Curing a loose rear end

[Steve Hart]

Whilst racing my Radical Clubsport at Silverstone last weekend I suffered with poor grip at the rear, particularly around the slow Luffield complex. It was quite a handful and I couldn't get any traction until I was almost dead straight. I'm a total amateur when it comes to chassis setup, but as several observers also commented on this apparent "looseness", I'm sure I'm not imagining it!

Anyone got any suggestions on what I could try? The rear chassis (unlike the front) is fairly orthodox coil-over-mono-adjustable dampers with wishbones that only allow camber adjustment, and a simple anti-roll bar.

I'm still running with factory recommended settings, but I know other Radical racers have med significant changes from these (I just don't know what they are!). All suggestions for directions to try gratefully received.

Steve

[enzo]

This could get fairly long and involved & I've got to get to the shop shortly.

If you would, post ALL of the factory setup info, as well as motion ratios, & corner weights, & I'll try to help a bit if I can.

You are probably suffering from a classic case of being sprung way too soft, allowing the rear of the car to move up & down & roll too much. Rear ride height is probably too high & the rear is rising too much under braking - the "having to wait until the car is pointed straight" tells me that. There can be a bunch of other things contributing to this also.

If you can, try to find the site for the Formula Ford Underground (it may be listed in the Links section) and read through all of the tech postings. It's been a fabulous site, with a ton of great info for beginners.

[Gerry Richards]

Steve,

by 'loose', do you mean that you were simply experiencing drastic oversteer, and that the car felt unbalanced when cornering? The classic solution in this case is to either increase the spring rate or anti-roll bar rate at the front to promote a little understeer and therefore re-balance the car, or to soften the rear a little to decrease weight transfer onto the outer rear wheel and provide more rear-end grip. However, you need to take into account the problem highlighted by Enzo of having the rear end too soft, and as he says, lowering the rear may stop the rear end rolling due to softer springs.

Hope this is of help

[bobdrummond]

Could the differential have anything to do with it? Or are you running a spool?

[Diabando]

I am petrified of making a complete idiot of myself here....but

Sway Bar?

Perhaps the more experienced tech people can expand on this thought?

[enzo]

If I remember correctly, that car uses a Quaif, so too much lockup is definetely not a problem (too little is normal for those!). A spool would cause push, not looseness.

Too much swaybar and/or too much spring would cause the car to be loose all the time, not just on exit.

The condition he describes is something we see all the time at this level of the sport - most cars are sprung way too soft, and the shocks are usually way too stiff.

Based strictly on what he describes, and there isn't enough information there to make a really accurate analysis, the scenario goes something like this : he brakes & turns into the corner ok, but while he is braking, the rear end rises a lot, probably well over 1 inch, maybe even more than 2 inches. At this attitude, he can't even touch the throttle or the car will go sideways. As the bump settings on the shocks are most likely way too stiff (set that way in order to try to control the car as there is not much control going on with the soft springs), the car will take a long time to settle down, and he has to wait until the car is straight before punching it.

Another possibility is that the rear is way too soft in roll, and combined with way too much negative camber, is causing the car to roll off the inside edge of the outside tire. This problem, however, will usually show up as a snap oversteer at mid corner, not as a throttle sensetive issue. If he is using radials, this is also not a likely scenario, as they are not as camber sensetive as bias ply.

[Steve Hart]

Hey this is great stuff...thank you very much!

Enzo you are correct about the diff, and spookily accurate in your visualisation of the symptoms (are you sure you weren't watching?!). I did have the feeling that braking was lifting the rear a bit excessively, as you have guessed.

At the time I changed anti-roll from full-soft to full-stiff, but this made little difference to the grip on the slow corner where it was worst. I made no change to the dampers. However I have sinced established that I was running significantly more rear negative camber than the factory suggest (zero is recommended - incidentally the slicks are the same Avons which F.Vauxhall (nee Junior) used).

I talked to the factory about it - they suggested that camber could be to partly to blame but, like Gerry, suggested I stiffen the front to neutralise the oversteer. I can't help thinking though that if too much front grip was the problem I'd have noticed it more in fast corners?

Based on what I've heard, I'm going to try lowering the car overall (and maybe decrease the rake a bit), re-set the camber and explore less damping & stiffer springs at the rear.

Thanks again....I'll be back....

[enzo]

Some hints on where to go, setup-wise:

1- Find out what your motion ratios are and calculate the springs necessary to have your wheel rate match the corner weight 1:1. This is your MINIMUM starting point - as you develop the car you will find that you end up with springs that produce wheel rates that are 1.5 to 2.0 times the corner weight. 2.0 can only be reached with a lot of shock development and high motion ratios (at least .8/1 shock/wheel) - any ratios below that require real heavy springs that cause too much bending load on the shock (and the attendant friction that kills mechanical grip), and the shock moves too slowly to give good spring control and adjustability.

2- While I don't know what shocks are on the car, I can give some recommendations as to a startig valving philosophy. You will want probably no more than about 100 - 120 lbs bump force at 1.0 in/sec shock speed, with a concave slope to the knee at that speed, with a digressive slope after the knee (to about 150 lbs max at 5.0 in/sec max). It will take a lot of bleed to accomplish this ( bleed holes in the piston or bleed shims). Keep the canister pressure to about 150 psi max if possible, again to decrease frictions, but more importantly, to keep the nose pressure to a minimum ( about 70 lbs, depending on the shaft size).
On the rebound side, valve linearly, again with bleed (actually, the same holes as used for the bump bleed), with the force slope line crossing the zero force line of the graph at about 1 inch/sec. You'll see this point only if you DON'T do an averaging graph. There is a ton that can be done with modern shocks, but farting around much with them is best left until you get the car sorted. I could also be way off on this guess work, as I don't know squat about the Avons you use - they may have vastly different requirements than the Goodyears & Firestones I usually work with.

This is only a rough guess as to a starting point for the shocks. A you develop both the car and the driver, you may find it necessary to stray from this, but I doubt that it will be very far. DON'T let someon talk you into forces much greater that these - that setup philsophy is ancient history!

3- At a 1:1 wheel rate/corner weight setup, you will probably set the front ride height at about 1.375 inches, and the rear about 1/4 inch higher. At 1.5:1, set the front to 1.1 ( unless the track is real rough), again with about 1/4 inch rake. Set the bars mid way in their adjustment range ( we'll just assume that they are somewhere near correct). With this as a starting point. you'll find your neutral mechanical balance point by raising the spring rate opposite the end that is giving the problem (for curing a push problem, raise the rear spring rate; for a loosness problem, raise the front spring rate). This tuning is assuming that you are getting a good temp spread on the tires, and no excessive wear on either the inside edge or the outside. - those problems have to be fixed first before you can find the true mechanical balance.

Anyway, just some suggestions as to where to start. Good luck !

[enzo]

Just curious as to how the car is going now. Any luck ?

[Steve Hart]

Car was definitely better last race, but had different problems with horrible pad knock-off. If I fix that then I'm racing at Silverstone again next weekend so will have a better idea what effect the changes I made will have (considerably lower car and a bit stiffer at the front and softer at rear).
Will let you know how it goes
Steve

[ma]

The only really effective way to eliminate pad knockback is to switch to floating rotors - all anti-knockback springs will do is slow you down due to the extra pad drag.

[Steve Hart]

Your probably right about the springs - I'm going to try residual pressure valves (at least one of the front-running teams uses them on all their cars) and see how it goes. I've been told (by an engineer from another manufacturer so I don't assume it's right) that the knock-off is caused by the inadequate design of the uprights, which allow some play to develop in the double-row bearings - he's seen it also in another Radical and replacement bearings were the same - there is indeed some minor movement in all wheels and the've only done 5 races. Anyway, I'll be looking at that over the Winter!

[Steve Hart]

The car was definitely a little better at Silverstone with slightly harder front damper setting & softer rear - I took 0.3sec off previous best - then a strange thing - after a run through the gravel on last lap of qualifying ripped off my splitter, I raced without it and took another 0.7sec off!! All this whilst left-foot-tapping the brake pedal on approach to kill the knock-off!

It was more inclined to understeer on power but slight lift and it tucked in really nicely, more controllable - I'm thinking that this is showing that less front grip IS the way to go, just moreso - what do you reckon?

[racer10]

I must admit I am not at all familiar with the car you are running, but a thought came to mind. On your rear end assembly; do you have a bar running from the top of the rearend housing to the frame. We run these in America. These bars drastically control the amount of traction the rear tires produce. Use a solid bar, not one with a mini shock absorber, these are not good for your type of racing. We ran these solid bars on our car, and the most minute adjustment of maybe 1/4 of an inch to adjust the angle of the bar makes a difference. Increase the bar's angle to increase traction, down to decrease traction.