Tyre swap - June 2005

Choosing

You may or may not remember from the build diary that my original wheels and tyres were an ebay bargain.  I knew that the Matador MP31's were not going to be great, but I figured that they would get me on the road at minimal cost.  Having now done a couple of trackdays, the limitations of the tyres were becoming more apparent.  Additionally, two of the tyres had always had badly scrubbed inside edges (I guess from some enthusiastic use on the previous front wheel drive car that they had been fitted to).  So, I decided it was time for a new set of boots, so began to do a bit of digging on various forums to help me make a decision as to what to go for.  There seem to be mainly two camps when it comes to kit car tyres:

  1. Use a set of track-oriented tyres that are road legal

  2. Use a set of 'normal' road tyres that happen to suit a kit car more than your average Michelin

In the first category are the following:

Yoko AO48R - great in the dry and OK-ish in the wet, life 2000-4000 miles, 60+

Yoko AO21R - great in the wet, can overheat on a dry track,  life 2000-4000 miles 60+

Toyo R888 - very new so not many users, but generally similar to the Yoko AO48R 50+

In the second category, the following tyres are used by kit cars on a regular basis:

Yoko A539 - generally rated as being OK wet and dry.  Most negative comments I have come across are when they are compared with tyres like the Yoko AO48r's, which is really an unfair comparison in my opinion.  Life 10000 miles, 25+

Bridgestone RE720 - Similar ratings to the A539, 35+

Three things put me off the first three tyres in the list- the initial cost, the short lifespan given the number of miles I am covering, and the wet weather performance.  I therefore decided to go for either the A539s or the RE720s.

While swapping tyres, I also gave some thought to tyre sizes- the Matadors were 185/55R14, but I now had an opportunity to change if necessary.  Some owners, including Jeremy Phillips at Sylva, have fitted wider tyres at the rear.  However, I was concerned about clearance to the rear arches, and to be honest after a couple of trackdays I was fairly happy with the balance of the car.  However, another option was to stick with the same width of tyre but go up a profile to 185/60R14.  This would still make clearances tighter, but had a few advantages:

  • 185/60 tyres seem generally cheaper than 185/55 (which is a fairly rare fitment)

  • General consensus for kit cars is that higher profile tyres work better, for the following reasons:

    • They are typically more progressive at the limit

    • They give some additional vertical compliance

    • Low profile tyres are typically fitted to reduce the compliance in the side wall of the tyre to stop the tyre moving with respect to the rim during cornering.  Given the lightness of most kit cars, this is less of an issue

  • They would fill the arches even more, which would look good!

  • They would give me a small increase in ride height, without changing my coil over shocks (I'm really at the max limit of the shocks at the moment, they could really do with being a half inch longer...)

  • Keeping the same size front and rear means I can rotate the tyres around the car to even up the wear rates

The only real risk with going for the 185/60R14s was that they would foul the rear arches or the cycle wings.  I did some rough measuring, based on the new tyres being approx 9mm larger in radius, and it looked tight, but I decided I could always reposition the cycle wings or slightly trim the return on the rear arches if this was a problem.  I then got some quotes and ended up getting a set of A539s for about 100 (plus fitting)!

Fitting

I got the tyres fitted at a local garage, taking the wheels and tyres in the tin-top just in case I had issues with clearances to the Mojo bodywork.  On returning home, I refitted the wheels.  The only modification I needed to make was to grind off about 5mm of the exposed thread on one of the bonded- in threads on the underside of the right cycle wing.  Before doing this the wheel could be fitted, but the tyre just rubbed the thread as it turned.  A 30 second spell with the angle grinder soon sorted that one!

Standing back to admire the end result, and the car looks noticeably more purposeful, with really tight (but not too tight!) clearances.  The fit of the cycle wings in particular was spot on, but I wouldn't want them any closer...

Driving

You always hear people saying you need to take it easy on a new set of tyres, and boy were the Yokos slippery for the first 20 miles or so!  Turning out of our road, and then slowing for the next downhill T-junction, I applied the brakes moderately, and promptly arrived (and thankfully stopped) at the white line with all 4 wheels locked up!  You have been warned!  The release agent that Yoko use to get the tyres out of the moulds is obviously very slippery...

During the first few miles I was taking things very steady, but this gave me the opportunity to absorb other benefits to the tyres other than outright grip.  

  • Yes, as I had hoped, the ride is noticeably better, with the higher profile tyres taking the edge of sharper bumps and ridges.

  • More surprisingly, the general tramlining and nervousness that I had felt previously on grooved roads and over cats-eyes was also much better.  This was unexpected, and a great benefit.  The construction of the Matadors obviously did not suit the car very well.  One of my theories is that the Matadors didn't like being run at fairly high camber angles (about 2 degrees all round), but that's just a theory...

With a hundred or so miles under their belt, I started pushing the tyres a little more, and they feel fine.  Overall, for a little over a hundred quid, a great improvement to the car, and not necessarily for the obvious reasons!

I'll see how the tyres perform at the limit when I get along to another trackday at some point in the future...

Front clearance to tyre

Rear clearance to tyre

Mojo with new boots

Update: I have now been running the A539s for a few thousand miles on the road, and completed a day at Curborough Sprint Track.  I am still pleased with my choice, on the track the tyres gave good grip, and the rubber was getting fairly warm and sticky over the short course.