zx Tyres?? 185/195?

robsons · Post by **robsons** » 06 Jan 2006, 23:14

Yup I have TD Volcane

Totally standard at the mo but it I get 1 bar anyway (straight from granny first owner). Rear axle just gave up on me at 100K and I've found a company downsouth who'll recon my axle (would be straight swap but they need the Volcane torsion bars). I'll let you know how I get on.

Despite 100K, weeping matrix and knackered rear arms I'm keeping the car as it handles so sweetly. If anyone's driven the std ZX, drive a Volcane, they're so well set up as standard. Very little understeer and can be easily provoked into oversteer with a lift off on a corner. I'd say it's pretty neutral handling and fun to play with on the edge.

There was a huge difference changing to the 15" wheels although I'd opt for 185's instead. I only got 195's as they came with the wheels. http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... B:RTQ:UK:1

sooty · Post by **sooty** » 07 Jan 2006, 19:24

Robsons,

With these tyres your speed will be more than 3% faster than your speedo, to be equal to the 185/60 x 14 you should use 195/50 x 15 then it will be 0.3% slow in other words the same.

So watch out when passing speed cameras

robsons · Post by **robsons** » 07 Jan 2006, 20:48

You're quite right (useful site http://www.miata.net/garage/tirecalc.html )
Not too worried as speedo's always overread anyway.

As for understeer you really need to understand car dynamics. inside wheel lift is more due to too much anti-roll than anything else. The car is designed to understeer (because it's safer for the novice) so there's quite a stiff anti roll bar up front.

When you corner, the car will roll around the roll centre. Weight will be transferred to the outside of the car and the inside will lighten. This is made worse by the anti-roll bar which will pass some of this force to the inside wheel and try to lift it some more. The result is the front end will drop in height but the net effect is there is less force on the inside wheel than with no anti-roll bar. The stiffer the bar, the more wheel lift.....And the less grip at the front. (You lose more grip from the inside wheel than you gain from the outside wheel) ....so the car understeers.

Now if you stiffen up the rear (roll) you also lose grip on the rear and the car is balanced. You get LESS grip but it's balanced. Put wider tyres on it and you rectify this ( up to a point)....(The Volcane)

It all get a bit complicated after this as the more the car rolls the more the tyre is presented to the surface at an angle

....If you are really interested in setting your car up properly check this site. http://www.dsm.org/menu.epl?item=352

It's all physics but it's very interesting

AndersDK · Post by **AndersDK** » 08 Jan 2006, 11:54

Speedo's have to overread by regulations in most countries. They always overread by at least 6-10%.

Right now I'm running 185/70 (winter) on my BX and the speedo now is spot on. Meaning I can forget about driving with needle over the mark to "compensate" as I used to.
I know because of those warning readouts found on the entrance of every small village here (city speed limit of 30mph).

jeremy · Post by **jeremy** » 08 Jan 2006, 17:08

Ah yes but are those roadside speed indicators accurate or do they underread by the same amount?