|
26 May 2001 |
This
subject is a regular visitor to the message board and ‘Knowledge’ section of
Mini Magazine for whom I write - fitting bigger wheels. Predominantly 13-inchers,
but 12s as well, the main thrust of the questions being 'what do I need to do?'
along with 'which wheel arch extensions will I need?' The answer has always
been somewhat 'clipped' purely because of the space availability issues both in
the magazine and on the message board, so something a little more detailed was
required.
Although
the Mini suspension was designed around the use of 10-inch wheels with
cross-ply tyres, a quick dash through history reveals that fitment of these
bigger wheel sizes isn't a recent, 'modern' application. Very early on in the
Mini's racing career it soon became apparent that something needed to be done
to make racing tyres last more than a few laps.
Many
scribes have inferred that the current boom in fitting 13-inch wheels is down
to the recent addition of them to mainstream production, i.e. the Sportpack
Coopers. I don't share this view - what a surprise! 13-inch wheels gained
popularity in the mid to late seventies with Mini racers because of tyre
choice. All that was really generally available in the way of 10-inch slicks
were the comparatively expensive Dunlop’s used by the Mini Miglia racers. A
wider choice of compounds was available in the form of ex single-seater front
wheel qualifying tyres at miniscule expense - in 13-inch only. To be competitive
in rallying and rally-cross, again the bigger tyre choice was 13-inch
orientated. So Mini racers started sporting 13-inch wheels. And what's good
enough for the racers…how so many trends start, and where I believe the
influence came from for Rover to go this route - they could see it was what
people wanted.
Initial considerations.
There are
a number of things that need serious consideration no matter which wheel/tyre
combination you decide to go for.
First and
foremost is suspension and steering component condition. The original 10-inch
combination is relatively 'forgiving' - masking slight component wear simply
because it has relatively low leverage effects and dynamics as a whole. This
encompasses everything affected by wheel/tyre weight and profile and the grip
afforded by the tyre contact patch on the road. Wheel/tyre weight affects
damper performance - their ability to keep the tyre in contact with the road
surface the main issue. It also affects acceleration and deceleration generated
by the 'flywheel effect' and steering by the 'gyroscope' effect.
All these
conspire to exaggerate any shortcomings in the suspension and steering
components. Slightly worn swivel pins, top arm bearings, bottom arm and tie-rod
bushes, track-rod ends, steering rack, and wheel bearings will become very worn
in a very short space of time. Tired dampers will not be able to keep the
wheel/tyre in contact with the road. This can be dangerous - it will adversely
affect safety, as you are completely reliant on those four small tyre contact
patches whatever you do in a car. If the tyre isn't in contact with the road
surface, you can't steer, stop, or go. As outlined earlier - a situation made
very much more difficult in a Mini (or any front-wheel drive car) as it's all
going on at the same end at the same time. Lower profile tyres have less
flexible tyre walls - affecting ride quality (making it bumpier), and less
compliant (will follow all ripples, bumps and cracks in the road surface).
Although it has to be said that a number of modern sports cars have huge
wheel/tyre set-ups to not only increase grip - the larger over-all
circumference 'bridges' some bumps/holes, thus 'smoothing' the ride out by reducing/eliminating
some of the smaller bumps.
Wider
tyres will usually increase grip that will apply greater leverage against all
the suspension locating bushes and bearings. Any that are worn will cause the
increased leverage to push and pull the suspension into unwanted
positions/angles. The result is a wayward car that doesn't handle well -
seemingly developing a mind of it's own at inopportune moments if not all the
time. The more grip the tyre develops, the easier it is for it to overcome the
baking capability of the set-up on your car - the extra leverage applied by the
bigger over-all diameter worsening the situation.
Up-rated
dampers are a must. The standard items are barely capable of dealing with the
standard set-up when brand new (a nod in the direction of 'comfort' by the OE
manufacturers) let alone when worn. Bigger wheels/tyres means more (un-sprung)
weight to control when encountering road surface imperfections
(bumps/pot-holes). The barest minimum should be 20% up-rated standard-type dampers.
Personally I would only recommend starting at Kayaba 'Gas-A-Just' dampers (my
current preferred type for cost/performance and fit), if not going to one of
the adjustable sports types.
The
message here then is to make sure your whole suspension is in first class
condition before contemplating the change. This is an ideal opportunity to fit
up-rated components if worn bushes need replacing. Up-rated bushes offer
greater resilience in both wear and movement, which will help provide greater
suspension location and control when a bigger wheel/tyre combination is fitted.
Essential to maintain the good 'handling' aspects believed to be endowed by the
Mini's original design. And make sure your braking system is capable of dealing
with your wheel/tyre choice. A change in pad/friction material (see Calver's
Corner articles on 'Brakes - pads functions and parameters' and 'Pad types and
choices) can often set this right.
Then we
have your own personal mechanical capabilities to consider. Do you have the
equipment, skill, or at least confidence to do the swap? In many cases this is
going to be more than just simply un-bolting one set of wheels and throwing the
smart new bigger ones on. Unless the wheel/tyre combination you have chosen is
known to fit without any other modifications to your car (like fitting OE
12-inch Minilite-style Cooper wheels to a Mini-originated disc brake set-up),
sure as eggs is eggs, some surgery of some-sort is going to be needed. Be these
fitting wheel spacers to modifying bodywork. If you can't, then you may well
have to pay somebody to do it.
All of
the foregoing can add considerably to the cost of the planned wheel/tyre
change.
12-inch combinations.
Undoubtedly
the easiest to apply from drum braked-cars upwards.
Because
these wheels were designed to fit disc-brake endowed cars, it will be essential
to fit wheel spacers to drum-braked variants. The reason being that the wheels
have a lot of 'in-set' to counter the wider tracking given by the disc brake
geometry. In most instances this is more because of a lack of clearance at the
rear than the front. Just how big the spacer needed is will depend entirely on
the aforementioned in-set of the particular rims you desire or have acquired.
The legion of different manufacturers makes it impossible to list whose needs
what, so this will only be assessable by trial-fitting the wheels/tyres and see
what gives. Alternatively, fitting a set of drums as used on the rear of the
disc-brake cars (part number GBD106) along with their matching longer wheel
studs (part number 21A1278) and retaining screws (SF604121) will negate this
problem entirely and give the car that 'wider', more purposeful stance. It will
also mean wheelarch extensions will be needed, but will complete the
aesthetics. For those that are abhorrent of arch extensions - it's the trial
and error route for you.
The only
other major problem when fitting 12-inch combos is tyre-to-arch clearance at the
leading edge of the front wheel arch. Not a problem when the standard fitment
145/70/12 tyres are used (unless something is out of line
body/subframe/suspension-wise in a major way), but can be an aggravation when
using the more popular, wider 165/60/12 low-profile types. Half the time this
is because the front caster angle is too great, generally caused by bent
tie-rods, worn/collapsed/AWOL bushes, or incorrectly set adjustable tie-rods.
There is no need for more than 3 or 3.5-degrees caster angle on a road-going
car.
The other
major contributor is subframe to body alignment, more prevalent on the later,
rubber-mounted front subframe cars. They are simply not always square and true.
This is why on some later Minis (mainly the 12-inch alloy wheeled Coopers in
the early '90s) have shim-plates between the front panel to front subframe
'tear-drop' mounting. It's not to 'square' the subframe, as some seem to
believe - quite impossible considering the magnitude of the bulkhead retaining
bolts - but simply to space the front panel out away from the wheel/tyre! These
shim-plates are available under part number 2A4292. Using them will normally
solve any slight fouling problems experienced. Having said that - there is a
point of no return - or actually a point where any more shims actually starts
to worsen the problem. As the panel is pushed forward, there is only so far it
can be pushed before the wing/front panel section starts deforming, pulling the
wing in. This causes the arch to tyre clearance to start reducing again.
Where the
165/60/12 tyres are used on the rear, it is generally necessary to fit a 5mm
wheel spacer shim with associated longer wheel studs. Both of which are readily
available (spacer shim part number SR4, and stud part number ENJO1039). And whatever
you do, don't be tempted to fit very cheap 155/70/12 tyres. I cannot describe
how they completely destroy the handling, and aggro caused when trying to fit
some of them to the car.
13-inch combinations.
The
13-inch wheel/tyre combination brings out the worst in everything Mini
suspension and brakes orientated - but to many the aesthetics far out-weigh all
other considerations. And there are many considerations.
Those who
believe they are fitting this combination for increased grip and handling
capability on a road car need to think again. Most of the 13-inch tyres are
manufactured for heavier cars. This means a harder tyre compound is used. A
Mini simply doesn't get the tyre up to its proper operating temperature, so the
increase in grip originally hoped for doesn't materialize or isn't maximised.
Wayward handling (unless suitable suspension set-up has been applied), heavy
wayward steering, and exaggerated bump-steer are further consequences. These
are caused by the offsets employed, necessary to get suspension clearance.
13-inchers are generally wider than standard rims, so the wheels are made with
the greater portion of the extra width applied to the outside of the wheel,
effectively pushing the wheel further out away from the car. This causes a
greater leverage to be applied to the already poor bump-steer geometry of the
Mini. The wider tyre needs different geometry settings to ensure the full
footprint is road-surface bound as the lower, stiffer side-wall does not distort
as easily as the 10-inch tyre types. Excessive camber - positive or negative -
will cause the tyre to loose contact with the road surface when cornering or
with extreme steering in-puts. This then causes the tyre to follow any
deviations in road surface.
The
offset dimensions employed can also mean wheel spacer shims are required in
certain combinations to gain clearance, and even the steering rack may need
replacing with the one used on the Sportpack cars - and these are definitely
not cheap, costing in excess of £100. These have built in lock-stops to reduce
the turning circle to prevent the tyre scrubbing the inside of the rear of the
inner front arch. Again, the types and styles are legion, so doing a
comprehensive and accurate whose needs what is impossible in a few pages.
The
increased footprint increases drag - responsible for reduced top speed, reduced
acceleration, increased fuel consumption (all neatly illustrated by the
difference in performance figures between the Sportpack cars and the 12-inch
shod variants, the 13-inchers suffering 6mph slower top speed and 0.6 seconds
slower 0-60 with less mpg).
This
larger footprint-induced grip/drag increase will help with braking, but only if
the brakes are equal to the task. The wheel diameter also gives a greater
('longer') leverage working against the brakes. The combined larger footprint
and greater leverage means fitting 13-inchers to a drum-braked Mini is an
absolute no-no. They simply over-come the applied friction capability of the
shoes. Disc brakes are a must. Even the 7.5-inch S type discs are a little
marginal in my opinion/experience unless steps are taken to maximise their
performance envelope.
The
13-inch combination is a good deal heavier in most instances than the standard
set-up so up-rated dampers are an absolute must.
Having
considered, dismissed or navigated round the aforementioned problems and are
still serious about fitting 13-inchers, one thing is certain - body surgery
will be necessary. Pictures speak a thousand words so look at the relevant
following diagrams to see what's involved.
Conclusion.
There is far more to fitting a set of aesthetically pleasing bigger wheels and tyres than meets the eye. A majority of which can potentially seriously affect the safety of not just you and your passengers but other road users too. Think long and hard on this before making the change.
 |
Click image to enlarge |
Whichever wheel/tyre combo you plumb for, up-rated dampers are necessary. Although any of the 'usual suspects' are up to the job, the Kayaba Gas-A-Just offers the best performance for the money and as a bonus is also considerable smaller in diameter than the others - giving less clearance problems.
 |
Click image to enlarge |
If current suspension and subframe bushes/mountings are flagging, take the opportunity to fit up-rated ones. Particularly the front subframe tower to bulkhead and front-panel to subframe items. May slightly increase noise transmitted through the shell, but outweighed by elimination of wayward front subframe movement.
 |
Click image to enlarge |
Stating which arches are needed for which wheel/tyre combos is impossible - not only because of the different wheel fitments, but personal taste too. Although it is possible to do the whole wheel fitment thing in a day, it's worth taking your time over to get it right and to your personal satisfaction. I'd love a pound for each person that's been unhappy with a hurried result and 'first guess' arches.
 |
Click image to enlarge |
Rover Sportpack steering rack and standard racks look very similar, but Sportpack one has reduced turning circle to prevent tyre-scrub on the inner rear front wheel arch section. Incredibly, many Mini Parts suppliers recommend - and some insist - this rack is fitted where prospective 13-inch wheels are asked for. The cost price however, is cripplingly and generally this isn't needed unless wheels with a deeper in-set (majority of extra wheel width is moved in towards the car) are used. Far and few between coz the wheel manufacturers just know you lot love wheels that stick out a-ways!
Front arch modifications for 13-inch wheel fitment
 |
Click image to enlarge |
First job is to jack vehicle up, place it securely on axle stands, remove wheels/tyres, arch extensions if fitted and front bumper, then create the new arch profile. Doing this free hand is fine for the artistic/experienced. For those not in either category, it’s advisable to make templates out of thin card or brown paper or such. Start just ahead of the arch peak, and use a gentle curve that passes through the foremost and lowest original plastic arch retaining rivet hole. If not, measure in 1.25-inches from the arch line at bumper platform curve level.
 |
Click image to enlarge |
Once you're happy with your profile, transfer this to the arch itself by taping the template in place and drawing round it. Remove template and flatten the original arch lip using a hammer and dolly preferably (gives a better finish), or Vise grips if not. Now draw a line free hand or stick a strip of 0.50-inch/13mm wide masking tape along the arch side of you new arch profile.
 |
Click image to enlarge |
The section of arch outside this second line needs removing by cutting it away with either a suitable hacksaw or sheet metal cutters. Which ever, be VERY careful not to cut yourself on the very sharp edges. Wearing thick gardening gloves, welder's gloves, or such will help here.
 |
Click image to enlarge |
Reform the arch lip along the new profile line again using hammer and dolly for a neat finish, or vise grips. Once again being very careful not to cut yourself. It isn’t essential to re-form this lip, but it adds a great deal of stiffness to the panel so is highly recommended. If you really don't want to re-form the lip, then the arch needs cutting along your first, new arch profile line. Once completed, grind/file the sharp edge off the new arch profile you've formed. Before going any further, fit the new wheels/tyres, lower the car onto its wheels, and turn the wheels lock to lock to ensure clearance. It's easier to deal with any extra trimming now before the job's finished off. Treat the cut metal area with at least something to stop it rusting, if not re-painting it properly.
 |
Click image to enlarge |
The only way to know exactly how wide the arch extensions you're going to need should be is to fit the wheels/tyres, and measure how far they stick out from the bodywork. Mini Spares Centre/Mini Mania have catalogues that not only show what the arches look like fitted to a car, but also their fitted dimensions. Others, you'll just have to ask.
 |
Click image to enlarge |
Don't forget to do something about existing arch extension holes if there are any. Either by using an arch extension that covers them up, or filling them in. The latter could be difficult/expensive as to do the job properly will require filling the holes in, then re-painting.