Tyre/Wheel optimisation

[veevet]

In a bid to get weight out of my car I'm thinking of investing in some light rims which are made to the buyers spec. In Formula Vee we run an old cross ply control tyre design, 120/590 -15 at the front and 135/620 -15 at the rear. The fronts have an overall width of 161mm (6.3")and a tread width of 120mm (4.7"). The rears have an overal width of 190mm (7.5") and a tread width of 138mm (5.4") according to Dunlop's specs.

Most of us currently run them on 15 x 5.5j wheels because that's what's commercially available off the shelf at the right size. They seem to work fine.

However I can now potentially order any rim width. Dunlop's own website says the optimum rim width for these tyres is 4.5" for both front and rears with the fronts being okay on rims between 4 an d 5.5 and the backs okay between 4 and 6. I phoned Dunlop to ask why 4.5 was the optimum and although helpful they couldn't really throw any light on how they had set the optimum and what difference running on different widths would make. My guess is that when Vee started and used Beetle OEM wheels they were on 4.5" rims.

So the question is if I opt for narrower rims what handling differences, losses or gains might I see. As I can specify a bigger offset I wouldn't have to lose track width, so it's really what will the change to the tyre geometry do?

[RPD Motorsport]

There is a number of issues here... So I will tackle them one by one as best I can with out getting to complex.
Basicly running a wider wheel will increase cornering stiffness and vice versa (where cornering stifness means the slip angle required per unit of lateral force). An increase in cornering stifness will mean you will need to run more caster, more KPI and possibly more camber (dependant on other things). A narrower wheel will do the opposite.
The lower your aspect ratio (ratio of overall width to wheel width) the more 'plunge' you will get. Thats when the tyre trys to roll off the wheel during cornering. The effect is that you will require more static camber, which may be less than ideal on cross ply tyres as a significant amount may be required.
A significant change in width will also change your cold tyre pressures and the time to get tyres to full pressure as a smaller/greater volume of air needs to be heated and expanded.
If your wheel change results in a change in track then it that may or maynot be good. Track width alters 2 things; the roll stifness and the weight transfer in cornering. Wider track reduces both. A reduction in weight transfer is always good (for resons I wont go into here...) but you may need to reduce your roll bar rate if you want to keep the roll stifness the same.
Then there is the fact that a change in aspect ratio will change the tyres spring stifness, which will require damer changes and if the %age change is different front to rear you may cause the resonant modes of the front and rear systems to shift causing very unstable results at speed.
Basicly my advice would be unless you have a good reason to change and what you currently use works - don't change. It effects almost everything else on the car. But if you really want to change, make sure you test it properly after - a day or 2 spent with somone who specalises in chassis dynamics will probably save you more monay and time than you thought possible!

[veevet]

Thanks RPD for the reply. I knew this was probably a complex subject and I'll be honest I can't get my head round all of the issues. I think my take home has to be, if the current set-up works don't change it. The confusion came from Dunlop having an "optimum" on their spec but as they can't recall where it came from I'll keep running at the same width and not lose too much sleep.

[dtype38]

Originally Posted by veevet

However I can now potentially order any rim width. Dunlop's own website says the optimum rim width for these tyres is 4.5" for both front and rears with the fronts being okay on rims between 4 an d 5.5 and the backs okay between 4 and 6.

I've asked a few tyre suppliers about this and they said to ignore the recommended optimum rim width for racing. Seems the published "optimum" is for best overall road performance and for track work a wider rim is better.

[veevet]

Originally Posted by dtype38

I've asked a few tyre suppliers about this and they said to ignore the recommended optimum rim width for racing. Seems the published "optimum" is for best overall road performance and for track work a wider rim is better.

Thanks Dtype38. What threw me was these are race only tyres so I assumed the "optimum" advised by Dunlop was race specific. However I do plan to stick to the wider rims.