Wheels & Tyres

I need wheels that are light-weight, becuase wheels are unsprung mass. They also need to be strong enough to cope with both track and typical UK B-road usage. It would be nice if they look good too. I need tyres that provide excellent grip in the dry but also have decent wet weather performance. I'm not planning to be driving this car on the roads in the rain though. When you've paid for track time though, I'll be out there whatever the weather. The tyres also need to be light-weight, again because they are unsprung mass.

Essential Reading:

Chris Longhurst's tyre bible
Fluke Motorsport's table of unsprung weights

Larger wheels often look better but 13" wheels are lighter and hence result in less unsprung mass, which should improve the handling. This is more important to me.

Wheels

The Ford Sierra and Escort hubs are four bolts at 108mm PCD. Typically, Fisher Sportscars

use 6" rims due to racing regulations. They fit 185mm front tyres on the front and 205mm on the rear. Having seen 205's on 6" rims, they don't look right too me and look too bulbous on side walls. On 7" wide rims they look fine.

Bearing in mind my power plant and future upgrades, I'm going to fit 7" wheels on the rear. Using 7" wheels means I will require a different offset to keep the geometry and clearances correct but, at this point I'm not sure what this should be. My thinking is:

The standard fitment is 6" rims with ET18 offset on front and rear. It is the racing regs that restrict people to 6" rims only.
Many people squeeze 205mm tyres onto these rims but they end up being slightly bulbous and stick out about 8-10mm from the edge of the rim. These seem to fit without problems.
A 7" rim with ET18 offset will be 12.7mm further into the car and 12.7mm further out, in effect being only slightly wider than the 6" rim with 205mm tyre on. It will also maintain the same 'balance' as the offset if measured from the middle of the wheel rim.
For SVA, the tyre needs to be within the vehicle plan form. I could borrow some 6" rims and tyres to get though the SVA test.
To clear the brake components, I've had to add a 10mm spacer with my 6" ET18 rear wheels. This means that a 7" rim is going to need a radically different offset in order to fit without a spacer.

Compomotive sell a very nice 13x6" CX-R wheel which weighs in at 4.6kg. It is also available in 7" width, which weighs 4.8Kg. It can be supplied with the required ET18 front offset. With a 153mm clearance radius, they should easily fit over the calipers and disks I'm planning to use. George Polley Motorsport offered the best deal on these wheels and also supplied the tyres fitted and balanced.

With hindsight: I would have liked to have ordered the right rear wheels from day one but I had no idea what was required. Learn from others mistakes!

Tyres

I need 185/60R13 tyres to fit my chosen front wheels and 205/60R13 for the rears. On a BEC car, tyres are inflated to about 16 to 18 psi. Based on my experiences with a Lotus Elise, these sorts of cars are incredibly sensitive to tyre pressures and the ambient temperature has a big effect on the actual tyre pressure. They need to be checked frequently. The tyre size determines the rolling radius, which needs to be fed into my gear ratio spreadsheet to check the transmission design.

I'm fitting Yokohama A048R tyres as they have 95% of the dry performance, 500% more wet grip and is much nicer on the road than the ACB10's. They weigh about 2.5Kg's more than the ACB10's though.

Avon ACB10 tyres don't offer the best wet weather performance and weigh in at about 6.1kg, which is a 1.2Kg saving over the Yokahama A032R's. Caterham seem to be very keen on the Avon CR500 because it is 25% lighter than the Yokohama A032R. Toyo 888's also seem popular lately.

Tyre Sizes

It isn't simply a matter of fitting bigger tyres on the back onto fatter rims! I've got 13 x 6" rims on the rear of the car and I'd like 13 x 7" rims to support wider tyres. I'm not convincved I need them mind, but I'm sure they will look better. They will weigh more. There is another downside though. The increase in diameter will reduce acceleration by over 4%!

Wheel Studs

The studs used on the hubs need to be of suitable length for the wheels and the nuts need to match the wheel design. Standard Escort and Sierra studs are not long enough. Fisher Sportscars

supplied suitable studs for the front and rear hubs, so I didn't have to do any cuttting. They will also fit them into the front alloy hubs for me.

With hindsight: I should have checked the length of the studs required before they were fitted. I ended up cutting them down with an angle grinder, which doesn't leave a clean edge.

Wheel Nuts

The nuts need to match the CRX wheel design and George Polley Motorsport

supplied some with the wheels and tyres. These are M12 with a 1.5mm thread pitch.

With hindsight: I should have bought some open nuts and not closed ones. I had to cut the ends off to get them over the overlong studs and this looks a mess.