, and simply use the motion ratio squared as per twin shock calculations?

2. The monoshock arrangement means that it isn't really anti-roll at all, but more like pro-roll with a stiffness... Because as one pushrod comes up (over a bump or through a corner) the other one is pushed down... So presumably the normal way of working out roll stiffness no longer applies?

Once I've figured out how to calculate both the above, I can combine them with calculations for the rear (twin shock with conventional torsion anti-roll bar) to get 'answers' like TLLTD, wheel frequency distributions and all of that (which of course aren't actually answers, merely suggestions that can be easily influenced to tell you what you want to be told... but that's not the point!)

Of course, the real world is complicated by the addition of preload and/or lock nuts on the belleville washers, preload and anti-droop on the coil (which I don't really understand the benefit of, but I guess it must be aero related), and of course damping. So much to learn when we start testing, but it would be nice to get an idea of how it's all going to work before it's close to being ready for the track.

I'd really appreciate your time to advise me if you can spare it.

Thanks, Tristan

 

[rrrocket]

It is a long time since I last looked at the front end of a Dallara, so could you refresh my memory. As I recall, the rocker has two pushrod pick-ups, the rocker shaft is orientated across the car and the rocker is free to move axially along the shaft, but constrained by a stack of disc springs either end. There is a single spring/damper mounted off the rocker to provide double wheel bump stiffness, but the disc spring stacks provide the roll stiffness. Is this the current arrangement?
I always thought that a lack of roll damping was a deficiency of the system.

 

[tristancliffe]

That is correct. The lack of roll damping at the front is probably a bit of an issue, but perhaps the friction in the disc spring stack helps a bit. Also, the car is really quite stiff, and I would imagine that the rear dampers do quite a bit of the roll damping as the suspension warps.

But for calculating wheel rate and roll stiffness I'm at a bit of a loss, regardless of the pros and cons of the system.

 

[Yawing]

So its a ride spring/damper coupled with a roll spring by bellville washers?

If so I would say in terms of TLLTD you neglect the ride spring and calculate the roll spring in much the same way you calculate an ARB for roll stiffness. if you split the anti roll system into 2 sides and estimate the wheel rate if the system was fixed in the centre (like an ARB welded in the middle to the car, as we are assuming symmetric roll, ie 0 displacement at centreline) then as you would a normal spring wheel rate using the track, work out the torque per degree roll.

for ride frequency it depends. If you are talking pure heave frequency then of course the washer system ca be ignored, but if you are talking individual wheel bump then you have to take into account the wheel rate addition of 1/2 the estimated wheel rate of the washer system above (as you assume the other wheel stays where it is so the system doesnt have 0 deflection in the vehicle C/L

 

[tristancliffe]

Yes.

Would that TLLTD method be valid, because as one wheel goes into bump the other is forced into droop, although the amount is reduced with the belleville washers. It's not an anti-roll system as such? But if you think it would be valid, at least for simple static calculations, then I'll go with that.

As for pure heave, again is that a valid way of calculating ride frequency in simple, static terms? As one spring is shared for both wheels, does one use half (or twice) the spring rate in the calculations?

Thanks for your help so far. I'm attempting to upload a picture of the monoshock arrangement, but it's not a very good picture (just what I have on my computer at the moment).

Tristan

 

[murpia]

The situation here may be confused by non-linear roll and / or heave rates. Fix the heave position using an adjustable length link (a tie rod with two rod-ends works well) instead of the monoshock. Measure the roll rate as Yawing describes. Repeat at different, fixed, front axle heave positions.

You should then have a table of TLLTD values vs. heave position. Then, from track data, you can match TLLTD with speed as the front axle heave position changes due to downforce.

Depending on how helpful the Dallara designers have been in terms of their motion ratios, you can reduce low-speed understeer due to TLLTD but maintain high-speed stability as the TLLTD migrates forward with speed.

Regards, Ian