The Imp Site

Imp rear suspension

Suspension, Dampers & Steering

Imp suspension

Index:
Handling
High to low pivot
Springs
Shock absorbers
Antiroll bar
Kingpins
Tracking
Wishbones
Steering

Handling

The handling characteristics of an Imp can be influenced eg. by tyre pressure or (front roll bar) weights. It can be set up for whatever characteristics are prefered.

  For those visitors arriving here via the wikipedia page on swing axles:
The Imp had a swing axle up front, not at the rear !
The Imp initially had too much understeer, due to a too late discovered legal failure: the headlights were too low - which Rootes solved by increasing the ride height.

An Imp with a little understeer will break away front end first. Set up for oversteer the back tends to get away first. It is said that with a rear engined car it is more difficult to regain control when the back end breaks out.
A set up which makes it possible to lift the front wheel coming out of a corner may look a little unstable, but it has the effect of putting more weight on the rear wheels and that's just where it is needed.

Huskies had uprated rear shock absorbers and rear springs were fitted, together with the Sport's strenghtened wishbones and trailing arms. This gave it handling characteristics not unlike a rally car.
The handling is superior to that of a standard Imp. You might try to run Husky rear suspension on your Imp and enjoy it.

The pre '67 Imps had a higher ride and positive camber.
This was changed because of the disadvantages:

 recommended author:
Competition Car Suspension : Design Construction Tuning / by Allan Staniforth. - Published by Haynes Pubns, February 1, 1995. - Updated : Dimensions (in inches): 10.05 x 7.24 x 1.02
ISBN: 0-85429-956-4 (Hardcover)

High to low pivot
References:
• John Simister, Impressions Jun. 1981 : Front suspension, Steering and shock absorber - modifications for road use and autotests Part. 3
• Dave Lane, Impressions Dec. 1992 : this article details the full conversion from high-pivot to the later low-pivot type
Front suspension; camber of the front wheel

After the mid '67 change for all Imps to low pivot suspension, standard Imp set-up was quite effective. But for even better handling and roadholding the car should be lowered further.
There are various ways of lowering the front, and more particularly: decambering it. Some methods are sucessful, others are potentially dangerous. Lowering an Imp will affect the amount of cool air that can pass underneath, and consequently it will not improve cooling.

front suspension

In the mid-60s various firms used to market 'decambering kits', which utilised the two pairs of steel extension brakets to attach to the pivot points and lowering them.
In terms of camber change and ride height, the effect was similar to, but greater than the factory alteration to low pivot design. With most kits the pivots were lowered 1½". In theory this is a good idea, but in practice too much strain got placed on the original mounting brakets. These could crack, particularly the rear one which holds the steering rack. Also the bolts that attach the mounting plates to the floor could pull through. (Incidently, these bolts ought to be accompanied by large washers to spread the load and prevent cracks.)

If you cut 1" of the springs of a high pivot Imp, it will be 1¼" lower, and 1" lower than a low-pivot Imp.

high pivot -
low pivot
ideal
ride height is 1¾" too high
ride height is 1" too high
- cut 1¼" of the springs

Some say that to stop the Imp from understeering in the wet, you stuff the front of the car with heavy equipment and use worn cross'ply remoulds. This would allow the rear of the car to slide around corners in any weather... (?)
However: although weight in the front does help reduce the wandering, it has its disadvantages: The more weight you carry, the slower the car is, and the heavier the steering and the slower the acceleration and the more petrol you waste.
If your Imp has wandering tendencies (while you do have radials fitted), check for wear:

A 250 lbs front spring, under a load of 490 lb would compress by about 2". By measuring the free length of the spring and subtracting 2" we can determine what effect that spring will have on the ride height of the car by comparing it with the standard spring laden length.

Using the values in the Haynes manual, a Sport front spring has a free length of 10.35", and a laden length of 7.84 . A compression of about 2½" if we divide 490 lbs (laden weight) by 2½, we get a 'spring rate' of ca. 195 lbs.

Springs

The load under which the spring is compressed, when measured to give the laden length of the spring (Haynes workshop manual):
Front: 490 lbs
Rear : 940 lbs
These values represent the maximum design weight per spring.
Figures given in adverts for springs are the 'spring rate', which is the load required to compress the spring by one inch.
eg.: rally springs
Front: 250 lbs
Rear : 550 lbs.

For race use apart from wheels and tyres virtually the only suspension work which was required was to change the springs to works 'Monte Carlo' settings (ie the same as used on standard sport and coupe models) throw away the rubber insulators between the spring and the abutment, change the dampers to short Spax or Koni's adust the front camber angle by modifying the king pin carrier to give 0.5 to 1.0 degrees negative camber.
The Spax do give the more comfortable ride though. Koni for racing, Spax for road...
In the wet you need softer settings.

Original Hartwell Tourings springs:
fronts - 9" unladen and they have a 250lb rate
rears - 8.4" unladen length (?) and 610lb rate.
Hartwells made these springs for a number of years, but then switched to using Montes.

R.A.C. springs are stiffer than standard and will make the car sit slightly higher until they have been well belted. Fitting these will result in your car having positive camber.
Martyn Jones. - Impressions Oct 1986

If the rear springs have settled about an inch, then think about a replacement set of dampers

Monte Carlo springs are available for £75. Or have virtually any length/rate spring suitable for an Imp made to order for approximately £12.00+ vat.

It depends on the Imp which combination of springs and dampers is the best.
See John Simister's article.

For road use Andy Bryson recommends the set-up he used on 1998 Targa Tasmania (in which he ended up 2nd outright): Monte Carlo springs all round with SPAX dampers Front with adjustable platforms. Front 2º negative and 2mm toe in, measured at the wheel. Rear 4mm toe in and 1-½º negative.
To get really sophisticated you can also do corner weights.

If you use springs that are shorter than the standard items then, when the suspension is on full droop, the distance between the spring cups is greater than the length of the springs allows them to move about. Resulting in crashing / clattering noise.
Use some check straps to prevent full droop - only at the front. They could be obtained from Team Hartwell.


Shock absorbers

Improve handling by fitting a better quality rear shock absorbers (Koni or Gas Spax).

If you uprate the shocks, uprate the mountings, too.

Normally, to lower a low-pivot Imp, there is no need for adjustable platform shock absorbers, as simply fitting shorter springs has the desired effect, without having to chop off large amounts the springs.

1976 Koni advert using an Imp
KONI heavy duty rear dampers are excellent and have a long live, too. They work well with uprated springs.
KONI shockabsorbers are reconditionable. KONI (Dutch company) do not make any Imp shock absorbers anymore and do not have them in stock anymore. Given old ones, they can recondition them (in nearly all cases). They would be fitting completely new internals.
Look for Konis on jumbles.

Spax or KONI adjustable dampers will probably give the best results. The choice really depends on how much you can spend. When you fit adjustables, position the adjusting screw facing the centre line of the car, otherwise you have to remove the wheel to adjust them.
Be sure to smother the adjuster in grease or Waxoyl to prevent the ingress of water, which would otherwise seize the unit solid.

If you use shock absorbers with an adjustable spring platform, you can use short Monte Carlo springs without cutting them. Simply let them sit ½" lower on the shock absorbers. You might use this set-up on a high pivot car.

Some Impers prefer the handling characteristics of Competition Spax to KONIs. For not much more that the standard Spax, you can get the Competition Gas Spax for both standard and lowered spring heights. The advantage of the Competition front item is, that it includes the facility to raise and lower the spring platform. This enables you to easily adjust the ride height and wheel camber. They also have a larger capacity, which means they work more efficiently and cooler and are thus less prone to fade.

Settings (more-or-less):
front: 3-4 clicks from the softest
rear: 7-8 clicks

Gas Spax; Competition 
part numbers:
Shock absorbers front Std. height: G752 AS200
Low height: G753 AS200
Shock absorbers rear Std. height: G755 200
Low height: G756 200

With lowered springs, you should opt for low height shocks.

Girling 'Rally Ride' shock absorbers
front: 2014
rear: 2013


Antiroll bar

To improve the cornering grip and traction and to permit the use of equal tyre pressures all round fit an antiroll bar. One with a diameter of 5/8" (16mm) is too stiff and would increase understeer. 9/16" (14mm) would be far better.
A front antiroll bar can still be found secondhand or you can adapt one from a modern car. One that will fit reasonably simple comes off a series-1 Renault 5 (about 12mm) (or Renault GT Turbo). It comes complete with flexible mounting blocks arm ends and central support.
The addition of an anti-roll bar from a Hillman Hunter to the front suspension further .
Rear anti-roll bars for Imps can be used but must be very weak or handling will be spoiled. Therefore usually not done. According to the experts a rear bar is not necessary, it's primarily for race use (?).

HILLMAN-SUNBEAM IMP & SINGER CHAMOIS
ANTI-ROLL BAR FITTING INSTRUCTIONS

With front of car raised, wheels in normal position (not hanging),
Drill wishbone in the flat web as close to the rear rib as possible,
2 holes 3/8" diameter at 29 1/2" centres apart same as the Roll Bar.

Next step - Take the 3/8 bolt, note how rubbers and fittings are
assembled on to the bolt. Take off the first three rubbers and
dis washers - place bolt with remaining rubber and disc washer
into wishbone hanging down from top. Replace two rubbers and disc
washers as previously assembled, the Roll Bar, then the last rubber
disc washer and nut. Do this on both sides. The back of the Roll
Bar must face the rear of the car. Then lift Bar with rear
mounting brackets fitted. Mark and drill floor to suit. Bolt up
with reinforcement plates over and under floor.

V.W. Derrington Ltd.
Automobile Engineers and Manufacturers
159-161 London Road, Kingston-upon-Thames, England
Telephone: 01-546-5621/2

V.W. Derrington instructions supplied by Robin Human, 7 Aug. 2011


Kingpins
If a wheel can be rocked up and down, the kingpin is worn. Up to 1/32" (0.8 mm) movement is acceptable between top and bottom: measured at the edge of the tyre. A how-to on replacing king pins and other bits.

If an Imp has negative camber up front, because it is a low-pivot Imp that has been lowered, a cure is to modify the kingpin carriers. File metal from the top, probably less than 1/8", and use a washer on the bottom.

Wheel bearings
If a wheel can be rocked up and down and side to side, the bearing is suspect. Spin the wheel and listen for excessive noise for further diagnosis.


Tracking

Incorrect tracking can give rise to all manner of peculiar handling traits as well as excessive and uneven tyre wear.
Imp tracking should not be set parallel. Correct tracking is 1/16" to 3/16" at the wheel rims (according to Millington) or 3/16" plus or minus 1/8" at the tyre thread (Rootes manual). Not enough toe-in will give less straight line stability. Too much will promote unnecessary tyre wear.
Be sure to reset the tracking after lowering / raising the ride height of your Imp (says G. Pearson), even is the change is minimal.
For race use the non-adjustable track rod should be replaced by an adjustable one and the front wheel toe-in set to 0 to 2mm with both track rods set to the same length. The rear suspension ideally should be set to 3mm toe in (under no circumstances should toe-out be permited on any rear wheel) and 0.5 to 1.5 degrees negative camber.

Track rod end wear may show as a slight clonk over bumps as well as wandering. To test a track rod end, simply grasp the track road in one hand, and the drum in the other and see if they move as one. If not, it needs replacing.

Tracking gauge
Get your own. Tyre shops know nothing about Imps and are just not interested.
A very cheap way of setting the tracking is described by G.Harding (Impressions Feb 1996, p.25). It involves tape, string and a good eye (?).


Wishbones, rear

All Imp rear wishbones are physically interchangeable if they are used with the type of handbrake cable appropriate for the year of the wishbone, rather than the car.
Rear off-side wishbones tend to rust far worse than those on the near-side, whatever the reason. Don't scrap good off-side ones.

Can't get the column of the rack -
Rust will seize the joint here. Actually I don't bother removing the outer column as well as on the Mk 1s I put the auto-cancel in the dustbin and twist the wires together; on Mk 2s I only do the first part. This allows the inner column enough movement to face the rack. People will cringe here - so be careful, but I've had no trouble. Lightly hammer a small chisel between the lugs after removing the pinch bolt to break the nut. The remove the chisel ! And from the inside of the car hammer the steering wheel towards you with the palms of both hands on the innermost of the spokes.
- Grahame Harden. - Impressions 1986 June

Suspension bushes
To fit (polyurethane) bushes, try using a G clamp. They come with instructions: let them soften in hot water and they go in no problem.
Chrysler Special Tuning in Coventry made Imp competition bushes
Hard rubber bushes can be bought eg. at Dutche Components in Axminster. Silicon lubrication helps to fit them.

Make sure that all front suspension bushes are to the correct torques. Those that pass through through Metalastic rubber bushes must only be tightened with the car sitting with its weight on the wheels (as in: not jacked up). Otherwise they will be constantly under stress and won't live their full life. (Worn bushes cause wandering).


Steering

Steering rack
Check the steering rack for wear by ensuring that when you turn the steering wheel, the front wheels move instantly.

Boss Dimensions
Cars that have the same steering wheel spline dimensions as the Imp: Herald, Vitesse, GT6 etc.
The main problem is clearance - the distance of the rim of the wheel relative to the dash. Some of the Triumph and Ford ones are quite shallow.

Lowering the steering column

Steering column upper bush (part no. 7103555 / 7103662 / 77080539)
Upper steering bush 7103662 has been deleted by Talbot early 1987. Felt pad 77080539 can be fitted in place of the bush. It has a more positive fit and is permanent. It is a little difficult to insert. It will help if it gets first soaked in light oil and if the surface is lightly coated with grease.

Upper steering column bearing - Temporary repair
Replacement for the upper steering column bearing (Part no. 7103555): a 1" wide strip of self-adhesive carpet tile. Make it a tight grip, otherwise the felt soon settles and there will be excessive clearance. The length of the strip would depend on it's thickness. Example: about 8" for two turns. Use a twisting motion and and press with a large box spanner, to wind it down inside the outer column until just below the top. As the self-adhesive backing was on the outside of the bearing, this effectively holds it in place. A soaking in engine oil completed the the fitting which took 20 minutes - only the steering wheel needed to be removed.

If you drive with a broken bearing, the wheel transmits road shock, and the self-cancelling action for the indicators can be disturbed. Later models with the multipurpose stalk could sustain damage to the return-wings, if used without adequate bearing support.


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Franka