Renault 5 Turbo Owners

It's difficult to drive a Renault 5 Turbo in high heels...

ROFL!

and you know this from experience?

I dont have one but a mate has a fast road one with 235bhp+ dyno'd

Yeah, everyone told me drag racing was fun. Let me tell you - it aint much fun putting all that makeup on.

0-60? anyi ideas?

I wouldnt like to speculate, in a little fwd car with a sodding great turbo on it, I expect around 6 seconds.

And a half. I'm sure I've read it's nearly impossible to get the power down to get a launch much better than 6.5 seconds to 60 with a FWD car without extensive diff / traction control trickery.

Saab does a 5.5 with an open diff and only minor launch control

On slicks mind. Your T16 has a rather better front suspension set up than most FWD cars too. In fact, I can't think of another double wishbone FWD car.

I've just had a look around the web and found a site that says they are

115bhp/860kg standard.(133bhp per tonne). That explains why my project car (188bhp) outdragged one once.

Mind you 235bhp/860kg = 273bhp per tonne which is 85bhp per tonne more than mine. Bugger .... can't fit a turbo in mine either because the bays too cramped.

What if the front wheels were 12 inches wide and 20 inches diameter .... that would increase grip, but also wind drag.

Obviously the gearing would go wrong, so maybe fitting a low ratio gearbox from a 1 litre car would restore that side of things??

No it wouldn't. You'd need really sticky / soft tyres to increase grip. The only reason wider tyres are grippier is because they have softer compounds. Narrow tyres don't have the same compounds because they wouldn't last 5 minutes. Anyway, you'd still have to deal with weight transfer, which is the main reason FWD cars end up scrabbling for grip.

Grip isn't directly proportional to tyre size, as such, it depends mainly on the weight pressing on the tyre. Detailed stuff below, if you're interested.

Given lots and lots of torque, the gearing becomes less important because you can get good acceleration whatever the ratio happens to be. Conceptually, you can put down more torque than the tyres can pass through to the road so a low / close ratio gearbox to increase torque / power for a given range of speeds, won't help a lot and will mean changing gear more often than you otherwise have to.

Moving the driven wheels to the back immediately increases driven wheel grip, though, because the car will transfer weight from front to rear when accelerating (and vice versa under braking) so all else being equal, a rear drive car can accelerate faster than a front drive can, since the harder you accelerate, the more grip you get from the driven wheels rather than the *less* you can get when accelerating more quickly in a FWD. This is because you are increasing (or decreasing) the weight on the driven tyres and this translates into the amount of grip you are getting from them.

Ok, the detailed stuff about tyres is here, skip this if not interested.

First thing to identify is coefficient of friction. In an idealised physics situation, the force due to friction is independent of the contact patch size, it depends only on weight. Real world can vary somewhat from idealised physics but the principle is still generally true and increasing the size of the contact patch does not begin to start making the frictional force higher, even though larger patches may involve higher grip in some cases for other reasons.

To look at this in more detail than that, wider tyres don't actually increase the contact patch anyway. The tyre on a car deforms due to the weight on it, put a ton of lead on the bonnet and the tyres will flatten down more than if you don't. This actually changes with tyre pressure, so that the contact patch size varies to spread the weight over the area needed for that pressure.

To put this in maths terms, a tyre that is at 24 pounds per square inch (PSI) and carrying a tonne and a bit of car, i.e. 2400 pounds for the sake of it, spread equally on all four wheels means the tyre is taking 600 pounds of weight at 24 pounds per square inch and producing a pressure on the ground of 24 PSI so this weight is spread over a patch 25 square inches in size.

If the tyre is five inches wide, it will produce a contact patch that is five inches long (this is using a square example, the real shape will be more oval but that's harder to visualise).

Increase the tyre width to ten inches, and it won't flatten down nearly so much, resulting in a contact patch that is ten inches wide, but only 2.5 inches long. Each square inch of contact with the ground will still be at 24 PSI because that's the pressure of the tyre and weight of the car.

You can lower the tyre pressure to increase the area of the contact patch, e.g. dropping the above tyre to 6 PSI would mean the five inch wide tyre goes completely flat and rests on the wheel rims, while the ten inch tyre will now have a square contact patch 100 square inches in area, ten by ten. But as mentioned above, this doesn't in itself increase the grip.

These tyres would be better in mud / sand, though, because the 6 PSI is low enough not to cut through the mud and get bogged down.

What you can now do, though, is use a material with a higher coefficient of friction. This is generally by using a stickier compound that wears out faster, but since you have larger tyres, they can do about twice the miles before they wear out compared to tyres with less material, so you can increase grip by enlarging the tyres and lowering the pressure, until you get to the sort of tyres that they use in Formula one.

Low pressure isn't all that good a thing, though, if you have ever driven along the motorway with tyres that are under working pressure, you'll have noticed they get hot, and this heat energy is coming from work done by the tyres flexing, energy supplied by the engine. Pump the tyres up first, and they will run cooler and with less rolling resistance. Hot tyres at low pressure is good for grip, but shortens the tyre life considerably and wastes energy (power) from the engine. So you can gain grip at the expense of less acceleration and greatly shortened tyre life, and you will find grip / ride is poor until you get the tyres up to working temperature (fine on a track, not quite so handy on a winter morning).

There's another consideration well worth taking into account, what happens in the wet? The contact patch shape and size depends on the width of the tyre and the pressure in it.

In the wet, the grooves in a tyre must drive water out from the middle of the track to the edges, so that at the back of the contact patch, the tyre is making contact with the tarmac and therefore providing lateral / longitudinal grip, otherwise you are aquaplaning with no real grip.

The wider a tyre is, the further your grooves must move the water to get it away from the middle of the tyre at the leading edge. Doubly so, if you have increased the width and left the pressure the same, the contact patch has actually got shorter. Therefore, as well as having to move the water further, you have to do it quicker so there is some part still in contact at the back.

All this combines to make tyre choice quite important. As a rough guide, many cars are shipped with sub-optimal tyres and increasing the tyre width typically gives a better performance in the dry with not too much harm to the wet performance. But some are already on the best rubber and there is a definite optimum that going beyond will only harm performance in dry and especially in the wet.

407.

That 450-500bhp Meastro Turbo that holds (held?) the FWD record at the Pod has no rear suspension to deal with weight transfer, so the back can't bog down. Didn't a Ti beat its record tho? Or did it beat the Ti that held the record?

180bhp can be got from an NA Saxo VTS motor, that fits into the AX bay ;)

the maestro is the faster, i dont think it had 500hp tho,

I nearly went that route ... then decided the insurance would be so much i wouldn't want to insure my seven series as well.

So i just stripped the ax bare, fitted a close ratio box (from a pug 205xs) and went to town on the alloy ax engine. Mind you, i prefer the sound of a bmw engine at 6000rpm, to that of a renault engine at 8250rpm. Plus the bmw engine is designed to do that continously ... where the tu3s ax engine wouldn't last more than a day.