Very early Imps had 3½" section wheels.
The standard rim is 4½"J x 12", used with a 155/82/12 tyre.
The 'J' simply refers to the shape of the rim's edge.
Imps work well on wider rims (and tyres) on the rear. Too wide is not good for an Imp, due to camber changes in the suspension.
Recommended: 5½x13 (with 175/60x13 tyres)
Recommended: 6x13 on the rear (with 175/60x13 tyres).
Possible: 7x13 at the back (with 185/60) is useable, but they are a little wide... perhaps a bit less offset.
From 4½ to 5 is an increase of 9.0%; to 5½ is an increase of 22.2%; to 6 is an increase of 33.3%.
Imp tuning & modification by J.D. Hansen: Wheel width is restricted to 5½" at the front and 7" at the rear by the space available. And there is no point in exceeding 5½" all round on a road car.
Tuning Imps by Willy Griffiths: You can fit wider rims without needing spacers and I do not recommend them. The best tyres can be fitted to 6 maximum section rims.
For racing, use equal rim widths and equal tyre sizes front and rear.
Rims ought to be just one piece and not the standard rim with a piece let in. Sawing a wheel in half to let in extra bits should only be attempted by a specialist.
With 5x13 wheels (like Panasport alloys) the front hubs are a fit problem, because the center cap does not fit onto the wheel after installed. The back hubs appear to be fine.
PCD, Pitch Circle Diameter, is the diameter of an imaginary circle drawn through the fixing holes. Also known as Bolt Circle Diameter.
4 " wheel hubs
maybe ? 4/101,6mm (= 4 bolts x 4 inch)
Rover Austin Metro
Has been done:
If you want to use steel rims from a metric car, chose some with the most favourable inside offset. (Perhaps the 5Jx13 of a nineties Opel Kadett). Then take them to an engineer to have them made fit for an Imp. He will completely remove the centre boss of the wheel (centre hub hole and wheel stud holes) and throw it away. Even the nearest metric size can't be made to fit.
Then a steel disc is prepared in a slightly larger circle than the hole in the wheel (¾" / 18mm thick). The correct PCD centres and the centre boss hole (2 3/8" / 60mm) are drilled.
The edge of this disc gets rebated to form a seat the size of the hole in the rim. Then the rim and the newly made disc are welded together. If the disc is offered up from the inside of the rim, the rear offset will differ about ¾" / 18mm from the standard.
If you don't like the offset, the rim hubwelds might be removed on a lathe, repositioned and rewelded.
The stud holes will require a taper to suit your wheel nuts.
A lot of wheel achieve most of the width from the hubface outwards. An Imp needs a wheel with quite a bit of inset. Wheels suitable for front-wheel-drive cars are often suitable for the Imp, but you'll have to find a manuafturer who is willing to drill a blank casting for a 4" PCD and 7/16" UNF bolts/nuts.
Racers have a greater proportion of the weight on the rear than a road car. They used to run 8 inch on the front 10 on the rear.
A topic that concerns rallying or racing Imps is whether the Imp ever had 13" wheels homologated. Competition Imps that run in the roadgoing class have to stick to what has been homologated.
Using 12" wheels restricts the width of tyre that can be use, and anyway the choice in 12" tyres is getting more and more limited.
The homologation paper for 13" wheels on Imps is dated 1st January 1967 and the FIA recognition number was 526.
Standard and/or period wheels
Alan Millington at Impetus always seems to have a good selection of suitable period wheels. Maybe other suppliers have too.
Vince Frost @ Merlin Motors (01352 - 720 280) has standard wheels in various conditions.
In stead of buying new wheels, you could have a set of secondhand ones powder coated. The coating fills-in any pitting/scratches etc. Anything to be powdercoated must be clean to bare metal for the process to hold and work well long term. This can be either by fine sandblasting or some other type of media blasting.
First check carefully for
Magnesium deteriorates with age and becomes very susceptable to stress cracks or things like that. So be careful when you trade for old Minilites and such.
A 5x12" Minilite weighs 8 lbs
Superlite alloys are available direct from the manufacturer in the UK
Weller steels are to be taken over by Polleysports
'Try it and see' can be more expensive than calculation, when you want to buy new wheels.
If you can start with a series of correctly measured values, you have a better idea of which adaptions (how many mm) would be likely.
'Bob from Dundee' says "the Imp had around 25mm of positive steering offset. This was achieved with the imp wheel which had a positive inset of around 25 mm." This could be probable for an Imp on 4.5" wheels.
Theoretical diameter and radius for 155-12-80 =
measured part worn =
compression under load is around 4%
according to workshop manual castor angle = 10 ° and
original 4.5 inch rim measured mounting flange to tyre wall =
Car on axle stands, axle height measured 307mm
mounting flange (drum brake) projected on floor
king pin centre line projected on floor
real offset =19+(tan(11.5)*(307-265)=
'Steering offset' doesn't get mentioned in too many manuals, not the workshop manual WSM141 either.
Though they do mention that there aren't any settings for the suspension and control procedures are only to check for accident damage. Quote: "Under normal service conditions, the only item requiring attention is the frontwheel alignment (toe-in). A full geometry check should only be necessary following the rectification of repair damage to the front suspension or steering."
But the offset is a factor in steering performance.
When fitting new road wheels, if the diameter remains the same, the wheel inset must remain the same.
If the wheel inset is not clearly defined, the steering could take on all sorts of unwanted effects.
With Imps there is half an inch difference between the early wheels (4") and later ones (4.5"). The extra inch simply added to the outside. Therefore a larger offset. Which would make the steering somewhat heavier and make steering a little more sensitive to obstacles.
Then there was the slight difference in height between the early crossply tyres and the later radials. Of course it is well known that tyre brands aren't too particular about the diameter specifications of their products - the diameter will be different with a new set of tyres.
I would appreciate your comments on an accessory that is being advertised more and more in motoring magazines - wheel spacers.
G. HOPKIN, Shirley, Croydon
John Simpson, Technical Editor of 'Motor Racing', Febr. 1966:
Perhaps as a result of the wide appearance of the racing Minis, it became 'fashionable' to use spacers, and while there undoubtedly IS some advantage in having a wider track, this is not nearly as necessary as one would think. For normal fast road work the Mini and the Imp have perfectly adequate roadholding with the standard track.
The use of spacers does in fact have drawbacks. Firstly, it adds to the unsprung weight, which itself detracts from roadholding. And secondly, it places additional strains on hubs, hub carriers and drive shafts, which does not exactly help reliability. The wheel centres themselves are also subject to additional stress.
Hub spacers and steering offset
Spacers between the wheels and the hubs are nasty things. In The Netherlands they are simply illegal.
Racers would apply such things on purpose, but they would keep a diary, making notes of all their trials & tribulations and the subsequent results on the circuit. These diaries are not often shared with the competition / general public. So most of this sort of information is never made available and much of what is available is plain wrong.
Steering offset and discs
Modern wheels for front wheel drive cars often have too much inset for an Imp. But more inset would be handy if you want to fit discs. Using standard Imp wheels in combination with discs would put the wheels too far out.
This may have been discused on the Coollist imp-list.
The real effects of changing the steering offset, and some recommendations
Allan Staniforth - Competition Car Suspension: Design, Construction, Tuning
an excellent introduction to suspension design, including a historical look and a peek into state-of-the-art. As an added bonus, it is interesting to read.
read page 108 and further
Allan Staniforth - Race and Rally Car Source Book
Less technical than Competition Car Suspension, this book is designed for those who are trying to turn a production car into a racer. Some fascinating case studies are included.
The leading guide to building or modifying a competition car.
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Wheels - Imp tech (this file)