Topic

Sports cars.... bigger wheels, wide low profile tyres, must be the best option, yes?. It is if your car was designed with them in mind. Retro fitting them to a car not originally designed for them can reduce performance, adversely effect stability, handling and can prove to be dangerous.

With the trend for cars to be fitted with larger alloy wheels as standard, which do generally look better, it is very tempting to go down the path of fitting larger wheels to your car. This can however lead to a number of problems.

Warning this next bit needs some time and consideration, leave now if you CBA.

Many of our overseas members, including Germany, Belgium and Australia often comment with some surprise at the freedom to modify we have in this country. In many countries the law says you cannot change overall diameter from original equipment specification. So why do they impose these restrictions?

This extract from a Sumitomo Tyre service bulletin gives a very simplified overview of just a few things people need to consider if thinking of going down the route of larger wheels (Plus sizing);

" To avoid re-engineering the vehicle one must stay within the load carrying capacity requirements, speed-rating requirements, O. D. (or revolutions per mile) tolerances, and weight standards established by the vehicle manufacturer. Other considerations may be necessary if the vehicle is equipped with Tire Pressure Monitoring systems.

Plus sizing may affect vehicle handling in several ways. The effects of higher lateral traction, or quicker steering response, may affect stability issues on dry roads. Also, Plus Sizing may affect vehicle handling, such as drivability in decreased traction conditions (example: wet or snowy roads).

Failure to consider the issues and follow safe guidelines may increase the dangers of reduced stability due to changes in handling, reduced hydroplaning resistance, reduced snow traction, or tire failures from overloading or underinflation.

When considering a plus size application for a vehicle, parameters include the Original Equipment Tire Size, Speed Rating, Load Capability, Inflation Pressures, Outside Diameter (Overall Tire Diameter), and Revolutions Per Mile, as well as the space limitations of the vehicle wheel well. The plus size changeover may be for looks or for performance enhancements. In either case, lower profile tires which have wider section widths, as well as higher performance tires, normally will increase steering response and overall cornering grip since the tire footprint shape (contact patch) is usually wider and shorter than the Original Equipment tire’s contact patch. To ensure proper vehicle operation the following guidelines may be considered in relation to the original equipment tire and wheel package: Overall Diameter–Within 2% of the original equipment fitment; Optimal is within 1% and usually the shorter is better for stability, turning, acceleration, and braking.

Load and Inflation–Ensure the load carrying capacity for the vehicle is not reduced with the new tires and wheels. Calculate the new minimum inflation pressure to maintain the load equal to or greater than the Original Equipment tire and inform the consumer of the adjusted minimum inflation pressures.

Speed Rating–Using equal or higher speed rated tires ensures the durability standards and performance characteristics are not diminished.
Rim Width and Offset–Ensure the rim width is within the rim range for the selected tire size. Also maintain the offset of the original equipment tire and wheel package.
Tire and Wheel Weight–The combined weight of the new tire and wheel package should be near the Original Equipment weight. Increased weight affects the Shocks, Springs, and Brakes as well as other suspension components.

Clearance
Since Plus Sized Tires and Wheels are usually wider than original, consideration must be given to wheel well space and any potential interference with suspension, chassis, or
bodywork. The new tire and wheel package must have ample clearance to avoid the
possibility of contacting another component of the vehicle through turns, dips, and bumps. Tire deflection, lock-to-lock steering, jounce, and rebound all must be considered.

Brakes
Concern for increased weight of the wheel and tire package must focus on the performance range of the braking system. As the rim diameter increases and the tire widths increase, the weight is concentrated farther out on the lever arm in a rotational mass, which increases the stress on the brakes (or decreases their effectiveness). Also of concern is the overall weight of the tire and wheel and the effect on the braking system. Consideration must be given to upgrading the braking system to maintain braking capability within operational limits.

Electronics
Since the ABS systems, ECS systems, engine management systems, speedometer,
odometer, transmission shift points, speed control system, and inflation warning systems use information based on the revolutions per mile of the tire, attention must be given to maintaining a tire RPM close to the original design. Staying within 1% is optimal. Vehicle dealers should be contacted to reset the electronic codes for affected electronics. Of special concern is any tire inflation warning system that may need recalibrating after a Plus Application. Air pressure is critical for optimum tire performance and tire safety.
Clearance for the monitors within the tire cavity may also be of concern. As the air cavity
reduces in height, the clearance to the monitors may be reduced and therefore may be more vulnerable to road impact damage, as well as damage during tire installation.

Location
(distance from the hub) may or may not affect the monitors’ performance. Contact the
vehicle manufacturer for clarification.

Alignment
Since the shape of the tire footprint has been altered (normally shorter and wider than OE) there may be a need to modify or restrict the range of the static wheel alignment within the factory specifications. When plus sizing be aware that the new tire’s footprint shape may require an alignment arrangement which minimizes scrub and/or camber thrust."

If you want to check how different sized tyres compare you can use one of the many online checkers. Click here to view one I have used before..

For more a more in-depth, analytical paper on this topic click on the link for the attached document below.

I would hazard a guess the F originally got 15" wheels down to cost alone..

Smaller the rim.. Generally the cheaper the tyres.. So a low budget roadster would suit 15's perfectly..

I know a fair few have swapped to 17s.. With mixed views about ride and handling..

Only know of Kamelon who's running 18s all round and has now swapped the fronts to 17s due to rubbing issues..

Diesel Destroyer wrote: wrote:

I would hazard a guess the F originally got 15" wheels down to cost alone.........

The Lotus Elise S1 used 15" wheels on the front and 16" wheels on the rear, The Porsche 911 of 1996 was fitted with 16" wheels front and rear, it wasn't down to cost.

Wheels size has increased over the years. Most people think they look good, which is why you see such large wheels on new cars - generally looks. It is a trend that really started about 15 years ago and can be clearly traced. Every year the average new car wheel size gets a little bit bigger.

Economy cars that used to have 13-inch wheels now have 15, 16 even 17-inch wheels. Twenty years ago even performance cars predominantly had 15 and 16-inch wheels, 17-inch was rare and 18 and above - extremely rare. Its mainly driven by aesthetics.

Having moved to 16" 11 spokes, to accommodate 304mm discs, in 2009. Last year bought an Abingdon for its wheels, hoping to restore the ride quality of our VVC. Noticed the the Abingdon ride improved on 15" wheels, when we removed its original 16's!

Having fitted 280mm rears, to compliment the 304mm fronts, we ordered a set of Mike Satur's 280mm fronts. Once we were happy with the 280mm all round braking we switched back to 15" VVC wheels - HEAVEN!

Having fitted lowering knuckles (30mm lower ride) with 4 wheel alignment to MGF Trophy settings the MGF offers excellent ride and cornering - crap handling though - must get around to fitting line locks to isolate the spheres...

One of the potential problems staying with 15"s and even 16"s is that as wheel sizes increase for common cars, there will be less demand for quality performance tyres in these now less popular sizes, and as such there will be little incentive for manufacturers to bother making them.
Ask any Triumph TR-7 or TR-8 owner, it is near impossible to find good performance tyres in the 13" sizes required for these cars now, and the situation isn't much better for those needing14" sizes either these days.
Once there was a huge selection of tyres in 13" and 14" sizes, but now most of what is available is only budget tyres for econoboxes or old bangers, as this is where the demand is in these sizes.
How long before this also applies to 15" and 16" sizes?

One of the first things I did was swap the rears for 16s.. So 16 rear 15 front.. Like the lotus..

I was new on dot org and a few of the self proclaimed experts insisted I'd crash and die as no one else had done it..

The car drove better than before.. But then I had swapped to T1Rs from the ditch finders that was on it..

No one even noticed the difference in size.. The square spokes are the only MG wheels available in both sizes.. And if you look at the spoke thickness you'll spot the difference..

I have run minis on 13s and 10s and prefer the handling on 10s..

I've not driven my F yet on the 18/17 combo.. But worst case scenario I'll fit 17 rears.. I can't go smaller than 17 due to the brakes fitted..

I still think (just me) 15s look like roller skate wheels under an F/TF

My car with 16/15 combo




Loved my 120bhp steppy here.. 2 days of paint correction and some decent wax.. Loved driving it.. Until it ate the CVT belt

Having commented previously on the much firmer ride having gone from 15 to 16 inch rims, I've replaced all 4 shock lower bushes (£18 worth of OEM rubber from Sussex Classic Car (great service)) and suddenly I'm not noticing the firmer ride anything like as much. In theory there can be some clearance issues even with stock rims and also bump stop problems....I've done nearly 2000 miles now on my 'new' rims and have had no issues. The 16 inch rims are 7 inches wide and this makes a difference too to both apperarance and performance. While I can see the attraction to bigger front brakes, I've new Mintex discs & pads allround (OEM size) and that combined with a through clean of the calipers has made a big improvement now that they are bedded in for less than £100 - so the original braking set-up is not bad if it is in good condition.

Richard

Diesel Destroyer wrote: wrote:

One of the first things I did was swap the rears for 16s.. So 16 rear 15 front.. Like the lotus..

I was new on dot org and a few of the self proclaimed experts insisted I'd crash and die as no one else had done it.....

Why should that have made any difference? Your 16" rear with 215/40 (assuming recommended tyre size) would have had only a 3mm difference in radius to the standard 15" rear running 205/50 tyres.

Keeping within ~ 2% of the designed radius will not cause any undue effects.

Problem on forums is if something is repeated often enough it becomes 'fact'.


People also appear to latch on to incorrect ideas a lot better than the truth.
I left a high school group to research the advantages and disadvantages of different ways of generating electricity.

Before I knew it I had a whole class condemning offshore wind farms. Their reason? They Kill Dolphins Sir!

Eventually I traced it back to one bright spark who had found research that a particular species of whelk had had it's numbers reduced within a 2m radius of the base of the wind turbines. He told his friend that wind turbines kill sea life, who told the others that they kill Dolphins. Within minutes 90% of the class were arguing that offshore wind farms must be stopped to save the Dolphins.

Quite agree.. Read loads of stuff on forums over the years which is complete twoddle..

One of my favourites is when people quote like something is fact.. Then you ask if they have ever actually done it.. And they reply.. Er no..

Anyone can guess... Real experiences are best

Nah, real life experience is over-rated. F A C T

I agree with Diesel Destroyer - real experience is valuable, the rest is jaw. Speaking of which, I would still like to know whether my 17" wheels all-round would be better with 215s or 225s on the back as I have 205s all round now. Real knowledge on the matter would be v welcome!

Here is what Rob Bell has to say on this question

"There is a long answer to this very interesting question, but I'll spare you it, other than saying that there are narrower front tyres for a reason - that being chassis tuning.

A narrow tyre will generate a slip angle sooner, and thus will be more responsive to changes in direction, than a wider tyre. It will also go into slide earlier too of course (generating understeer if fitted to the front wheels).

The traction circle of the rear tyres is challenged by the drive being applied through them, so it makes sense to ensure that there is more rubber on the ground for the rear wheels. Also bear in mind that the MGF has a 40:60 rearward weight distribution (unlike most other vehicles on the road that are front-engined).

So in honing the chassis dynamics of the car, the choice of tyre and its properties are one of the things that are considered when designing the suspension. And then we come along, the owners, and fit different brands of tyre and even different sizes of wheels!

Largely, the MG is reasonably tolerant of slightly wider front tyres. But I find the 215 section on the front causes excess tramlining and numbs the steering excessively.

On 15" wheels, with 205mm section rear tyres, I wouldn't really want to go above 195 (OEM 185)
On 16" wheels, with 215mm section rear tyres, I'd aim to keep to no more than 205 at the front.

It is a compromise however, as it depends on what size tyres are available from the brand you are looking at. Goodyear F1 GSD2s of course do come in the correct sizes, but their availability is often limited. Going for the same size all round works, and worked well when I was using Bridgestone S-02s on 16" rims (205 section tyres all round)

BTW for 17" wheels, I can't make any recommendations because I haven't tried them. But keep narrower fronts, and I think you'll find a better balance."