The Trials of Trials: Spring deals

March 8th 2013

On the last event we had some problems I described in the previous blog. One was the extra weight of Ian compared to previous passenger, Jackie, that the car had been set up for. I got the 125 lb/inch springs from Bryan that he had dug out. They are new and unused. He had actually fitted 100lb/inch springs to the car and found them ideal for the crew weight they were running.

Shot of the passenger side coil/shocker

I fitted the 125lb/inch spring to Ian's side but have left the 100 lb/inch spring on my side. I weigh about the same as Bryan and so reckon that as Ian stays mainly on his side (and when he leans to my side it's because that is the uphill, and lighter side) it's his side that needs the extra resistance to stop the coil/shocker bottoming on the bump rubber.
I measured the clearance on the push rod; at rest; with me in the car; and my neighbour (who weighs 18 stone, like Ian) in the car.
The compression was 1" with me in the car (12 stone) and 1.5" with my neighbour. Which sounds about right. But it only left 1.25" before bottoming. This was at rest, so under trial conditions the compression when hitting a bump or at the bottom of a hollow would easily have the suspension locking up. That leaves only the tyres giving any suspension and this will have an effect of grip. Probably reducing it, and possibly having an unwanted steering effect

With the new spring in place the "at rest" measurement was increased by an inch. With me in and my neighbour these went down by 0.8" inch (1 inch previously) and then 1.2" (1.5" previously) so the stiffness seems to have gone up by 20% (or is it 25%? my maths doesn't stretch to this).

I also raised the spring platform by 1/5th of an inch to allow a little more in hand and pre-load the spring a touch. Whilst I was at it I wound a bit of resistance into the shocker setting. They are Spax adjustables, but presently with the adjusters backed-off.
It looks like this may be a move too far as this side of the car now feels substantially harder. However I can unwind the adjuster easily from under the car.
We need to test this on my friend's fellside farm before committing it to the heat of competition.

I have also had problems in getting the car to turn sharp uphill corners which I have mentioned previously. The front end just doesn't bite and the car slithers straight on. Part of this, as many helpful voices have suggested, is due to me not using my fiddle brakes enough.
However there are moments when you want the front end to start to make the turn before you want to be piling on the power and these are the moments that catch me out.
Bryan (the previous owner) mentioned that he had found it understeered a lot and he thought it lacked any Ackerman angle effect on lock.
I knew what this was as it's something I've been involved in before. It's the mechanism to get the inside wheel to turn a smaller radius circle than the outside wheel.

Measuring it back at base I found that there was no toe-out (that would indicate the ackerman effect) on lock. Researching this it appeared that the steering arms are not quite right in this installation and would need bending inwards. There is a reason for this, but it needs diagrams etc to show and I can't be arsed.
It also appeared that moving the steering rack rearwards can increase the rate at which the ackerman angle is applied as you put on lock.

I didn't fancy bending my only pair of steering arms and possibly knackering them (heat required) so decided to try moving the steering rack backwards first, as a trial.

Excellent. I’ve now moved the steering rack back by 2” and this has resulted in some measurable Ackerman angle. On lock the rims now have toe-out amounting to 1” (rim to rim) on full lock. But neutral steer in the straight ahead position.
Actually measuring the rims back and front demonstrated about an inch difference. As the rims are 17” it can be expressed as a percentage, but I’ll leave it to those who understand geometry to work that out.!

(Yes the string really is in contact with the rim. It looks strange because of the offset in the angle I have taken the shot from. The string is also higher than the hub, as that sticks out and makes touching both the rims impossible)

So, you can see from the pics that the inside wheel is turned more sharply than the outside one. It works the same on the opposite lock as well. This fits in with the information I have found on the ‘net about increasing the Ackerman angle.
All I have done at the moment is to mount the rack on the rearmost hole on the axle bracket using the front leg of the U-bolt.

Obviously I need to make some substantial pads which bolt to the existing brackets and provide a further hole further back to take the other leg of the U-bolt.
I also need to make the pad thick enough to lift the rack a bit as the radius arms are now very close to the rack casting and may contact it on full suspension droop. Something I have yet to measure.
The steering column needs shortening by the 2" I have moved the rack.

The tie rods now approach the steering arms at right angles to the car centre-line when in the straight-ahead position.. Before they sloped backwards. On lock these angles change and it's that that creates the ackerman angle.

Before

Now they lead to the steering arm at 90° to the car centreline. This is the crux of the advice on moving the steering rack.

After

Some people have suggested altering the castor (caster?) angle. I thought that not possible but it looks as though that could be achieved by adjusting the A_frame and radius-rod rose joints. This would improve self-centring. As the car is used at low speeds on non-sealed surfaces I can't see this is a particular advantage. I am not going to start messing with that until this initial change to the rack has been properly installed and tested on an event. If at all.
All I’m looking for is an improvement in turn-in on loose surfaces.

Looking at the before and after from above it may seem that I have achieved the toe out at the expense of ultimate lock. The outer wheel now doesn't follow such a tight radius as the inside wheel. However as the outer wheel will now actually steer the car instead of ploughing straight on, that is only a theoretical loss.

Before