I thought I posted this reply a day or so back, but it didn't show - I have been working long hours, so maybe I forgot to hit send or something. Anyhoo, here is the post: I'm simplifying the tech stuff below - it's 'right' there's just a few minor other issues which affect overall grip (tyre hysterisis for one) but relatively speaking it is minor, and the 'main' issues are hte ones I mention below. believe it or not, tread width isn't the big deal here. It's friction co-efficient of the tyre tread compound (apart from chassis/suspension tweaks to maximise downforce/grip) Here's the deal - IF (and it's a big if) the tread compound is identical - then a 235 will provide exactly the same grip as a 195 width tyre. The reason is simple enough. Grip is a result of teh combination of 2 things - downforce and friction co-efficient. If you have the same tread compound, but halve the width, then there is double the 'load' on each square cm of tread, so it basically has the same grip. Obviously you can get to a thin enough tyre where it'll just overheat too quickly. And it'll also wear out quicker (since at half the width, it'll have half as much tread area to wear down). Since thinner tyres will experience that, very generally, the tyre makers run harder compounds in thinner tyres, and can 'get away with' softer compounds on wider tyres and still have acceptable tyre life. So in general, wider tyres 'might' grip more, but it's only because they were made with softer grippier tread, not just due ot the width all by itself. The reason I mention this is pretty simple - it is probably now obvious - it's not so much a matter of finding 'a' tyre in 225 or 195 or whatever the widest width is, the REAL trick is to look at each individual tyre in the sizes that will fit, and find the ones with the stickiest tread compound. Now, even people who compete in short sprint races can overcook their tyres, so you have to try and find a balance between tyre life, the traction you need, and how long they will be good for 'thrashing' around before they start to go off a little due to overheating. As a good example - this is why the smaller cars (relatively speaking) tend to do so well compared to the big ones in Improved Production - the tyre sizes are too narrow (relatively) for the bigger cars and they overheat after a couple of laps, so the drivers have to nurse them a lot more. They can't 'really' use grippier compounds, because the tyres/compound are a regulated item. But on a streeter, you can find a happy medium (if you are willing to go through tyres perhaps every 5-10,000km) So now on to chassis/suspension. One of the critical things (aside from axle tramp, and there's plenty of discussions on here about caltracs/anti-tramp bar fabrication and fitment) to do is to make the car's intertia help to push down hard on the rears when the car launches. TO do this, you need the front leaf pivot point to be higher than the axle centre line. then the leaf moves slightly 'uphill' toward the front. This is important because when the axle is pushed forward (as you launch) it'll try and 'lift' the car (since that slight angle translates into a slight upward force). So if it is trying to lift the car, then it has to do it via 'pushing down' on teh rears. The car won't (usually) actually lift up at the rear, instead as the rears push forward, the inertia of the car body/chassis tries to push rearward, and so this then pushes down harder still on the rear tyres. The car itself won't (when setup right) massively lift or squat down at the rear but the FORCE applied pushing down on the rear wheels will be maximised. The reason i bring _that_ up is that often if a car is lowered, you can end up with the axle centre line being higher than the front leaf pivot point. If that is the case, instead of it 'increasing' downforce, it actually 'unloads' the tyres and makes it easier for them to lose traction and continue not to grip. So you need to either have the car ride height set high enough at the rear to allow the front leaf pivot point to be higher than the axles, or if that is too high, you'd have to look at shifting the front leaf attachment/pivot higher up in the chassis (not exactly an easy option a lot of the time). At the very least, keep 'lowering blocks' well away from it. Aside from the lowering blocks setting the axle centreline too high, the further you raise the axle tubes up away from the leaf springs, the more 'leverage' the diff has to twist and wind up rthe springs, so it'll also increase axle tramp (which itself will compromise launches). The same thing applies to various 4 link/control arm rear suspensions, the math angles are perhaps a little different, but the overall principle applies. so as bad as it sounds, often the loss of traction is just as much to do with a car being 'too low' and ending up with undesirable suspension geometry as a result. When it comes to circuit racing, you'll find that you might end up wanting it a little lower than would be ideal for straight line, and as a result you'll get good cornering/mid corner speed, but might start to compromise corner exit speed as it just won't grip enough to leave the corner hard enough. TO that end, obviously you'll have to test and come to the best combination of mid corner vs corner exit speed and straight line grip off the mark. For most circuit racers, grip off the line isn't as important, as you only do that once - at the race start, so a lot of them sacrifice their launch potential for better lap times for the rest of the race. Obviously you can also look at stuff like less rear sway bar and or thicker front one (if they are fitted front and rear) to try and get better grip out of the corner (but it might affect turn in a little - hey everything on a performance car is a combination of compromises!). Additionally, you could also look at adjustable rear shocks, and in particular (if it is still too slippy out of a corner) look into softening the rear rebound damping. This will tend to make the car get more grip out of the corner, but ironically because it grips better and doesn't mildly oversteer out of the corner as much, it can actually 'feel' slower. But it will be quicker lap times (obviously experiment to find the ideal setting). What else - I highly recommend 'tune to win' by Carroll Smith. It discusses car suspension design and optimising for racing. Admittedly it is a few years old now, but 99% of the info in there is absolutely gold. About the only thing it doesn't have in quite the same detail as we might like is shock absorbers (it does discuss them pretty well though) and this is simply because at the time the book was written, fully adjustable shocks existed, but were miles away from the current stuff. Also - the tyres he discusses were race tyres of the era, and differ from street oriented radial tyres. The general info is all accurate, but the inflation pressures are night and day - street radials don't grip better with low pressures, etc. I'd actually recommend all of Smith's books. Drive to win is all about how to get sponsorship etc, as well as how to drive. It's also amusing in some parts. Tune to win is everything about setup/adjustment for optimal track times. Engineer to win is about race car construction - various metallurgy stuff, relative strengths/thicknesses of mild steel vs chromoly (for example, chromoly is stronger so thinner wall thickness can be used, but when it is in a collision, it is more likely to break and leave sharp edges, but mild steel will tend to flex/bend instead. So it can actually be safer, albeit heavier). He also did a book on nuts bolts fasteners. I've got the books. Someone mentioned to me that tune to win might be available as a torrent on bitsnoop, but I haven't looked and I absolutely am not suggesting or condoning any illegal download. Apart from it being 'unfair' in a general sense, I honestly reckon the books are so packed with info, that they are worth every dollar ten times over. Maybe someone out there might (hypothetically, but not in the real world) download the torrent, just to check it out, and then are so impressed by it, they purchase the book(s)

jmac wrote:

believe it or not, tread width isn't the big deal here. It's friction co-efficient of the tyre tread compound (apart from chassis/suspension tweaks to maximise downforce/grip)

Here's the deal - IF (and it's a big if) the tread compound is identical - then a 235 will provide exactly the same grip as a 195 width tyre. The reason is simple enough. Grip is a result of teh combination of 2 things - downforce and friction co-efficient. If you have the same tread compound, but halve the width, then there is double the 'load' on each square cm of tread, so it basically has the same grip.

Obviously you can get to a thin enough tyre where it'll just overheat too quickly. And it'll also wear out quicker (since at half the width, it'll have half as much tread area to wear down). Since thinner tyres will experience that, very generally, the tyre makers run harder compounds in thinner tyres, and can 'get away with' softer compounds on wider tyres and still have acceptable tyre life.

So in general, wider tyres 'might' grip more, but it's only because they were made with softer grippier tread, not just due ot the width all by itself.

The reason I mention this is pretty simple - it is probably now obvious - it's not so much a matter of finding 'a' tyre in 225 or 195 or whatever the widest width is, the REAL trick is to look at each individual tyre in the sizes that will fit, and find the ones with the stickiest tread compound.

Now, even people who compete in short sprint races can overcook their tyres, so you have to try and find a balance between tyre life, the traction you need, and how long they will be good for 'thrashing' around before they start to go off a little due to overheating. As a good example - this is why the smaller cars (relatively speaking) tend to do so well compared to the big ones in Improved Production - the tyre sizes are too narrow (relatively) for the bigger cars and they overheat after a couple of laps, so the drivers have to nurse them a lot more. They can't 'really' use grippier compounds, because the tyres/compound are a regulated item.

But on a streeter, you can find a happy medium (if you are willing to go through tyres perhaps every 5-10,000km)

jmac wrote:


so as bad as it sounds, often the loss of traction is just as much to do with a car being 'too low' and ending up with undesirable suspension geometry as a result.

choppedsprings wrote:
or you could just buy a factory 4wd sunny.otherwise known as nissan GTiR

tappet wrote:
CA18 powered datto picking the wheels up in a huge way on a 205 radial and H patteren and still running a 10.4

jmac wrote:
I'm simplifying the tech stuff below - it's 'right' there's just a few minor other issues which affect overall grip (tyre hysterisis for one) but relatively speaking it is minor, and the 'main' issues are hte ones I mention below.

believe it or not, tread width isn't the big deal here. It's friction co-efficient of the tyre tread compound (apart from chassis/suspension tweaks to maximise downforce/grip)

Here's the deal - IF (and it's a big if) the tread compound is identical - then a 235 will provide exactly the same grip as a 195 width tyre. The reason is simple enough. Grip is a result of teh combination of 2 things - downforce and friction co-efficient. If you have the same tread compound, but halve the width, then there is double the 'load' on each square cm of tread, so it basically has the same grip.

Obviously you can get to a thin enough tyre where it'll just overheat too quickly. And it'll also wear out quicker (since at half the width, it'll have half as much tread area to wear down). Since thinner tyres will experience that, very generally, the tyre makers run harder compounds in thinner tyres, and can 'get away with' softer compounds on wider tyres and still have acceptable tyre life.

So in general, wider tyres 'might' grip more, but it's only because they were made with softer grippier tread, not just due ot the width all by itself.

jmac wrote:

Now, even people who compete in short sprint races can overcook their tyres, so you have to try and find a balance between tyre life, the traction you need, and how long they will be good for 'thrashing' around before they start to go off a little due to overheating. As a good example - this is why the smaller cars (relatively speaking) tend to do so well compared to the big ones in Improved Production - the tyre sizes are too narrow (relatively) for the bigger cars and they overheat after a couple of laps, so the drivers have to nurse them a lot more. They can't 'really' use grippier compounds, because the tyres/compound are a regulated item.