Zetec suspension suggestions

[Steve W]

While at Brooklands on Saturday I managed to break the Di Dion tube on my car. The car is 91 supersprint with adjustable dampers and spring platforms of some type. The car now has a Zetec fitted of aprox 200 BHP. I'm not happy with the handling too much understeer and since the Di Dion tube is going to be replaced it would be a good time to sort it all out. I use the car almost entirely on the road but with a couple of track days a year. Can anyone offer advice on springs, damper roll bar recommendation? Also should I get an LSD fitted?

 

 

[StewartG]

I would suggest stiffer front springs 240lbs ish a 9/16 red bush front anti roll bar and you'll need adjustable platforms all round and a 9/16 rear anti roll bar. Set the front ride height measured at the back of the front wheel between the chassis rail and the ground to 130mm and the rear ride height measured at the front of the back wheel to 155mm. Fine tune handling with rear ride height and rear anti roll bar settings, stiffer anti roll bar(drop-links further forward means more oversteer. Lower rear ride height means more rear grip and less oversteer. When you've changed the ride height take the car for a quick spin to settle the suspension before re-measuring. An LSD is nice but costly and get some decent tyres AO32Rs.PS I'm no guru this is all standard text book stuff but it works.

 

Edited by - stewartg on 4 Sep 2000 19:52:44

[Steve W]

Well you sound like an expert to me. Chris Wheeler at the Seven workshop will be doing the work and I will pass on your advice. You mention the springs how about the dampers. Should I just leave what's on there ? I've ordered a set of 13'' wheels from the site. 6'' front 7'' rear. I was going to go for 185/60 front and 205/60 rear on 32R's. How does that sound with your suggested setup ?

 

 

[StewartG]

Chris wheeler also looks after my car and when they arrive I'll be on the same wheels and tyres. I'm told the existing Bilsteins whilst not perfect are not too bad and I've no experience of alternatives.

[Peter Carmichael]

This all looks like good advice to get you into the ballpark. The problem with the dampers that you already have is that you don't really know what spec they are because you (and I) don't have access to a damper dynamometer.

 

The affects of roll stiffness on oversteer tend to show up at high speed. The ride height effects dominate at low speed.

[Steve W]

Thanks again for your advice Peter. I think if the budget will reach I will change the dampers aswell. As you said I'm not sure what they are.

Looks like I've found a friend willing to sell a spare LSD.

Are caterham the best place for new springs and dampers or can anyone suggest a better or cheaper supplier.

 

 

[StewartG]

See you at the Wareside pub meet tonight maybe, Chris is usually there and the more beer you buy him the cheaper his prices are

[Steve W]

See you there in my aircon vectra. The Seven is running but ridding about 35mm two low at the back. At the weekend I may have a go at freeing up the spring platforms at the back and raise it up.

I think I will have to buy Chris a hell of alot of beer before this becomes cheap.

 

 

[James Rogers]

Stewart:

 

In the set up you suggested are the chassis heights taken at the wheel or the tyre. If answer is obvious please excuse me.

[Anna Rexia]

Have you thought of Gossard (of wonderbra fame) or Berlei, or even Playtex if you're really worried about your spare tyre!

[StewartG]

It's the distance between the ground and the underneath of the chassis rail measured level with the back of the front wheel or level with the front of the back wheel. The good lord seems to normally send rain on the first wednesday of the month in the evening of course today he didn't which was why circumstances conspired to make me work late.

[Steve W]

Thanks for thr advice Anna. In fact I'm so worried about my spare tyre I'm going to have it removed ????

[StewartG]

Have you got spring compressors to take the springs off the shocks?

[Steve W]

Well I don't but I'm sure that Chris does. If the dampers all look OK do you think I should just replace the springs ? Currently I don't have a rear anti roll bar is that a must. It's a lot of cash but is the wide track front worth the money.

Sorry for all these questions but as you can tell I'm no expert on this.

 

 

[StewartG]

Don't replace the dampers they're probably essentially the same as those on an R500 yes an arb is essential and no the wide track kit is probably not worth it.

[Alex Wong1697456877]

Opinions vary on the rear ARB. On my car, if I move it to the softest position, it definitely understeers more so it would appear to be beneficial, yet handling experts like Juno say that it's detremental and that a well setup car with the right springs and dampers without an ARB is better.

 

In other words, I don't know!

 

The widetrack seems to improve high speed stability at the expense of lower speed agility. I've yet to hear anybody claim a great improvement on changing to it.

 

 

Alex Wong

alex.wong@lotus7club.co.uk

www.alexwong.net

Home : 44-(0)121-440 6972

Fax : 44-(0)121-440 4601

 

Edited by - Alex Wong on 9 Sep 2000 00:45:27

[StewartG]

Alex does your softer arb setting give you more understeer or less oversteer. Intuitively it seems that grip is better than balancing front slip with back slip and presumably all this slip must wear out tyres. I wonder when a tyre is at its grippiest, schoolboy physics vaguely reminds me that once two surfaces start to slip the co-efficient of friction goes down rapidly (having said that a bit of slipping still feels good). On the wide track comment I expect that most people fitting this use the same springs and dampers so of course the front will feel less agile as the longer wishbones have more leverage to deflect the springs and the mass of the wheels etc is harder for the dampers to control. Once again intuition would suggest an improvement with widetrack but only with careful optimisation. P.S. The bit in brackets is the tongue-in-cheek bit.

 

Edited by - stewartg on 10 Sep 2000 07:29:19

[Alex Wong1697456877]

Softening the rear arb causes more understeer and it's easily noticeable at high speed. At low speed, it just rolls more easily!

 

I think the key difference here is between slip angles and slide. No technical backing for this at all but I thought slip was caused by the rubber not losing friction but grabbing increasing amounts of tarmac laterally due to the cornering forces without breaking free. Slide on the other hand is what happens when you pass the point of maximum traction. I would have thought the fastest you can go is just before the point one becomes the other.

 

Peter, do you know what I'm on about? Can you explain it more clearly (or put me out of my missery by telling me I'm talking a load of geriatric shoemakers!)

 

 

 

Alex Wong

alex.wong@lotus7club.co.uk

www.alexwong.net

Home : 44-(0)121-440 6972

Fax : 44-(0)121-440 4601

[Steve W]

If it's not too much effort (hense cost) I think I will go for the rear anti rool bar and change the front arb. The dampers all look Ok. After a couple of hours fiddling and cleaning off 9 years of dirt I've managed to adjust the rear spring platforms.

So plan A: check rear springs, fit 240 lbs front springs, front and rear 9/16 arb. Oh and a new steering rack and hopefully an LSD. Fit new 13'' wheels with 32r's and adjust ride height.

Any comments and thanks for all the advice.

 

 

[Peter Carmichael]

>>>Alex does your softer arb setting give you more understeer or less oversteer. Intuitively it seems that grip is better than balancing front slip with back slip and presumably all this slip must wear out tyres.

 

This is a huge subject, which conjures misunderstanding like few others. I will try and put a few coherent thoughts together.

[Peter Carmichael]

A quick answer to Alex. Yes I do know what you are on about and it isn't cobblers.

 

Slip angle... is difficult to visualise. tongue.gif

 

The tyre is made of a thickness of rubber tread. When rolling, a different tread section is coming into contact with the ground at every point in time. Have a go at visualising a block of tread as it travels onto the ground, entering the contact patch and leaving the other side, all the time while a sideways force (cornering) is acting on the tyre.

 

The tread block approaching the contact patch is unstressed. As it takes up the stress of the sideways load it is going to deform (shear) sideways. The force it transmits to the carcass of the tyre is related to how much it shears because the tread is (imperfectly) elastic. It is also related to how fast it is asked to shear - see what I mean about imperfect elasticity. The tread block is in fact a little spring/damper unit in its own right, generating heat to dissipate cornering energy and preventing peak localised loadings from breaking the gripping contact with the ground leading to the lower grip levels of what you have termed 'sliding'.

 

In taking up its new sheared state, the carcass of the tyre has moved relative to the tread in contact with the ground; the carcass moves to the outside of the hypothetical bend, the tread block (which is gripping the road perfectly) hasn't moved. Eeek! This means that the carcass is progressively travelling sideways compared to the rolling direction of the wheel. The sideways deformation taken over the length of the contact patch gves you a slip angle . However, this is only half the story. As the tread block passes out of the contact patch, it is going to spring back to its unstressed, undeformed state, however because of its imperfect elasticity, it does not spring back pinging across the road surface scraping itself to bits as much as it might.

 

The spring /damper effect of the hysteresis in the tread compound can be examined a bit more closely. The tread block is 'hitting' the ground vertically as well as this slip angle sideways deformation. If the tyre were perfectly elastic, the energy from this impact would spring back and give uneven loading in the contact patch with uncontrolled resonances reducing grip at certain road speeds. The other thing you can quite easily visualise is that the rubber will only deform a certain amount before it tears. If there is enough contact pressure (heavy car, small tyres), the contact patch will keep gripping up to this point of tearing, which leads to poor tyre wear and reduced grip. This is avoided by reducing tyre contact pressure with a larger tyre giving a larger contact patch. Choosing the right tyre size is related to contact patch loading and not going to such a large size that the energy per unit area is insufficient to get the tyre up to operating temperature. Tread thickness affects heat dissipation and the ability of a tread block to cope with distortions of a given size. The tread block acts like a rising rate spring, so if you ask too thin a tread depth to cope with large distortions it will behave like a suspension setup that is constantly running on its bumpstops - i.e. reduced grip.

 

Another popular misconception is related to tyre pressures when you run different size tyres. An incorrect but intuitively enticing statement is that the larger contact patch obviously needs a lower air pressure behind it to match the car's weight. This ignores some very significant facts. Air pressure alone does not support the car. The spring rate of a tyre is not related to air pressure. The effective spring rate of the air in the tyre is related to how the volume of the air is altered by displacing the contact patch; it doesn't alter very much at all, therefore the air bag itself is not much of a spring. If it was, then you would notice a significant increase in tyre pressure from lowering a car off a jack onto its tyres - you don't. Here is what is actually going on:

 

The air in the tyre loads up the carcass so that the sidewalls and tread are in tension, both radially and circumferentially. The sidewalls are elastic (not very much), but as they are loaded up with the air pressure they stretch. When the car's weight is supported by the tyres, the sidewalls in the load affected area (between the contact patch and the rim) are deformed reducing the amount of stretch and therefore reducing the amount of elastic tension. The sidewall cannot provide much support when it is in compression, but from a pre-loaded tensioned state, a reduction in tension gives much the same effect. Tyre springing comes down to the stiffness of the sidewalls. Steel is much closer to being perfectly elastic than rubber, so a radial tyre can't make use of the internal damping effects of the rubber compounds used. However, a radial can provide much better control of the shape of a wide tyre tread. i.e. crossplies (ACB10s) have better internal damping in the sidewalls which gives better grip, but as they wear they tend to balloon in the tread area, leading to accelerated wear of the centre of the tyre. In wide sizes and for long (road car) periods of service, radials can provide more consistent control of the contact pressure.

 

Tyre pressure controls pre-load of the tyre sidewalls and can affect the contact pressure distribution, particularly with crossply tyres.

[Peter Carmichael]

If you thought that last post was long you should see the handling article (book) I've got cooking...

 

There was a more specific question about anti-roll bars and understeer/oversteer which I haven't answered yet. In my last post I outlined how high peak contact patch pressure could lead to tearing of tread compound and reduced grip. When a car is cornering there is a weight transfer which means that the outside tyres could end up loaded to the point where they tear (microscopically) with reduced grip. Because the overall weight supported by the tyres is not changing, the only way to get a higher contact pressure is to borrow it from one of the other wheels. The comparative roll stiffness from front to rear makes this happen.

 

For example. If you have front suspension set up to the point that it can be considered rigid and soft rear suspension, as you apply roll force to the car, the only weight transfer will take place at the front causing, in the extreme case, high peak tyre contact pressure on the outside front tyre and the inside tyre just lifting. The contact patch of the outside front tyre will be tearing and have less grip (mu - the coefficient of friction will be lower) than two evenly loaded tyres even though it is carrying twice its orignal weight. The rear will still have an even weight distribution because the car will not have rolled having been restrained by the stiff front end. i.e. you will get understeer

 

Keeping the car's weight evenly spread over all four tyres gives the maximum grip. Anti-rollbars allow you to tune one end compared to the other. A soft, evenly-balanced car will (almost) have the same grip as a stiff, evenly-balanced car. Stiffness of itself does not reduce the grip that the car has on the road.

 

 

[Peter Carmichael]

Widetrack causes reduced weight transfer and therefore better grip. i.e. a smaller weight difference side to side, spread further apart is able to counter the same rolling moment. The rolling moment is dependent on the height of the centre of gravity and the cornering load.

 

There is a rule of thumb which suggests that if you can reduce the centre of gravity by an inch it is worth five inches in extra track, but the specifics probably involve starting with a typical road car rather than a Seven.

 

The Seven's handling trick is simply down to its low centre of gravity. The Seven design compromises on ground clearance for the sake of handling, so we cannot really expect normal road cars to come close. That something like a 911 GT3 comes close shows you where the development budget has gone...

 

[millsn]

Ref: Zetec suspension - I have a xflow at the moment and am going zetec.Point is that I've done loads of changes on the car ref dampers. So far, my best advice is don't change from standard. However, ther is no defintion of standard. My car ran '92 spec dampers which Caterham shouldn't do any more. Again, however, if you go for the new versions there are two pitfalls to avoid. The easy one is to avoid the widetrakc front dampers which are longer in the body. Caterham regularly send people the wrong (longer) ones even if you give them all the right info. Next is that the new dampers that should fit have a wider top link so won't fit. The solution is to send your dampers to Prodrive (Catreham can suply the number) and get them revalved. The spec you have is nothing like the valving they use these days so a change is worthwhile. I went to Leda dampers and am still trying to finalise the valving so avoid this route unless you really, really like playing.

 

Nigel Mills - xflow

[Steve W]

Well the cars all back now.

Suspension changes are great. Along with the 13'' rims and A032r's the car feals like new. Much more in control and neutral and the LSD puts the power down much better. Roger King has done a magic tune on the engine and it's running great.

Now all I need is some good weather ?