Don't underestimate the importance of the tyres. A car's tyres are responsible for a large part of the handling, comfort and safety. Doing 50 mph on a dry road, the influence of the tyres isn't much, but the more difficult circumstances of the road, the bigger the influence of the tyres on the handling.
| Contents:
Width Diameter Camber angles Tyre sizes (a table) Tyre markings Changing the wheels round Recommended tyres Tyre Pressures Law And a few links to sites of tyre manufacturers at the bottom of this page Wheels |
Good tyres wil increase your safety (particularly in panic situations).
What makes tyres good tyres ?
Tyre width varies from manufacturer to manufacturer. Although the tyre may be marked as having a certain width, its actual width could be a lot different.
Most Imps still run on the 4½" x 12" rims and use the standard 155/82/12 tyre. An improvement can be made by changing to a 5 x 12 alloy rim. Wheel rim width controls the section of the tyre and therefore the the amount of rubber that can be put on the road. Wide rim wheels are sensible: the more rubber on the road, the greater the road grip.
The disadvantage is that wider tyres increase wheel bearing wear and make the steering heavier. Don't forget: the more rubber on the tarmac, the greater the rolling resistance (increase of drag) and the more power they absorb and petrol they waste.
If you change to 13", you can still opt for 155/70 tyres. Your reason might be that fairly narrow tyres cut down on the squirming and bump-steer you would sooner encounter with wider tyres, as the centre of contact patch moves around. (The Imp's front suspension design is a factor when choosing tyres.)
Too wide a tyre distracts from its roadholding. One shouldn't fit wider than 185/60x13. The Pirelli P6 and the Goodyear NCT both have 175/60x13 available, but these have very flat tread.
Tyres with a flat tread aren't really suited for the Imp. Because of the suspension design, it needs a tyre with a round-shoulder. That way you can avoid a very sudden/ unexpected break-away. Rounded shoulder tyres work more consistently, due to camber changes in the suspension.
And a soft compound is best.
Generally speaking: a very flat tread and square shoulder will give sharp steering but a quick breakaway; rounded shoulders and tread will not be so precise, but more controlable.
If you want really high performance tyres or if you are going to use your Imp in competition, you will have to think about a change to 13" wheels and ultra low profile tyres. You will have to buy wheels with a 4" P.C.D. and the correct inset/offset. This is important, otherwise you'll get strange handling and clearance problems.
185/60/13 is considered ideal by some Impers. These will give the same gearing virtually as the standard tyre. If clearance is a problem, use a 175/60/13 (Pirelli and Fulda used to do one...). Ultra low profile tyres can work well on an Imp, although they can be a little twisty, not as good at absorbing bumps and they last a lot less. However the advantage will be obvious if you try them. They also look mean !
'Effective' Tire Diameter by Bowling
| Positive camber is when the top of the wheel leans outwards and negative is when the top of the wheel leans inwards. |
One factor in tyre performance is the suspension camber angles. Lowered springs for instance, often give far too much negative camber (especially at the front). This can be fine on tarmac, but on snow or loose gravel it is another matter.
To get the best from a tyre, the maximum amount of of tread must contact the road and excess camber defeats this. Early pre-67 Imps can have their front positive camber easily removed by stub axle modification. And cars with extreme negative camber (i.e. Monte Carlo or low Hartwell springs) can be similarly altered or adjustable platform shock absorbers can be used.
One possible view on springs, absorbers and camber:
For best all-round road use, giving about ½ - 1° negative camber standard springs or Monte Carlos at the rear; plus Montes at the front, but with the negative camber removed; and a set of Konis.
Service Bulletin August 1966: Excessive rear tyre wear due to excessive rear wheel toe-in.
| Tyre size | Rec'd rim size | Revs per mile | Rolling radius |
|---|---|---|---|
| 155-... x 12 | ..." | 966 | |
| 155-70 x 12 | 4 - 5½" | 1013 | |
| 155-82 x 12 | 4 - 5" | 963 (966) | |
| 165-70 x 12 | 4 - 5½" | 980 | |
| 175-70 x 12 | 5 - 6½" | 955 | |
| 155-... x 13 | ..." | 920 | |
| 155-70 x 13 | ..." | 974 | |
| 165-70 x 13 | ..." | 933 | |
| 165-65 x 13 | ..." | 969 | |
| 165-60 x 13 | 4½ - 6" | 1006 | |
| 175-60 x 13 | 4½ - 6½" | 971 (983) | |
| 185-60- x 13 | 5 - 6½" | 955 (961) | |
| 195-60 x 13 | 5½ - 7" | 938 | |
| 205-60 x 13 | 5½ - 7½" | 917 | |
| 175-50 x 13 | 5½ - 7½" | 1043 (1060) | |
| 195-55 x 13 | 5½ - 7½" | 965 |
Changing tyre size and/or profile has a quite significant effect on your car's gearing.
Rule of thumb: one size wider means going down one size in aspect ratio to maintain the rolling circumference, so
155 x 13 roughly equals 165/70 x 13, or
175/70 x 13 roughly equals 185/60 x 13.
Going up a size on the wheel rim adds an extra factor. As an example, on a Cavalier fitted with standard 165 x 13 (or as they are sometimes badged now 165/80 x 13), on 14 inch rims they use 175/65 x 14 or 195/60 x 14.
In 155/82 x 12, the 82 refers to the height of the sidewall in relation to the width of the tread. In this example it is 82% of 155 millimetres (=127.1mm).
The figures in the table are approximates, and will vary (slightly), depending on the manufacturer. Not all tyres with the same size marking are the same size!
If you would compare a Pirelli P6 185/60 x 13 with a Firestone S660 of the same marked size you would find the Firestone tyre wider and chunkier!
| Tyre size | Rec'd rim size | Revs per mile |
|---|---|---|
| 145-82 x 13 | 4 - 5" | 935 |
| 165-70 x 13 | 4½- 6" | 936 |
The standard Imp transaxle top gear gives: 15mph / 1000rpm
Fitting 165/60s would lower your gearing: 15.4 mph / 1000rpm (nearly 3% higher)
185/60x13 tyres don't actually raise the gearing, but they slow the car down, because of increased drag.
| Speed | old marking | eg. | new marking | eg. |
|---|---|---|---|---|
| Speed < 113mph | SR | 155 SR 12 | S | 155R 12 76S |
| Speed < 130mph | HR | H | ||
| Speed > 130mph | VR | V |
Obviously the speed rating only applies if the tyre is in good condition, correctly inflated and within the load range.
In 155R 12 76S, the R denotes radial construction, 76 stand for the load rating (in this case 400kg).
The TWI (Tread Wear Indicator) is a mandatory marking for the North American market. It hasn't everthing to do with the softness of a particular tyre. It is purely a test, undertaken on a track, against a benchmark figure of 100. It measures the level of wear produced during this test.
A tyre with a TWI of 140 will not necessarily be any 'stickier' than a tyre with a TWI of 200, for a given set of criteria, it will just wear out faster.
However, when tyres approach TWIs of 280-300, then it is usual that the tradeoff is less grip courtesy of their high milage characteristics.
It is recommended to change the wheels (including the spare) round about every 5,000 miles to even out wear. But the construction of radial ply tyres limits what can be changed.
Radial tyres are neutral when new, but when rolling, the pressures on them cause the weaving within to 'tuck up'. This means that you're not supposed to use them rolling in the other direction (chalk an arrow on them). If you change a tyre from the left side of the car to the right you may find that your steering wheel trembles, and balancing the wheels won't help. If you change the tyres over, keep them on the same side.
Van wheels and Mk 1 wheels are not interchangeable. The van wheels are 2 lbs heavier.
Every tyre has its own particular qualities
| MPH/1,000rpm | standard 155/12 | Yokohama 165/60 x 12 | |||
|---|---|---|---|---|---|
| 1st 2nd 3rd 4th |
4 7 11 15 |
3.7 6.4 10.0 13.7 |
|||
The Yokohama 165/60 x 12 is a great Imp tyre, but it will reduce your gearing drastically.
Perhaps not what you want on the road, but for Hillclimbs and Sprints, it may be just the thing.
Consider it as an alternative to buying a close-ratio transaxle.
Klébèr tyres ?? (advert 1971)
Correct inflation of all four tyres is crucial to a car's road behaviour.
It is worth experimenting a bit with the pressure as even 1 psi. can make a noticeable difference, particularly on the rears.
The book of Imps by John Thorpe: The rear end tyres must be kept to approximately double the pressure of the front tyres. Otherwise the stability of an Imp seriously deteriorates.
The recommended pressures are 15psi. up front and 30psi. at the rear. These can be increased by 5psi., resulting in better steering (at the cost of a harder ride and more noise).
Manual:The tyre pressures quotes are for Dunlop C.41 or Goodyear G.8 tyres. If tyres other than these are fitted, they should be inflated to the pressures recommended by their manufacturers.
Use 18psi. up front for radials.
Tuning Imps by Willy Griffiths: With the front wheels upright or with negative camber, there is no need for the huge difference in tyre pressures (as there is with positive camber).
For fast road work I have found the best roadholding to come from either 28lb. front - 32lb. rear or the same pressure all round (depending on the handling characteristics you aim for).
For racing a fully lowered car, with ½ to 1° negative camber in front and 1½ to 2° negative at the back, use equal tyre pressures front and rear.
Too low a pressure also means unnecessary tyre wear.
25psi up front and 35 at the rear is not a bad choice.
If an Imp wanders while the air pressure is correct, have the balance checked. Properly balanced wheels improve the car's handling and it lengthens tyre life.
When Autocar experimented in 1964 with the Nerus Hillman Imp on Dunlop SP tyres, they found that 20 p.s.i. front and 32 p.s.i. rear gave the best handling. A tiny bit of understeer, but almost neutral.
The legal minimum for tread depth is 1.6mm.
When finances do not permit a needed new set of tyres, remoulds or second-hand ones are what you need. Also (of course), when you buy a pre-owned car, you buy pre-owned tyres.
Regulations (hah!):
Likely it wasn't very necessary to put these stipulations to paper, as surely no motorist needs to be protected from the obvious. However, keep in mind that you're not allowed to sell just any tyre in the E.U.
Though there are good remoulds around, in the long run it's probably better to buy new tyres (but budgets can be limited sometimes).
Arktic, Arktrans, Avon, BF Goodrich, Barum, Bridgestone, Continental, Cooper, Dean, Debica, Dunlop, Falken-Ohtsu, Fate, Firestone, Fulda, GT Radial, General, Goodride, Goodyear, Hankook, Kleber, Kumho, Marchel, Marshal, Matador, Michelin, Nankang, Nokian, Pirelli, Pneumant, Roadstone, Sava, Semperit, Stunner, Suntic, TOP, Tigar, Toyo, Trayal, Uniroyal, Viking, Vredestein, Wanli, Winter Tact, Yokohama
Tyresome ? / by Martyn Jones. - Impressions 1986, June
This file is in part based on Martyn's article.