If I drive round a nice wide road island and gradually increase my road speed, the car feels like the rear slip angles are increasing and that if I continue, the rear is going to step out. There is no trace of understeer. I'd rather have a more neutrally balanced car where the car remains in the right attitude to the road, determined by the steering rather than the throttle. I'd rather the radius of turn increase with speed rather than the back end go out.

How do I achieve this?

Less front camber, more rear?

Starting with the easiest ---

Reduce front tyre pressure increase rear

Soften the rear springs and/or stiffen the front springs.

Fit a front anti roll bar.

Fit wider tyres to rear than front.

I'd start with the spring rates, softer on the rear, stiffer on the front, for a kick off.

(edit, post overlapped britishtrident!)

[Edited on 9/3/06 by NS Dev]

Thanks for the tips.

Reduce the front tyre pressure / Increase the rear? How does that work? I would have thought that lower pressure would increase the contact patch, increasing grip.

*Generally* it is normal for a tyre to give a higher cornering stiffness (more lateral force per degree of slip angle) at a higher pressure (within sensible bounds). That is why manufacturers alter recommended tyre pressures for different vehicle loading conditions, ie fully laden most tin tops will have a more rearwards weight distribution, and therefore the tyre pressures are recommended to increase quite a lot at the rear, and a little at the front, to try and maintain a similar(ish)handling balance.

These rules don't always work though. I use tyre test data in my work, and have seen some tyres that don't change much with pressure, and others that do the opposite to what you expect...

Hope that at least explains the theory!

Cheers,

Neil

Edited to say: I have recently been playing with spring rates on the Mojo, partly to improve the ride (was very firm to start with) but also to try and make it a bit more docile (rear end tends to dominate on the track!). I've dropped the rear rates quite a lot, have yet to try it in anger on a track but feels a bit more friendly on the road... See HERE if you want more info...

[Edited on 9/3/06 by nre]

[Edited on 9/3/06 by nre]

Yeah altering the tyre pressures worked better with crossplies but still good for the first port of call.

OK. A good first attempt then is tyre pressures. I'd just guessed at 18 psi all round (cold). The effect is not bad and I don't get the back end coming round. If I can improve the feel of the car then I'll be happy.

What should I do first, increase the pressure in the rears, decrease the fronts or a little of both?

[Edited on 9-3-2006 by smart51]

Tyre pressures may well alter the balance but if your pressures are correct then you are doing this by sacrificing grip, which is not the way to do it.

You should always try to increase the grip on the end that is not working.

There is generally an optimum tyre pressure. Basically if you are too hard the centre of the tyre touches the road, if you are too low then the outdside edges do the work (at extremes of pressure). Therefore you should be somewhere in the middle. The average of the outer temperatures should equal the middle.

Next spring rates. basically you want the softest springs that keep the car off the road and the suspension off the bupstops on anything but te largest bump.

Then balance the car with a front ARB.

In your case my first fix would be to soften the rear springs, unless of course it is hitting the road or bump stops.

Always try to increase grip tobalance a car and not kill the grip.

John

That makes sense. There is no recommended tyre pressure for sucha light car and so I just took advice from here. The springs are the ones supplied by MNR for the kit. For now I'll assume they chose the right ones. They were labled front and rear so I know I haven't got them the wrong way round. I'll adjust the rear pressure and see what effect that has.

I found an infrared thermometer good for optimising tyre pressure. Someone on here told me about it. Sorry can't remember who, but it seemed like good advice.

I went for uniform temp across the width of the tyre after a run. Sadly my i.r. thermometer got nicked (with my rucksac) from a works pool car on an overnight site visit in Hackney 18 months ago. Otherwise I'd have lent it to you, honest!

Maplin still have them in their online shop I think.

Maplin have infra red thermomoters on special at 29 quid but the old fashioned thermistor probe type work fine also - but you have to be quick to take the measurements.

For a starting point I did a long run down the A1 then pulled into a layby or service station and as quick as possible jumped out and measured the temps.

I started hard then let a bit out until I had uniform temperatures.

John

I measured the temperature of the tyres after a 5 mile run, just to get a feel for the starting point of my tests. The tyres were 18 psi cold and 19 psi warm. The temperature of three of the tyres was 2°C warmer in the centre than the edges and on the other was 4°C warmer on the inside edge than the outside edge.

What does this mean and do I need more pressure or less?

If it is warmer in the centre than the edges this indicates too much pressure. however 2 deg is pretty close to being OK.

If it is warmer on the inside edge than the outside edge this indicates negative camber (assuming all the wheels are aligned correctly). This is quite normal and is why you should take the average of the outside edges when comparing to the middles.

Just one tyre different from the rest indicates something odd with the set up.

John

the one tyre that is different was actually at 17 psi cold and 18 psi warm. Rather than inflate it, I just wanted to see what difference it would make incase I had to let a bit of pressure out.

I reduced the pressure to 17 psi all round and did the same trip as yesterday. nice even gradient of temperatures accross all 4 tryes. the fronts bulge a bit more at the bottom than before. As for the roundabout test, I'll leave that to a dry day.

The roads were dry again today. That 1 psi drop in tyre pressure changed the balance so that the front tyres started to slip first. Amazing what a small change does. the car felt slightly less settled so more experiments needed. I should be able to get it sorted nicely. Thanks all.

I run 18psi at the front, 20psi left rear and 22right rear. Sounds odd, but if the right rear is any less, I get the left hand front locking up prematurely.

sounds like you need the tweak the right rear spring platform up a touch Mark.

I have the rear left and right platforms set the same, but I weigh 17st just out of the shower!

Is it better to have the platforms asymetrical or the tyre pressures?

Really got to tweak the corner weight to suit the cars normal loaded condition ie. with the driver onboard

Weigh each corner put it into a spread sheet to work out (1) the total, (2) front total (3) rear total, (4) left total, (5) right total from that then work out the correct weight for each cornerbased on precentages.