| Tyres - it's a number game... |
| The bulk of this information was shamlessly ripped from a forum thread submitted by Alister "Fliptop" Philpot. Thanks Al, All credit to you |
| Changing tyres is a matter of a) understanding the numbers and b) compromise. Wider tyres would be the natural progression on a performance car (or any modern car nowadays) but it depends on what you want to achieve and what wheels you have as to how far you can go. First, you need to understand two significant numbers on the tyre. The standard tyre is known and, sometimes, marked as 155/15/80. 155 = width = 155mm 15 = wheel size = 15 inch 80 = the height of the sidewall as a percentage of the width. E.g. 80%, in this case 80% of 155mm is 124mm It is important to understand that the sidewall height is a proportionate value and not a definitive as a 185/80 tyre will be much taller than a 155/80. Car Bibles has a really good section on tyres. Higher sidewalls increase the rolling circumference, meaning higher gearing which may offer slight increases in speed but at the expense of handling. A higher profile sidewall is more likely to flex when cornering, giving a 'spongey' or 'washy' feel. The 95 / 96 had two popular rim widths: 4J and 4.5J both should be fine with the limits that the car puts on the width of the tyre but too wide a tyre on the 4J rim may not be such a good idea Lets take a look at the increase on rolling circumference. All calculations are compared against the standard 155/80 at 70mph. Bare in mind that this is largely academic as the tyre squashes under load so it is not an absolutely scientific argument! 155 80 15 = 1976.06 mm circumference = 70mph 165 80 15 = 2026.33 = 71.78mph 10mm wider and a shade taller. Perfectly acceptable increase in width with slight increase in height, meaning a little grippier, a little faster but a little flexy. 175 70 15 = 1966.64 = 69.66 Much wider but slightly lower profile, therefore more contact patch, better handling but a smaller circumference, therefore lower comparative speeds. Seems ideal in terms of grip and handling but expensive and hard to find! 175 65 15 = 1911.66 = 67.72 A nice wide tyre, again, lower profile so handling is improved. Cheaper than the above but much smaller circumference and a much greater strain on the shoulder of the tyre. These are Mini Cooper tyres, easily available and dirt cheap. If you have 4.5J rims then 175/65s will be happier, and you can even go to 185/65 but you have the same issue of sidewall strain as 175s on a 4J rim. 185 65 15 = 1952.5 = 69.17 About as wide as you can go. Loads of grip, not a significant amount of sidewall flex or reduction in circumference. Likely to rub, especially in the rear wings but these can be pulled out a smidge with a gentle tug. Likely to rub on full lock. Best go to a frinedly MOT station who likes 96s! 185 70 15 = 2010.62 = 71.22 Probably wont fit! There is downslide to running wider tyres: Yes, they grip more but they can let go quicker than 155s. The standard tyres tend to be quite progressive in the way they lose grip and you have plenty of warning, the lower profile tyres, because the sidewalls are stiffer, tend to give less warning, and when they do let go you're travelling that little bit more quickly... This is especially relevant in the wet. Oh, and v4s lose their amazing snow-plough capabilities with wide tyres. I am undecided which I will go for. I won't be doing anything until the tyres need changing or I get new wheels but I will probably go for a 175, for road use. Rallying? Not a clue but they will know on Saab-V4 or UK Saabs! |
 |