VLM 850M, the Stapleton Motors Imp from Frans Van der Breggen

The Imp Site

Andy Dawson

Dawson, A.
Handling 2. More theory and the Imp / by Andy Dawson. - Cars & Car Conversions 1975, April. - p.48-50


2. More Theory and the Imp

LAST month I looked at the pure theory of suspension and roughly what each component of geometry does. This month I am going more deeply into the theory and then showing how easy it is to alter the geometry of a simple car, in this case the Imp.

This month I am going more deeply into the theory and then showing how easy it is to alter the geometry of a simple car, in this case the Imp.

As a brief resume, we are assuming that the basic layout of the car is fixed, and that we are carrying out modifications that are basically FIA Group 2. The things that we can change are:

To make the car do what we want it to, we must play with the weight transfer at each end and thus the slip angle of the tyre.

Imp suspension assembly; Front and rear


Fitting wider tyres

First priority, and rightly so, in most suspension improvements is to fit wider tyres, which reduce the slip angle by reducing the contact pressure. There are however problems:

  1. The larger the tyre section, the higher the car will be, and thus the greater the weight transfer. So we are back to square one.
  2. Also the larger the tyre the smaller the scrub radius and the lower the stability.

The answer is to fit low profile tyres, which are lower than they are wide by roughly the quoted percentage. A 185/70 has the section height of a 165 tyre, but the width of a 185. Take the Kleber V 10S range as an example:

TyreSection Height Section width
155 x 13124 mm157 mm
175/70 x 13  125 mm176 mm
175 x 13139 mm178 mm
195/70 x 13139 mm197 mm

Rim width

The rim width is very important as it determines the amount of bracing that the tyre has.

  1. Too narrow a rim and the tyre will move over the rim and give very sloppy handling
  2. too wide and the tyre will be over braced and give no feel with a sudden break away.

The actual width which is best for a particular car, is dependent upon the suspension stability (the degree of lateral location). But as a general rule the rim width should be 80% of the section width.

Tread pattern

The other important tyre consideration is the tread pattern and construction.

A slick pattern (or lack of pattern) is very stable and progressive.
A Hakka type rally pattern (many small square tread blocks) is very unstable, has very high slip angles before break away, and gives plenty of feel. For road tyres block tread patterns are becoming more popular, due to their increased feel over rib patterns, but the rib pattern offers quieter running and more stability for Uncle Joe to go shopping on. As said last month, roadholding is important for circuit work and therefore a stable tread pattern is best. But for rallying and loose work, the feel from a block tread is more important.

lack of pattern many small square tread blocksrib patterns
very stable
very unstable
has very high slip angles before break away 
plenty of feel
quieter running
more stability
circuit workrallyingroad
therefore: stable tread  
therefore: block tread
for increased feel:
block tread patterns become more popular than rib

Tyre construction

There are two common types of tyre construction.

  1. radial
    With a radial the cords that make up the tyre carcase are laid along the tread.
    This gives good stability, but a harsh ride and little feel.
  2. crossply
    With a crossply the cords are laid across the tread and so don't stabilise the tread to the same extent, but they give more feel.

Basically radials are best in a straight line and crossplys best for cornering. But with radials being developed at the present rate, and having steel and glass fibre bracing (instead of the more usual fabric cord) they now give equal (if not better) cornering grip, but with a very fine limit.

For competition, loose surface tyres and tarmac tyres differ again due to the different requirements. For tarmac most racing tyres are crossply as straight line grip is relatively unimportant, although Goodyear are playing with radial racing tyres for very high powered cars, and very heavy cars, as the radial runs cooler.

For rallying straight line grip has always been considered most important. And with very powerful cars wear is a problem. So most rally tyres are radials, including the rally racers made by Kleber and Michelin. As rally cornering speeds go up, so we are beginning to see more crossply tyres appearing. On the 1974 RAC the Works Escorts were on 15 inch Dunlop crossply tyres, and Tony Pond has been using a crossply Goodyear for some time.
Kleber only make radials, so I shall make no comment!

Wheel angles

The only other consideration when choosing a tyre and wheel combination is wheel angles. If a car has a lot of camber change in roll, then there is no point in using a very wide tyre and rim, as only part of the tread will be on the road at anyone time. There is a way round this, and that is to use a wide tyre with a narrow rim, but this is rather like taking a sledge hammer to break an egg and there are better ways of doing it.

On the Imp

So let us look at the Imp as a specific example and see what our theory gives us as ways of making it handle better. I was able to borrow the car that I had used on the Tour of Britain from Frans Van der Breggen of Stapleton Motors as a test bed for our theories. And I must begin by explaining some of the modifications that had been made to the car. These are general for good handling and not just for the Imp or competition.

    roll centre,  semi-trailing arm rear suspension
Fig 11.   With semi-trailing arm rear suspension, the roll centre is found by projecting a line through the wishbone mountings to the wheel axis (as seen from above and from the front). The crossing point is then projected back (on the front view) to the opposite tyre centre
and the roll centre is on this line at the centre point of the car.


The suspension on the Imp is by swing axles at the front, which are in the shape of a wishbone with twin inner pivots, and there are spring damper units mounted inside the wheel arch. The wishbone is a steel fabrication with a simple casting fixed at the outer end, which has the king pin inserted in it. At the rear there is a crossmember bolted to the underside of the rear seat which has the front of the transaxle mounted on it and the semi trailing arms protruding backwards from it.
There are coil springs sitting in the middle of the trailing arms and separate dampers alongside.
The geometry is such that the effective swing axle length is the same as the track which is as Fig 11 last month.

When Uncle Joe and Auntie Flo are buying a car they are interested primarily in comfort and quietness, so the manufacturers put great big horrible rubber bushes into the suspension to absorb noise and shocks. These bushes allow the suspension to move about and under heavy cornering they deform and mess up the geometry.
The answer is to fit harder rubber bushes or spherical bearings such as Rose joints.

Rubbers in the wishbones (braking stability)

On the Imp the front rubbers in the wishbones hardly affect the handling, but they do affect the braking stability. On this car (and my old rally car) I fitted the rubbers that are available from Chrysler Special Tuning and are 50% stiffer. And the rear ones I shortened by 1/8 inch and fitted 1/8 inch packing washers behind the pivot, thus stopping the links from moving fore and aft under braking.

This is in fact Group 1 legal as methods of fixing are free - and all that I had officially done was add washers.

Rear rubber bushes

At the rear the rubber bushes cause the trailing arms to twist under heavy cornering, giving both positive camber and toe out. Neither of these is good for roadholding or handling so the bushes have to go. Chrysler sell harder ones from Special Tuning which are a big improvement.

But the real answer is to ream out the eyes and fit PTFE sleeves over the steel inserts of the rubbers, and make up bronze abutment washers to stop the trailing arms moving from side to side.
This is obviously beyond Group 1. And is also impractical for a road car, as the PTFE bushes need replacing fairly regularly and they rattle like hell on the road and drive you mad.

A tip here for people running Imps:
stiffening the bushes as above also helps prevent the Rotoflex couplings breaking as the side to side movement of the drive shafts is less. but it does make the rear crossmember break in the centre more quickly.

On early Imps, before the Californian and Stiletto were announced, the front pivots (and therefore roll centre) were 1 inch higher than present production models. The answer with these cars is to fit 1 inch shorter front springs and then assume they are the same as present production; or alter the king pin carriers by removing 1/8 inch from their inner mounting and putting an 1/8 inch washer above the carrier to bring the wheels to almost vertical. And remember that the roll centre is 1 inch higher.

From now on I will only talk about the latest production specification.

The front roll centre is midway between the wishbone mountings, 9 inches above the road. The standard spring rate is 195 lbs/ in. with a free length of 10.35 in. and with the geometry as it is the wheel rate is 105 lbs/in. At the rear the roll centre height is 4.8 inches. The spring rate is 460 lbs/in with a free length of 9.4 in. and a wheel rate of 150 lb/in. The front camber angle is 1° negative and the rear ½° negative; toe in is zero front and 0.26 in rear. The castor angle at the front is 9° and the steering axis inclination is 12°. Obviously all the above is at static ride. Scrub radius is one inch and there is no Ackermann effect, the front wheels remaining parallel at all times.

roll centremidway between the wishbone mountings9 inches above the road
standard spring rate195 lbs/ in.with a free length of 10.35 in.
wheel rate105 lbs/in.with the geometry as it is
roll centre height 4.8 ins.
spring rate460 lbs/inwith a free length of 9.4 in.
wheel rate150 lb/in. 


camber angle1° negative 
toe in0 
castor angle 
steering axis inclination12° 
camber angle½° negative 
toe in0.26 

The spring chart shows the springs available, their rates and the resultant roll centre height assuming that the standard mountings are used. They can have packers fitted under them to raise the car. But to lower it, we must use shorter springs.

There is a wide range of dampers available. Some with adjustable platforms for the front, so that ride height can be adjusted without packers. Demon Tweeks (of Tattenhall, Chester) lent us some dampers to try from their range. These included 25% uprated versions of the standard unit, Spax. Koni and Armstrong adjustable. These being available with or without screw adjustable platforms for the front. The other units available were Girling Rally-ride and special Konis. Chrysler sell an extra-heavv duty Koni for use at the rear on rallies which is in fact to a setting that I worked out for my Imp. The other Konis available are specials for racing which are independently adjustable on bump and rebound.

The only other modification that is commercially available is an anti-roll bar from V.W. Derrington of Kingston Upon Thames, who also sell springs which are similar to those sold by Chrysler.

The idea of getting all the parts which are available together was to find settings for road, rally and racing use. Or to be more accurate: confirm that the Chrysler recommendations are in my opinion the best.
The tests consisted of driving round a 100 ft circle; through a wiggle woggle; and along a twisty piece of test track. The circle showed how good the roadholding was; the wiggle woggle showed the controllability and the piece of test track showed how well it all worked.

Front SpringsRate
Free Length
Ride height
inches from
Roll centre
Standard19510.35 -9.0
Monte Carlo1959.60-1.08.0


Rear SpringsRate
Free Length
Ride height
inches from
Roll centre
Standard4609.40 -4.8
Husky5109.45+ 0.55.0
van5509.90+ 1.55.5
Monte Carlo5508.50;- 0.94.4
RAC5509.40+ 0.655.1
race7507.50- 1.83.9
    standard Imp suspension; Sunbeam VLM 850M
The standard Imp on the steering pad. Andy Dawson: 'Enjoying myself?'
 The Imp in fully modified road specifications. Sunbeam VLM 850M
The Imp in fully modified road specifications. Andy Dawson: Really enjoying myself!

We began with the standard suspension and with the Avon Wide Safety crossply tyres inflated to their design pressure for the axle load, which is 18psi front and 30psi rear. The handling was fairly neutral, oversteering with power off and understeering with power on but the angle of roll was very high. I found it very difficult to keep a constant speed round the circle indicating that there was insufficient understeer to make the car stable.

First modification was the fitting of a pair of RAC front springs which are 25% stiffer and should have increased the understeer by increasing the weight transfer at the front. The result however was that the suspension jacked at the front causing the outside tyre to take up a heavy positive camber and lose adhesion. It was like driving a jack in the box with the front going up and down and losing and gaining adhesion.

Next on the list to try was to lower the front to reduce the jacking effect. I reverted to the shorter, but standard spring rate, Monte Carlo type, to try to keep the ride acceptable. The oversteer was increased, but the car was more stable.
The lower front roll centre had increased the weight transfer at the rear and the rear outside wheel was rolling to positive camber.
The remedy to this is to stiffen the rear to reduce the roll, but that also increases the weight transfer and thus the oversteer. By lowering the rear as well, all can be brought under control as the roll moment is reduced front and rear.

Monte Carlo springs at front and rear gave a stable understeer condition with not much roll and slightly reduced 'power on / power off' attitude changes from standard. I tried the different dampers with these springs, and for road use came to the conclusion that either the Demon Tweeks 25% uprated or the Girling Rally-Rides were plenty stiff enough and yet gave a good ride.

For rallying the road settings are too low, and it seemed to be better to increase the attitude changes under power. RAC springs raised the car 1 in. front and 1 ½ in. rear compared to Monte Carlos. The roadholding was much reduced, both front and rear wheels jacked slightly giving positive camber on both outside wheels.
The answer was to lower the rear roll centre by lowering the inner trailing arm pivots ¼ inch. This increased the roll, increased the negative camber at the rear and the final outcome was that the car understeered dramatically.
By reverting to standard front springs the car was made to handle really well, oversteering with power off and understeering with power on. There is also a side effect improvement from doing this:

  1. Weight is transferred to the rear wheels when power is turned on, and the inside front wheel comes up in the air allowing the driver to widen the road!
  2. Also it reduces the need for a limited slip differential, which would make the car difficult to drive down the straight.

Dampers - Rallying

As far as rallying dampers are concerned the fixed setting units l found to have insufficient rear damping. Both the Spax and Armstrong adjustables were capable of being set up to give the car transient oversteer, but from past experience the rears are not capable of absorbing the heat generated by special stage rallying. So the only answer is to use the special large Konis at that end.
When I worked out the setting for this unit we increased the bump considerably to get transient oversteer and the results are very good.
At the front it is worth using dampers with adjustable spring platforms, so that the handling can be tuned to give the exact amount of under- or oversteer required by the driver. Raising the front increases understeer and lowering it reduces understeer.

Race settings

For the race settings I didn't have any decent racing tyres and I wasn't willing to chop the wheel arches about to fit them in even if I could have borrowed a set. The answer which I came up with on the Avon road tyres is in fact very similar to the set-up used by some of the leading race drivers.

The ideal for racing is to get the car as low as possible and yet keep enough suspension movement to keep it off the bump stops over the undulations of a race track. Just inserting the Chrysler race rear spring gave too much negative camber and set the drive shafts at too large an angle. I inserted a van spring packer (pt. no. 7095007) below each spring and this raised the car over ½ inch from the Chrysler racing height which I found to be ideal. In fact it was this set up that I used on last year's Tour of Britain for the race tracks and we found that the tyres wore evenly across their tread showing that the wheel angle was right.
The low stiff rear springs gave tremendous oversteer due to the high cornering power and the fact that the car rolls very little.

With the Chrysler race front springs, the front wheels were very negative and gave too much understeer. But raising the front on the adjustable platform dampers raised the roll centre and thus the understeer, so ride height made very little difference. The Tour of Britain answer was to fit RAC front springs and screw the platforms down so that the front wheels were 1½° negative. The result was neutral handling and very little change in attitude with power on or off.

A better answer to the problem would be to modify the king pin carriers to give a reduced negative camber with the low ride height and thus be able to use the stiffer race type front springs

    Imp racing dampers,  front and rear: Standard, Armstrong and Koni
Standard, Armstrong and Koni racing front and rear dampers. Note the rebound adjustment at the top of the rear Koni and the ride height adjustment for the front.

I found that the best damper setting was with the front Konis set on their minimum and the rears set at 3 out of 6 half turns up on rebound (25kg at 84rpm) and 4 out of 12 clicks on bump (5kg at 84rpm).

For the Tour I also played with tyre pressures and ended up with 30psi front, and 50psi rear before pattering set in - although I reduced them to neutral for the rally stages to increase the feel.

So there we are. We can make the car do what we want, just by playing with spring rates and lengths, and for rallying by altering the rear roll centre. The basic principles that I have used are relevant for all cars.
Next month I will delve deeper into the problems by looking at the Mini, which is more complicated.
I will also look at steering geometry. Something that is more relevant with the front wheel drive car than the rear engine/rear drive Imp, as it is the front wheels that are doing most of the work with the Mini and the rear wheels with the Imp.
The following month I hope to be able to put the lessons learned with the Mini and the Imp into practice on an Escort and see if we can make that work better.


oo - 00 - OO - 00 - oo




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