Oversteering 7

[Robster]

I recently had a great day at Kemble in my live axle, 1800 Xflow. I know that they tend to be a twitchy at the back end (I have a LSD fitted)..but am not convinced that this is the true story. I have fitted pre '96 De Dion springs (stiffer) but am on the original Spax dampers, 3 years old now, wound up at the back quite stiff.

 

When cornering hard, the car grips well and can be held nicely on the throttle to hold a 'drift' through the bend, but when taking off the lock, it feels as if the cars weight swings to load up the opposite wheel. I mean that if I turn hard right, come out of the bend, straighten up the wheel, the car's reaction is to oversteer right during the bend, then left on the exit(as the load comes off the left hand wheel in the bend and seems to bounce to the opposite). Is this due to the dampers rebound not coping with the spring rate??? Is it due to no ARB being fitted to these cars....?? I suspect it is inadequate dampers, but would appreciate some advice. The oversteer is fairly sudden as the lock comes off, but is catchable, although at high speed a bit nerve wracking!

[ashaughnessy]

Well, I don't know what causes it but I'm pretty sure it shouldn't be doing this. My supersprint live axle doesn't behave like this at all. I presume we're talking about dry weather behaviour? What kind of drifting/sliding are we talking about? Are we talking about a moderate drift, or are you looking through the side windows to see ahead?

To check the basics, what tyre pressures are you using (measured hot), and what tyres? How many clicks do you have the dampers set to at front and back?

 

If you get into a very leery slide and you have to put on an extreme amount of opposite lock to hold it, I think the car would snap back violently if you weren't careful with it, especially if you lift off suddenly. But the way you describe it sounds like it happens all the time.

Anthony

[philwaters]

I think stiff springs on the rear will make a Live axle skip around a lot. Can you fit De Dion springs to a live?? I thought they were very high rates (170lb) and not suitable for a live car. I run 115lb rears and think they are about as high as I want to go. If I adjust the dampers to too 'hard' setting then the back does skip about and is very prone to oversteer and is generally not very confidence inspiring.

I suggest you check that the dampers are not set to high in terms of damping, I run my Spax at around 2-4 clicks I think and more like 4-6 on the front and find this works well.

 

Phil Waters

"Darling, DO you love the 7 more than me?"

"Driving OR Fixing?" wink.gif

[Dave H]

It sounds like you may have the backend set up to firmly. I have the standard Spax, set on the softest, and do not experience the problem you have. My springs are fairly soft as well - original '86 issue, not sure what they are.

I'd also check tyre pressures and the state of the bushes on the back axle, they all have an effect.

 

Dave H

[cdg]

After competing in the Kit Car classes in speed events for several years, I have seen this problem many times. It is virtually always due to excessively hard dampers at the rear, sometimes in conjunction with too soft a spring/damper set-up at the front. The car gets pitched into "roll oversteer" and demonstrates exactly the traits that you have mentioned. Without wishing to sound patronising, it is usually amongst drivers new to competition, and is very quickly sorted out.

 

Try your car fully soft at the rear (or first click max.) and say five clicks on the front, although obviously this may depend on your front spring rate. I think you'll find your car's handling is transformed. If you go too stiff on the front you'll find it glued to the road on fast drifting bends, but will understeer like an 1800 Morris Marina through tight corners, losing loads of time.

 

As a general rule, with a rear drive live-axled car like a "7" the car's handling is dictated by the spring/damper set-up at the front.

 

Chris.

[Robster]

Thanks for the advice. Yes, the rear dampers are set fairly hard, the front ones being softened until the car stopped understeering too badly. I fitted the pre 96 de dion springs on the advice of Redline, as the original classic springs are way too soft, meaning ride height is very low on A21 tyres, and body roll too bad (also , the springs are too long if you look at the number of coils compressed into the damper unit). The new springs are not too hard as the ride is better.

 

I will try to soften the dampers all round as I am sure that this will help as suggested. They were set harder to reduce body roll in the longer corners.

[philwaters]

Sounds like a good plan to me.

 

My car rolls slightly at the back due to the soft dampening, but the front stays very stiff. I like this set up as it allows the live axle to ride the bumps whilst the stiff front gives very quick turn in response. I am running 310lb front springs. However, changing springs will normally require adjustable playforms to get the ride height back - as you say.

 

Hope sofening the dampers solves your problem.

 

Phil Waters

"Darling, DO you love the 7 more than me?"

"Driving OR Fixing?" wink.gif

[millsn]

Just check that you've got 35mm or so of rebound travel and lots of bump available. If you run out of rebound (droop) you might get over steer problems as a result. (One wheel lifts)

 

Nigel Mills - 2.0 Zetec carbs

[Peter Carmichael]

Eek!

 

Dampers can't control roll in long corners.

 

Dampers can control onset of roll , but they won't resist a steady roll moment. That's what springs (and ARBs) are for - attempting to stiffen up the dampers to that degree will end up with bizarre handling.

 

I agree with the expert advice you seem to have been given above. A couple of things you mention are misconceptions which it is worth getting some clarity on, the above being one of them.

 

Is this due to the dampers rebound not coping with the spring rate???

 

At the onset of roll, one side of the car is compressing and the other extending, so one sides pair of dampers are using their bump valving and the other side their rebound damping. The opposite happens when you straighten up after the corner. If you are massively overdamped at the rear, it is going to slow the rear suspension taking up a level position on the exit of the bend. The rear axle has appropriate roll steer built into it to keep the car stable in the bend, but when it is still roll-steering and the car is unloaded coming out of the corner it is going to give an inappropriate steer in the wrong direction as you have discovered.

 

Springs store energy. Dampers dissipate energy. Together they form a filtering system that spreads out peak loadings and keeps the car in contact with the ground. The tyres are also a filter system, taking out the highest buzz frequencies.

 

The car's suspension movement can be described by four modes which will be familiar:

- Pitch (braking/acceleration - front pair and rear pair of spring/dampers operate in opposite sense)

- Roll (cornering - left side and right side spring/dampers operate in opposite sense)

- Heave (high speed undulations; jumps! - all four springs and dampers operate in the same sense)

- Warp (bumps)

 

The first three of these keeps all the wheel contact patches in a flat plane. Warp is the only mode where there is no flat plane intersecting all the contact patches.

 

All of these modes have a characteristic frequency that could be considered to be ideal. Heave is slow (about 1.2hz for a sporty car), Roll and Pitch are medium (let's you move on/off the brakes/accelerator or change direction quickly), Warp is fast (you want the suspension to soak up the irregularity and be ready for the next hit). You are trying to optimise all of these settings from just four spring/damper unit, one at each corner.

 

If you start with appropriate springing for a slow Heave response, you will have too slow a Roll response. This is where anti-roll bars come in. They stiffen up the roll, without affecting Heave or Pitch. They also stiffen up the Warp mode, which gives a poor ride quality and some skittishness over bumps. On a perfectly smooth surface, a stiff-in-roll car will have better grip than a soft-in-roll car just because it will maintain its contact patches better on the road surface, so all this stuff about stiff cars having less grip is nonsense. Generally ARBs are used to trade some grip from the front to the rear or vice versa until balance is achieved, but the overall grip level doesn't change that much.

 

The filter characteristics of each mode are determined by the spring rate (in that mode), the damping rate (in that mode) and the controlled mass/moment of inertia (in that mode).

 

Why moment of inertia? Well in the pitch and roll modes, the masses of the car are rotating and it is that movement that is being controlled. If you can concentrate the masses in the centre of the car, it is easier to achieve a high pitch frequency while maintaining a low heave frequency. This is probably why the Blackbird-engined Caterham is so phenomenal and kart-like in braking. The absolute weight difference compared to a k-series Superlight is small, but it is arranged more in the centre of the car. The same goes for Roll, but most of the masses are already arranged along the centre of the car so there is little opportunity for improvement.

 

Dampers have high and low speed characteristics in both bump and rebound. The slow speed damper force is approximately proportional to speed, but dampers open up extra valving at some point in the speed range, so that a plot of damper force against speed has a characteristic elbow. The position of this elbow and the steepness of the plot before and after the elbow are all characteristics of a damper. You have independent valving for bump and rebound, so all in all there are six characteristics of concern which you cannot twiddle independently with a single knob on the outside of the damper unit. The dampers may also behave significantly differently at temperature compared to cold and there is always the bugbear of friction. A damper is a complicated device and selecting them appropriately is a black art.

 

The starting point is a good dynamic model of the vehicle. For this, it is not enough to know the mass of the car. You need to know the mass distribution. You need to know the suspension geometry in order to understand the orientation of the roll and pitch axes. You also need to have a good idea of the compromise solution you are aiming for - is the car for track use or for the road. Driving style comes into the fine adjustment at the end, but the basics need to be right from a fundamentally technical basis.

 

With four spring/damper units, one at each corner, and with anti-roll bars front and rear you cannot get ideal suspension behaviour. The design compromise is for the ride quality to suffer.

 

With adjustable dampers, the right setting for Heave may be the wrong setting for Roll. Some experimentation is necessary, but for every click you head away from ideally damped Heave, you are compromising the chassis' all round behaviour.

 

The easiest way to cope with adjustable dampers (as has been advised) is to start with full soft and progressively stiffen up a click at a time until you eliminate the floaty underdamped sensations. Any clicks beyond that head you into territory where the subjective sensations are difficult to pin to objective modes of behaviour in the chassis. Suspension is supposed to move - if it doesn't, you are too stiff.

 

Bit of a non-specific waffle, but I hope it is useful background info.

[ashaughnessy]

Peter, interesting stuff. You said:

"On a perfectly smooth surface, a stiff-in-roll car will have better grip than a soft-in-roll car just because it will maintain its contact patches better on the road surface, so all this stuff about stiff cars having less grip is nonsense."

 

I wouldn't like to disagree with this (I have no idea one way or the other) but why is it common to soften the anti roll bars and soften the dampers when the track is wet? Thinking about it, and about all the other stuff you said, I reckon the answer is very complex, but it would be interesting to hear it.

 

Anthony

[Eugene]

A little point, but one of the best handling cars was the Lotus Elan, and it had no front anti-roll bar. I have fitted the weekest one possible to my live axle Seven, in conjunction with much stiffer springs and run soft damper settings. The cars handling was transformed as can be attested to by Roger King and Chris Wheeler at The 7 Workshop. More info can be found at http://freespace.virgin.net/shaw.clan. Follow the links to the 'archive pages'.

 

Small Boy - with Big Toy!

See Eugene here

[ashaughnessy]

Eugene, I wouldn't disagree with what you've said, but if you search through old postings on this site you can find any or all of the following:

"I fitted larger diameter wheels and it transformed the handling"

"I fitted smaller diameter wheels and it transformed the handling"

"I fitted a larger ARB and it transformed the handling"

"I fitted a smaller ARB and it transformed the handling"

"I fitted stiffened springs and it transformed the handling"

"I fitted softer springs and it transformed the handling"

etc...

The point being perhaps that you can't take a single component in isolation, you have to look at it in conjunction with all the other parts of the car and also the driver's preference and style of driving.

My own experience was that on a test day at Oulton park, I transformed my car's handling by gradually stiffening the dampers front and rear until I had 7 clicks rear and 5 clicks front (or possibly the other way round). This doesn't mean everyone should run these settings, just that they worked well with whatever springs, tyres, ARB, etc. I had on my car. On the road I put them back to the softest setting because otherwise it jumps around too much for comfort.

Anthony

[Eugene]

Anthony, I agree. Thats why i said it was a little point. Read my full report on my website to see what I went through to get to where I am today...

 

Small Boy - with Big Toy!

See Eugene here

[Peter Carmichael]

but why is it common to soften the anti roll bars and soften the dampers when the track is wet?

 

Eek! Wet stuff. I'm still learning. I tend to work it out from first principles from the contact patches back.

 

A tyre's grip characteristic is measured in terms of slip angle and vertical load. On a rolling dynamometer you run a test for varying vertical load and varying angle and measure the side force and drag. As the angle increases, the side force increases until you get to a point where the tyre stalls; at this stage more lock does not equate to more steering effect.

 

As you vary the vertical load, the grip characteristic also varies.

 

In the wet, the stall effect is very abrupt. At the point where the tyre stalls, the car runs wide, in effect taking the tyre deeper into its stall characteristic. In the wet you don't want to start the tyre sliding because you have fewer chances of recovering it. A stiff suspension set up causes quite high peak loads. Onset of cornering is more abrupt and where there is a peak weight transfer of load onto the outside wheels there will also be a peak negative weight transfer from the inside wheels and a net loss of overall grip; in the dry, this doesn't matter because the peak passes quickly and whatever slide begins is quickly contained within the forgiving grip characteristic of the tyre. In the wet, the momentary loss of grip can be enough to start a slide that takes the tyres deeply into stall from which no recovery is possible.

 

A soft suspension softens the onset of weight transfer avoiding peak loads and avoiding momentary loss of grip. Progressive loading up under cornering makes it much easier to control the car without ever getting into the situation of a lurid slide where the tyres are working very inefficiently.

 

That said, I ran my car at Brands Hatch in the pissing wet on Monday on the same setup I use in the dry and on worn, hard compound 6 inch ACB10 tyres. The car gave such clear indications of where the limits were that it was very easy to drive right up to them. I was happy to let a novice driver (Jo) have a go on her own because the car was still astonishingly forgiving - we are talking about ~500BHP/tonne, a novice driver, a dry set up, dry tyres woefully short of temperature, standing water and an unfamiliar circuit. Jo was hitting ~105mph on the straights where I was getting up to 120mph skipping over the puddles, but both of these recorded speeds show a certain confidence in the car's handling responses.

 

Richard Ince in the meantime had softened his ARBs and fitted Avon race wets which got his lap times down into the region of 1:52. My best laptimes were of the order of 2:04. To put these in perspective, on my first ever session at Brands Hatch two years ago with the 1.6 Supersport engine, I recorded dry laps of 1:49 with a passenger.

 

So a wet set up is faster in the wet (big surprise), but a dry setup need not be considered dangerous.

[ashaughnessy]

OK Peter, as we have your attention :-)

 

If we start out with a reasonable set-up and we want to fine tune it (let's say on a dry circuit, it's easier). Where do we start? On my car (live axle supersprint) the only adjustables (easily adjustable at the track) are:

Tyre pressure

Damper settings

Spring platform heights

Front toe setting

 

I would start off playing with tyre pressures. As I understand it, starting from equal pressure all round, reducing the pressure at the front/raising the rear will reduce understeer tendency, reducing rear/raising front pressure reduces oversteer tendency? Or is that total rubbish?

 

Then I'd play with damper settings. Again, stiffening the rear/softening the front would help reduce understeer, softening the rear/stiffening the front would help reduce oversteer? Or have I got that the wrong way round as well?

 

If I was desparate, I might play with ride heights. Reducing the rear would reduce oversteer tendency, raising the rear would increase oversteer tendency. I would avoid this, as it would upset my corner weight balance.

 

If I felt the car felt funny, I might play with the toe. Toe-out might help turn-in, but might make it feel less stable. Would toe-in make turn-in less dramatic, which might be useful if the back was too unstable?

 

I haven't got an adjustable ARB so can't change this.

Anthony

[Peter Carmichael]

Tyre pressures: There is one right answer, judged by even tyre temperatures taken with a pyrometer. If you go above or below this pressure value you will get less grip. Most of us don't have pyrometers and don't have the appropriate pit crew expertise to use one effectively, so check your hot tyre pressures when you come off track rather than the cold ones going out on track, aiming for even pressures all round. (on 6in ACB10s I aim for 21psi, on the larger sizes ~23psi)

 

Damper settings: Equally, there is ~one right setting. Very easy to get lost in damper adjustment. Start fully soft and click at a time adjust until the floatiness disappears - no more. (I have non-adjustable dampers, thank God.)

 

Spring platform heights: Yes, yes, yes. If you alter the rear the same on each side you don't screw up the corner weighting and this has a radical effect on rear roll steer and handling. Up for more oversteer/turn-in. Down for less oversteer.

 

Front toe setting: Start straightahead and if after all the mucking around with the rear ride height the car refuses to turn in and then the tail comes out under power mid corner, toe it out a bit and drop the rear ride height a bit to compensate.

 

That's about as simple as I can make it.