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Andy Dawson

Dawson, A.
Dawson's Dodges, Part 3 / Andy Dawson. - Cars & Car Conversions 1976, July. - p.63-65,67
Part 3 in Andy Dawson's series on how the experts prepare competition Imps. In this concluding episode, the master deals with drive shafts, bodyshells and suspension

 

Dawson's Dodges

LAST month I completed the engine and went through the transmission. However, I didn't quite get to putting the power on the road. Between the transaxle and the driveshafts are Rotoflex couplings, rubber doughnuts, which allow angularity and plunge of the driveshafts with suspension movement. Doughnuts are probably the most publicised and common Imp failure, and yet I only broke one in all my days of using Imps.

There are two types of coupling:

In the past people have tried all sorts of ways of stopping the couplings breaking, but none have been 100% successful:

It would appear that when a cure is found for the Rotoflex, then the extra loadings transmitted from the wheels breaks the transaxle.

My simple cures are to make sure that the coupling is properly fitted and that there are no sharp edges to dig into it.

  1. Radius off the ends of the driveshaft and the transaxle output shafts with at least a quarter inch radius, but you must be careful not to encroach on the seatings for the coupling inserts.
  2. Stone off the mating faces, and
  3. make sure that you fit the couplings with the protruding sides to the flanges, to ensure that there cannot be any fretting.
  4. Always fit new bolts with new couplings, and
  5. make sure that they are a good fit in the holes, even if the holes are slightly oval, when there is a chance that the bolt will come loose and cause coupling failure.
  6. Fitting the bolts the wrong way round is a useful precaution to prevent them coming loose,
  7. and when you do this you can replace the bolts with 7/16 by 2½ inch Allen screws.

Fitting the Rotoflex couplings, whether standard or modified, can be a bit of a chore if you don't know the easy tweak. If you get the bolt that you are trying to fit at the top and line up the driveshaft just below it, putting a 5/8" ring spanner on the head of the bolt will allow you to lever the other end down and into the hole in the driveshaft. Eight minutes to change a pair was the par set by the Chrysler comps mechanics, although I've heard of people that have taken four or five hours to fit just one.

There are two types of actual drive shaft, the flimsy Mk 1 type and what is known as the Sport type. although it has been fitted to all Imps since 1967. It is this thicker shaft that should be used on any car with over 60 BHP.

Rear hubs

Another source of trouble in the final stages of putting the power on the road, is that the rear hubs have a habit of coming loose and falling off. The problem is twofold:

  1. Firstly, the tab washer which holds the nut on is soft and compresses, thus allowing the hub to chatter and wear away the splines.
  2. The other problem only applies to race cars:
    the spacer between the bearings compresses under heavy side loads and again allows the hub to come loose and chatter.

The remedy for both problems is to fit the hub onto its spline with Loctite 270, having first cleaned the mating surfaces with trichlorethylene, and replace the tab washer with a Super Snipe pinion washer. You will then need to drill and split pin the nut to make sure that it doesn't come loose.
The one snag with fitting the hub this way is that you either need a very big press - or a hacksaw - to take the hub off to change the rear wheel bearings.

Wheel nuts

A small point which is worth mentioning here is that the standard steel wheels can break up and/or cause the wheel nuts to come loose due to flexing. The cure is to use old type Sunbeam Rapier wheel nuts which are much larger and have a Nyloc type of locking. Part number is 12164-08.

Bodyshells

There are four types of car shell, early and late, saloon or coupe. The only real differences are the shape and weight, the early cars having quite a bit of excess metal which didn't seem to make them any stronger, just heavier.

The only mods needed for a race car, are to accommodate bigger wheels, or to remove weight if required. At the back you can fit 13" wheels with just a bit of hammer work at the front bottom corner of the wheelarch. But at the front you need a bit more than just a big hammer. The problem is that the tyres will foul at the back of the arches when the steering is turned, and the back of the arches are the door hinges. A simple cure is to fit Mini external hinges and cut away the standard hinges.

But a better method is to cut away the bodywork and mount the hinges ¾" further back, having first cut down the hinges in the door by the same amount.

    Front suspension: Top showing bracing plates the top of the damper mounting.
bracing plates the top of the damper mounting
 
  1" tube welded into chassis rails. Also note wishbone skids.
Strengthened front suspension mount showing one inch tube welded into chassis rails.
Also note wishbone skids.

Welding for strength

For serious rallying, there are a number of mods which should be done to the shell.

1.
At the front, the suspension pulls out below the cold air box. The cure being to weld a piece of 1" square tube into the channel section between the chassis rails.

2.
The front shock absorber top mounts tend to bow upwards after a bit of hammer. The cure being to weld in bracing plates across the top and down the sides, with enough bow in the plates to stop them fouling the springs.

3.
The rack mounting plate, whether for a high or low pivot car, should have all its seams brazed up, and washers brazed in round the front mounting holes.

4. A tweak which is worth doing, is to weld the retaining nuts for the adjustabie wishbone plate onto a strip of steel. This means that if you have to change a wishbone, then you can loosen off the adjustable plate, without having to drop the rack mounting plate or fiddle about to get at the nuts to tighten the adjustable plate back up again.

5.
The four rear rack mounting plate bolts should have a strip of 1" by 1/8" steel between them and the floor to stop them pulling through.

6.
Also it is worth welding in heel plates on a stage car, as the front of the floor takes a lot of punishment.

7.
At the back, the problems are that the suspension crossmember pulls out of the floor, and that the back of the car tends to bend in the region of the spring pads. The cure for the crossmember pulling out is easy: weld in 10swg plates on the vertical and upper faces (Chrysler part no CTS 1220), and bolt through the rear seat pan, with spacers filling the gap between the pan and the crossmember. One 3/8" bolt through above each arm mounting should suffice.

8.
The crossmember itself should be strengthened with seam welding and a 10swg plate across the top between the inner arm mountings.

9.
On stage rally cars you should also lower the inner trailing arm mounting holes by 3/8" to lower the rear roll centre and make the car more stable.

10.
To stop the back from bending it is worth welding in a piece of angle iron along the chassis rail in the region of the wheel arch, and plating the spring mount area with a 2" strip of 14swg.
The back will still bend after a lot of use, and the only real cure is to put a body jack across the wheelarch every few events to push it back up again.


    mm
The Group One homologated (really) front link which strengthens the bottom damper mount.
The dampers themselves are fitted with adjustable spring mounts to alter ride height.

11.
The front wishbones bend in the region of the 'well' inboard of the shock absorber mountings. The cure is to weld in bracing strips which extend the shocker mountings across the well. These strips are in fact homologated Group 1.
11.b.
For really rough use it is worth cutting the wishbones open and welding in 1" by 1/8" strips along the vertical edges, as extra bracing, and then closing them back up again.

One problem which the Imp will always have, and it is something of a safety factor, is that the king pin carriers bend if you hit anything hard with a front wheel. It isn't worth trying to strengthening them as the wishbone will bend instead, and they only take a few minutes to change when they do bend.

12.
The rear suspension arms are best strengthened internally, something that is already done on the Export van arms. For racing or rallying the standard arm isn't strong enough. The minimum that should be used being the Sport/van arm. This can be brought up to Export specification by welding strips of 10swg along both sides.

13.
Also for competition it is worth welding straps over the top of the bearing housing.

14.
If you are fitting big diameter rear dampers, then you will have to relieve the arm beside the bottom shocker mountings. A number 10 nozzle and a big hammer being the method.

There are no real tweaks as far as the steering is concerned, the steering being quick enough anyway.

15.
Export cars are fitted with stronger track rods, and it is worth fitting the adjustable versions of these to both sides. Alternatively, the Quinton Hazell replacement track rod is almost as strong as the Chrysler export part.

16.
In 1969 the steering arms were changed to ones with bigger tapers, and it is these and their relevant stub axles which should be used for all competition as the others have a habit of breaking at the wrong moment. I know. When it happened to me, I went 130 end over end yards through the trees.

Handling and brakes

Springs

The Imp was the first car I looked at in my handling series, back in Triple C's April, 1975 issue, so I won't go through that again other than to re-iterate what springs to use for what. For fast road use, Monte Carlos; for road rallying and stages with a heavy car, RACs; for stage rallying a lightweight car, Standard fronts and RAC rear; for racing, 350 lb/in front with an anti-roll bar and 700 or 800 lb/in rears.

 front springsrear springs 
fast road useMonte CarlosMonte Carlos 
road rallying and
stages with a heavy car
RACsRACs 
stage rallying a lightweight carStandardRAC 
racing350 lb/in700 or 800 lb/infront anti-roll bar

Camber

The only cars that should have their front camber altered, other than by lowering, are the early high pivot cars. The kits which lower the pivots to give negative camber, don't give the correct steering geometry, so the best way is to modify the king pin carriers.

Chrysler have special washers (7102823) which are the correct thickness to bring an early car in line with a late low pivot car. To fit this you just file the bottom off the inboard king pin carrier mounting, so that the washer can be fitted on top.

Shock dampers

As far as dampers are concerned, for road use either the Chrysler heavy duty or Girling Rallyride are adequate, but for rallying they haven't got enough capacity to keep the back end under control. The only dampers that have sufficient capacity are the extralarge Konis, and some time ago I worked out a setting with them that really makes the Imp handle, this is known as the SP 1 setting.

The fronts don't really matter, in fact all my early rallying was done with heavy dutys, and I only changed when I wanted to be able to alter the front ride height. Thus either the Koni or Spax unit with adjustable platform are the answer.
I always set the front ride height so that the front wheels were vertical under braking and had the dampers as soft as possible without getting patter. The rear Konis, by the way, should also be on their minimum setting, although 1½ turns up can be advantageous with racing tyres.

For race Imps there is an ultimate damper in the two-way adjustable Koni rear, but don't bother wasting £90 on them if you aren't sure that you can set them up. Personally, I would use a set of short Spax White Spots.

 Shock damperverdictnote
roadChrysler heavy duty
or Girling Rallyride
adequate 
rallythe extralarge Konis
the only dampers that have sufficient capacity
to keep the back end under control
SP 1 setting
(front: either the Koni or Spax unit with adjustable platform)
racethe two-way adjustable Koni rearultimate damperexpensive (£90 in 1976), so only if you are sure
that you can set them up
raceshort Spax White Spotsadequatewhat Andy Dawson would use

Steering wheel

One important facet of the handling is wheels and tyres, but before I get onto that, let me say that the size of the steering wheel is even more important. The Imp has 9' of castor, so a brake grabbing or a big pothole can cause the steering wheel to be torn out of your hands, the result being a modified roofline.
DON'T fit tiny steering wheels, 14" is ideal.

Tyres

Back to road wheels: I said earlier that 13" are difficult to fit, so the advice I am giving here relates to 12".
For loose rallying, 4½" or 5" with a 155 section tyre are sufficient. Any wider and the car becomes very unpredictable due to the camber changes.
For tarmac rallying or road use, 6" rims with a 175/70 tyre or my old favourite the 450L12 Dunlop racer. We found that with a more modern race tyre the car was much too twitchy, and had a breakaway that was uncontrollable.

Brakes

As far as brakes are concerned, there seems to be a myth that discs are needed at the front, but in my experience they are more trouble than they are worth, mainly due to the 2" increase in track with them. Drums with VG 95 at the front and AM8 at the rear are adequate for most uses, although for stage rallying I would use VG 95s all round and a mixture of one car and one van wheel cylinder in each front brake.

If you find that you must have discs at the front, as you are overheating the drums, then you will need a balance bar master cylinder set up to give even braking, plus the added complication of twin servos. The disc conversion, which uses a Mark 1 Viva hub and disc and a Herald caliper needs special adapter plates, which are available from Andy Chesman at Greetham Engineering, Edgwick Road, Coventry, or alternatively the Chrysler conversion, which uses an alloy caliper, is available from Chrysler Comps.

Undershielding

A final quickie about undershielding, something that isn't easy with the Imp.

At the back there is only one guard that does the job properly, and that is the big (and very heavy) Chrysler unit. All the other guards that I have seen have had their mounts in the wrong place, and have bent the chassis rails.
The alternative, for cars without a rear radiator which needs protection, is to bolt a ¼" thick sheet of Dural to the underside of the engine and along the transaxle. Whichever type you use, always remember to put some foam rubber between the engine and guard to stop stones from getting in and being punched through the sump.

- oo - 00 - OO - 00 - oo -

 

It is strange that I have at last written this ditty as the Imp is going out of production, but the fact that the engine will be continuing for a few more years in the front of the Avenger, and that more and more people are realising that the Imp is a cheap and reliable way of having some fun, means, I hope, that it will be of use.

 

oo - 00 - OO - 00 - oo

My thanks to all the people who have helped me gain my Imp knowledge, especially at Chrysler Comps

and my bank manager for sponsoring my rallying in my Imp days.

 



 

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© Franka
Version of: 25 Jan. 2014
File started: 8 Jan. 2014