BMW Buyers Guide - Wheels and Tyres
Why Are Wheels Different Sizes?
Different wheels suit different cars. Older BMWs, such as the 2002, came with 13" steel or alloy wheels to suit their small wheel diameters and the higher profile tyres of the 1960s and 70s. The E30 three series was supplied with 14" rims which became 15" on the E36, 16" on the E46 and 17" on the E90. The trend is to larger wheels, 18" are now the norm with 19" on some //M models.
So why the increase in wheel size? Tyre technology and fashion are the driving forces. In the 1970s even supercars like the Lamborghini Countach only had 14" wheels, most Ferraris had 15". As tyre design has advanced lower profile tyres with stronger sidewalls have become available. Overall wheel diameter is still broadly the same but modern wheels with low profile tyres and larger metal rims offer a lower "slip angle" due to the lower flex in the wheel as a whole, this improves steering response.
One problem with low profile tyres is that because they offer less sidewall flex the driver gets less warning when the tyre is about to lose grip and start sliding. Higher profile tyres give more feedback before breaking traction. So a 17" wheel and 40 section tyres may offer more ultimate performance but just be sure you know what happens when you lose traction with little or no warning.
So the difference between 15, 16, 17 and 18" inch wheels is basically due the profile of the tyres you want to fit. Bigger wheels also let you fit larger brakes, you must use 18" wheels on an M3 for example.
As a general rule 15" wheels are fine for most older BMWs, 16" are a worthwhile upgrade for enthusiastic drivers who want faster steering response and better handling. On more modern cars 17" are really all you need, anything bigger is just for looks.
With 18" or larger wheels you'll feel every bump and learn a whole new respect for potholes. The trouble with large wheels is small tyre sidewalls, it's very common to find curb rash on larger wheeled BMWs because the sidewalls offer no protection. If the wheel gets damaged moisture gets into the cut and the clearcoat can start to peel. The tyres for these larger wheels are progressively more expensive too.
One big downside of larger wheels is that they also tend to have wider tyres. In the UK this is why the country grinds to a standstill at the first snowflake. I've overtaken a Range Rover that was stuck in the snow with all four wheels spinning and diff locks on, I was driving my E36 328i touring with snow tyres. It was on silly wide summer wheels and had no grip. You don't need anything wider than 225s on most BMWs, 235 on the larger cars at a push. Anything more than that is going to have horrid grip in the wet or on snow and ice. In the rain it'll aquaplane sooner than a narrower tyre.
Some BMW owners have separate sets of wheels for road and track use. This give you practicality for everyday use and saves the real performance for the place you can use it. Others, like me, run nice alloys in the summer and an older set with snow tyres in the winter.
Tyre Types
All tyres have a rating printed on the sidewall such as 225/50VR16. This is decoded below.
|
225 |
50 |
VR |
16 |
| Tyre width in mm. | Sidewall profile. Here the sidewall is 50% of 225mm or 112.5mm | Speed rating. BMWs need H,V or Z. The higher the better. | Diameter of the steel/alloy section of the wheel in inches. 390 is a size in mm used on TRX wheels. |
The total diameter of the wheel above is (16 x 25.4 ) + ( 225 x 0.50) x 2 = 631.4mm.
You can use the JavaScript calculator below to compare the sizes of two wheels, very handy if you're thinking of an upgrade.
Don't forget that as tyres wear the wheel diameter will decrease by up to 14mm. This can be a problem on 4x4s as it causes the wheels to turn at different speeds and can wear the transmission. Some manufacturers recommend replacing tyres in sets because of this (expensive) so check before you buy.
If you decrease the overall wheel diameter by fitting lower profile tyres you are effectively increasing the final drive ratio of the car and improving low end acceleration a little. This has the side effect of making the speedometer read high though and decreases cruising fuel economy.
Running very wide tyres isn't a good idea. The weight of the car is spread over a larger area resulting in less pressure for a given area of tyre. This causes the car to aquaplane at lower speeds in the rain and have problems cutting through ice and snow in winter.
Z rated tyres tend to have stiffer sidewalls than V rated and handle better. Avoid remould tyres as they don't last. The previous owner of my 635CSi fitted remoulds to the back end, they handled dreadfully and only lasted 9,000 miles! The front TRXs hadn't worn nearly as much in the same period.
Always ensure your tyres are at the correct pressure. Incorrect pressures result in poor fuel economy, longer stopping distances and increased or uneven tyre wear. For enthusiastic drivers a few extra psi won't hurt, just don't over do it. Don't forget to check the spare either, especially if it's under the boot floor like the Z3.
It's easy to forget about winter conditions.
Snow Tyres
Rear wheel drive cars need a little help in the winter, especially if they run tyres wider than 200mm. I used 195 section snow tyres and steel wheels on my E30 318iS in the winter instead of the normal 205 section alloys.
So what makes a snow tyre different? Normally they are narrower to increase contact pressure on the ground. They use a softer rubber compound which is more pliable at low temperature and can dig into the road surface, this also makes them wear terribly in warm weather. Lastly, snow tyres have more open tread patterns and deeper tread to disperse water and grip through snow.
Snow tyres have a lower speed rating than normal tyres and tend to have softer sidewalls which spoil the car's handling. I can't wait to get mine off every spring but the difference in stopping distance and traction is worth it.
Always fit four snow tyres to a rear wheel drive car, never two. Getting an old set of used wheels is a good idea because it saves your nice alloys from the salt and muck. You also don't have to pay someone to swap your tyres twice a year.
Tempted by xDrive or a 4x4? In most cases snow tyres are more effective, trust me - I live in a ski resort. Four wheel drive helps you get going and has stability advantages when moving but it won't help you stop. Four wheel drive and wide, worn summer tyres are no match for rear wheel drive and the right winter tyres.
TRX Tyres
In the 1980s BMW fitted tyres known as TRX to some higher performance models. TRX were advanced for their time and used a special design of wheel and tyre lip to hook the tyre to the rim for improved high stress cornering performance. To avoid the dangers of fitting normal tyres to a TRX rim they were all made in metric sizes which only a TRX tyre would fit. Only Ferrari, Jaguar and a few other manufacturers used TRX and it became the Betamax and 8-track of tyre design.
415mm TRX alloy wheel.
TR means "Tension Repartie" (spread tension). Michelin's design called for a shallower rim that allowed freer movement of the tire sidewalls under compression but provided a progressively increasing lateral stiffness during cornering. This was thought to be a boon for the high-end luxury-sporty car market as it in theory would allow for a more comfortable ride without sacrificing cornering ability through controlling the slip-angle with sidewall stiffness. In essence BMW thought they were providing the ride quality of a 70-series tire with the handling benefits of a 60-series profile.
Modern tyres have better tread patterns and rubber compounds than TRX. TRX are also quite expensive and harder to get hold off than modern tyres. When the TRXs on my 635CSi wore out I bought and refurbished some used 15" cross spoke alloys and bought modern tyres.
TRX tyres may be spotted by a designation of 220/55VR390 or 415 on the tyre sidewall.
Buying Wheels
You tend to get what you pay for where wheels are concerned. Cheap wheels will have a cheap finish and poor quality alloy as a rule. With 18" rim this results in a wheel that buckles and chips easily. If you must get new wheels either pay the money for good ones or get a proper BMW used set. BMW owners magazines like the Roundel or BMW Car have good classifieds sections with used wheels and ebay is full of them.
I bought used 15" BBS cross spokes for my 635CSi for 35 GBP per wheel. After a good clean I decided to respray a few of them. I bought the correct silver paint from the dealer and applied a few light coats after 1000 grit wet and dry paper. I then used a few coats of clear lacquer and voila! BMW dealers will sell you stick on centre roundels cheaply to finish things off.