Car stuff I learnt: The tyre conundrum

 

In 18 years of car ownership, I never really paid attention to my tyres! I was always happy with the tyres that came with the car, and I subsequently left them in the capable hands of service technicians, replacing them whenever they said I needed to.

My Mini came with run-flats. Being a small car, with no room for a spare tyre, that made complete sense.

But as I started to spend more time on the track, I knew I needed a change.

But which tyres? Should I stick to the options listed on the car door? What about those fancy Michelins and Yokohamas? All track day peeps seem to love them, but I can't get them in the listed sizes. Does this mean I need new rims too? 

What rims should I get? Bigger wheels look cool. And you get fancy rims with lighter alloys. But bigger the wheels, the heavier and slower they are, so perhaps if speed is what I'm after, I need smaller wheels!

Should I buy brand new rims, or second hand, given my mini may not have too many years left in her?

So many questions! So many sleepless nights! So many tyre conversations with track buddies. So many pages searched on google. So much learning!

Answer first: I did not change my rim size and I replaced my run-flats with the amazing Michelin PS4s (pictured above, ignore the rashy rims) in a different size to what was listed on the door. Yes, we can totally do that!

How did I get to the answer? All those sleepless nights spent googling of course!

Here is what I found. 

My run flats were 195/55/16 87V. For those as clueless as me when I first started looking into tyres, this is what these numbers mean:

195 is the "width" of the tyre in centimetres. That is the length of a horizontal line drawn across the part of the tyre that comes in contact with the ground. In general, the wider the tyre, the more grip we get when driving.

55 is the "aspect ratio". This is the height of the tyre's sidewall expressed as a % of the tyre width. Typically the lower the aspect ratio, the lower the height of the sidewall. Lower aspect ratios help with better handling and improved cornering performance.

16 is the "rim diameter". Admittedly I did not change my rim size and hence did not study a great deal about how different rim sizes can affect performance. However, I did learn that larger rims apparently improve aesthetics of the car by reducing the gap between the car's body and the wheel, but smaller wheels are lighter and hence might be faster with better ride quality. People that know about wheels and rims, please feel free to leave your thoughts in comments as I'd love to learn more about the science of rims!

87 is the "load rating" which represents the heaviest load properly inflated tyres can carry, when the car is being driven at maximum speed. Fitting tyres with a lower load rating than what is recommended by the car manufacturer can be extremely dangerous (and I think illegal). The tyres may not be able to handle the weight of the car and result in a blow-out at high speeds. No-one wants that! Fitting tyres with a higher load rating may have some benefits in terms of longevity but does not really have a major impact on a cars performance on or off track.

V is the "speed rating" which represents the maximum speed a car can be driven at for optimal tyre performance. Load and speed ratings go hand in hand. Fitting tyres with a higher speed rating can improve  the car's handling, cornering and stopping performance, but it probably only makes sense when the car has been modified to drive at speeds higher than when they came rolling out of the factory.

Besides the tyres fitted to my car, the following tyre sizes were also recommended by MINI:

Unfortunately, my preferred Michelin PS4s were not available in any of those sizes. But I realised, that changing rims was not my only option. It is possible to fit tyres of a different size to the ones listed, as long the new tyres still fit existing rims.

Tyresizecalculator.com is an excellent website with a Tyre Size for Rim Size Calculator that provides the range of tyre sizes that can fit existing rims. These were my options:

 

I had already decided on Michelin PS4s by comparing various tyre brands on tyre reviews.com and through conversations with people in the know, so it was fairly easy to see which of the above sizes were available in the PS4 range.  

However, I had to pick the size that was right for my car. Changing tyre sizes does not just affect the vehicle's geometry and suspension, but the car’s speedometer and odometer readings and safety controls like ABS and traction control are all calibrated to a specific tyre size. For these reasons, it is usually considered acceptable to change tyres provided the tyre diameter is within -2% to +1.5% range.

 

Given I wasn't in the mood to perform math calculations, Tiresize.com helped me narrow down the choice to 205/50/16 tyres which were only -1.2% lower in diameter to my current run flats - well within the acceptable range. The website also helpfully has a table showing how this new tyre size would impact my speedometer readings.

Wheel-size.com is another useful website that goes one step beyond, providing calculations on how the new tyre size affects the cars geometry, suspension, measurements and performance across a whole range of factors such as steering precision, wet / dry grip, ride comfort etc. 

So that's how I landed on my new tyres! I'm sure I have barely scratched the surface in wheel and tyre science, but hope this post will save you some time when you are on the hunt for new tyres.

xoxo,

Dark Racer.

P.S. A special mention to Bob Jane Hoppers Crossing from where I purchased my brand new shoes! They know their tyres and the customer service was exceptional. I'm never buying tyres from anywhere else again!