Hi. Are larger rims/tyre combination any better for rallying? I currently run 15 inch alloy rims (on my PRC 1600 rally car) so that I can buy near new tyres off the 4wd guys. No major problem with clearance, although it is tightish at the front end. Added advantage is that I have more clearance inside for brakes and lower profile tyres.

Car is run in the Tasmanian Rally Series and the roads over here are fairly/ very good. All the 4wd run at least 15 inch, but is there any reason why 14 or 13 inch would be better on a Datsun 1600 rally car?

I have noticed that the rolling diameter of the 15 inch rally tyres is almost identical to the rolling diameter of some older 13 inch rally tyres (although newer tyres probably have a smaller profile). Apart from the gearing aspect of having smaller diameter tyres (thus helping gearing down the car, so that it accelerates quicker), are there any advantages of 13 or 14 inch or any reason why I should use these over 15 inch?

What do other competitors use? Mainly 14 inch as a compromise? Or are 13, 14 inch used so that small diameter tyres are used to gear the car down further? If so what tyres do you use to get the lowest rolling diameter?

Mark

My view is as follows:

You want the best tyre that you can get that does not compromise your brake choice or gearing.

Good 13 inch tyres are pretty hard to get in a decent pattern.

14 inch tyres, you can get good patterns, have good 280mm front rotors on the front, and with a 4.9 or 5.1 diff ratio, still keep really good accelaration. Heaps of mag wheels avaiable for Datsuns

15 inch tyres are easy to get, however a 15 inch rim with a tyre is a fair bit heavier than a 13 or 14inch equilevant. You can have really big brakes, however any bigger than 280mm and they are serioulsy big brakes. 15 inch tyres have a larger circumfrance and as such reduce accelration. I think ideal diff ratio is then 5.1 or 5.3

I use the Bridgestone steer tyres on the front and the drive tyres on the back using 14 inch rims. These always have plenty of grip. I use a 5.1 diff ratio and think that 15inch rims would drop off accelaration a bit. It would increase top speed, but I have never been flat out in 5th yet, so thats not an issue.

Thanks for your input StanzaRallyMan. I have a 4.9 diff with overdrive 5th gear and yes never use fifth gear. Thinking of changing back to 14 inch for improved gearing.

Can you get a 5.3 ratio, never heard of them?

Anyone with any more input?

IMO (not worth much if you ask Dave... ), the real issue is GRIP.

15" rally tyres generally have a bigger contact patch (widths approx. from 175 to 225) than 14" tyres (widths approx. from 165 to 185) and 15" tyres come in a far bigger range of compounds. I used to run 14" wheels on my 1600 rally car and paid the same price as I do now for 15" tyres on my 180B rally car, the main difference is I can get 4 or 5 different compounds in the 15" tyres (as opposed to 1 or 2 compounds in 14" tyres).

More Tyre GRIP = better acceleration, higher corner speeds and better braking*.

IMO the added grip (especially from better compounds) far exceeds the extra weight of the slightly larger wheel and tyre. Just look at how much tyre compound technology has affect Formula 1!!!


* Assuming all other things are equal.

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Dazda wrote:

"IMO (not worth much if you ask Dave... ), the real issue is GRIP.

15" rally tyres generally have a bigger contact patch (widths approx. from 175 to 225) than 14" tyres (widths approx. from 165 to 185) and 15" tyres come in a far bigger range of compounds. I used to run 14" wheels on my 1600 rally car and paid the same price as I do now for 15" tyres on my 180B rally car, the main difference is I can get 4 or 5 different compounds in the 15" tyres (as opposed to 1 or 2 compounds in 14" tyres).

More Tyre GRIP = better acceleration, higher corner speeds and better braking*.

IMO the added grip (especially from better compounds) far exceeds the extra weight of the slightly larger wheel and tyre. Just look at how much tyre compound technology has affect Formula 1!!! "

Most of that I agree with, however, a larger contact patch, does not always equal more grip.

I used the Bridgestone 14 inch tyres on the front and was told to use the Bridgestone mud tyre on the front. I was very sceptical. The tire is a LOT narrower, however the tyre has MUCH MUCH MUCH better turn in, stability under braking and it has a lot smaller contact patch.

A guy in our car club who runs an evo 6.5 found that a slighly narrower tyre worked better than a real wide tyre. I think it is part to do with the pressure on the road which tends to be a determine the amount of grip/traction.

In relation to diff ratios, for the H190 you can still buy BRAND NEW crown wheel and pinions, and yes you can get 5.1 or 5.3.

Remember this, to increae the torque at your wheels, change your diff ratio!

StanzaRallyMan,

You are right. Your point is more to do with having the right tyres for the vehicle and the conditions - gives you more GRIP. I cannot argue with that at all. This is why the WRC guys are always complaining about not making the right tyre choice.

The problem is in rallying (as you know) it will always be a compromise - one corner will be hard packed (like tar) and the next could be mud... Most rallies are run in generally dry conditions (now that I said that the next 20 rallies I run in it will piss down with rain) hence my comment about bigger contact patch.

For the record, I too use (and always have used) 'steer' tyres (mud tyres) on the front of my RWD rally cars for better 'turn in'. There are compromises with that too - less braking grip, less mid-corner grip, etc.

My point to mkyle was that there is a far greater choice of brands, compounds, widths, profiles, etc with 15" tyres. IMO, this gives you a better chance of getting the right tyres for the car and the given rally conditions - therefore more GRIP.



Sean,

I was taking GRIP in terms of 'the ability of the tyre to GRIP the road surface'. In rallying this means with more sliding than on tar - obviously. At this point it starts getting into 'slip angles' and a whole lot of stuff I haven't read enough about yet, sorry. Beef, thehelix, Baz, etc can probably shed lots of light on the matter for you.

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It really comes down to what suits your car/budget/driving style.
when 2wd was supreme the best tyre available(IMHO) was the dunlop SP52R with soft compound.(195/70/14) the only tyre we found that had enogh grip to get the power of the stanza on the ground.
Speaking to Russel Stukey on this subject a few months back, he lamented the passing of this tyre.
the choice now is up to you.
15" will give you the best choice of rubber.
If you want the "best" then be aware that we used to change tyres every 80 K competitive.
The softer compounds are generally the best, but also the most costly to run.
Like Dazda, i too prefer muds on the steerers.

http://www.stuckey.com.au/tyres_details ... ame=SP85-R
check these.

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Thanks guys. You have all been very helpful.

I have an R180 diff with 4.8 LSD. Any idea on the possibility of getting a 5.1 or 5.3 ratio to suit my LSD and casing? And if so how much and if anyone knows of any for sale. Or do I need H190 to get to 5.3? How much for a H190 with LSD?

Thinking about staying with the 15 inch rims, but trying to get the highest ratio possible.

Thanks Baz, re Stuckey tyres. Very interesting re outer diameter of tyres. Not much between 14 and 15 inch. 4% gear down if I went 13inch 175/70/R13, but probably wont clear the brakes.

You can definitely get 5.1 Crownwheel and Pinions for R180s. I know, 'cause I have one. I actually ordered it from Nissan, but am unsure of there availability now (it was 2 years ago when I got mine).

I have run 15's with a 4.875 and 5.1 diff ratio in my 180B, and enjoy the grip that "new" technology tyres provide. The selection of tyres (both new and secondhand) that are available make it worthwhile for me to stick with 15's.

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