Rims & tyres

[SEL_69L]

Styria, as you may remember, I had you raise the ride height on my car by about 20mm about 4 years ago, so that my car would not bottom out over my driveway hump. The wear pattern on my tyres since then is absolutely perfectly OK since then. The outside diameter of the 215 65 15 tyres on my car are within 0.6 of 1 percent of the correct specification for the standard fit tyre of 215 70 14. I have been running the car with 36 psi tyre pressure for the last eight years.

To calculate the OD of a tyre, multiply the tread width by the aspect ratio x 2, to allow for top & bottom, and add then wheel diameter to it.

(215x.65x2 + 15x25.4) = 660.5mm
(215x.70x2 + 14x25.4) = 656.6mm, works out to only 1.95mm difference in ride height

The difference in this case 0.59 of 1 percent, which is negligible and well inside the tolerance for tread wear.

[Oversize]

Tyre sizes specified on the sidewalls are nominal values only & are dependant on the rim they are fitted to, temperature & pressure. The only sure way to determine the OD is to check the NJOY Tyre Fitment Guide. Interestingly it doesn't mention the 6.9 or 215 tyre, rather a basic 450SEL with a 205/70R14 95H/V fitted. The OD of this tyre is 644mm. It lists a 6.5 x 14 rim; were these standard on a 450, or just the 6.9? Options listed include 225/65R14, 235/60R14 & 235/55R15 (obviously on replacement wheels). The Tyre & Rim Association lists a 215/70R14 as having an OD of 658mm. In Victoria the maximum legal change in OD is +- 15mm. The load rating must not be less than stated on the tyre placard 1610lb (97)! Out of interest, has anyone actually measured the wheel arches to find out what could theoretically be fitted?

[Styria]

I am not fully conversant with the limits imposed by body width and suspension components that may be affected by excessive tyre/rim width. I do know that 2.35.40.18s fit quite comfortable, and I think the rims are eight inches wide - I should really go and measure the width, shouldn't I ? Well.....

As you may be aware, I have refitted the standard size 14" Bundts to the rear of Gleaming Beauty, with 2.25.65.14" Toyo Tyres, but I am still running the 'fancy' Speedy rims with the 18"ers on the front. To say the handling is 'different' is an understatement.

Traction in the wet, at the rear, is chancy at best. We have had a bit of rain in Sydney during the last couple of days, and I have come to treat the road surface, when wet, as a bit of an ice rink. Seriously, I could turn this thing around (Gleaming Beauty) 180 degrees in the twinkling of an eye lid. In the dry, the 'chirping' of tyres is very easily accomplished - not that I am all that impressed by that. Admittedly, the Toyos aren't all that new (still plenty of tread, though), but the difference in handling capabilities and safety between the aftermarket set-up and "original" (perhaps XWXs excepted) is remarkable. Presently, I am waiting for the second new spare rim (as illustrated in earlier posts) to be ready for pick-up and once new Nankangs are fitted, that should restore the safe and capable handling. Regards Styria

[John S]

Styria, do you think that the reduced traction in the wet is related to the very wider and very much stiffer 40% tyres you have fitted?

The 215/70 tyres originally fitted allow the tyre to remain in contact while the diagonal swing axel suspension does its job.

[Oversize]

Styria,

I'd suggest the poor handling is due to the fact that you've not only reduced the width of the rear tyres, but also increased the profile of what's on the front. This modification is common with the youth of today for easy drifting & burnouts. The smaller, higher profile tyres are also cheaper to replace...... Did you notice a big improvement in the way your car handled when you upgraded to the 18s all around?

[Oversize]

Scratch that, seems you already hinted to a big yes! In what way was it better??

[BAR]

Forget the calculator: just drop into this web page:

http://www.exploroz.com/Vehicle/Tyres/SizeCalc.aspx

[SEL_69L]

Just make sure that you have the right offset for the wheel you choose, if you are going to change them. With the front wheels, the axis of the kingpin should pass through the centre of the tyre contact patch on the road.

The higher the aspect ratio the more compliance the tyre has, and can offer to keep the tyre in contact with the road but can lead to more case distortion with hard cornering. Obviously, a trade off, and helps you understand where you are in the ride / roadholding compromise.

Larger diameter and wider wheels, although they look better to some folks, tend to be heavier, and can lead to greater unsprung weight, which leads to more work for the suspension and a harder life for suspension bushings.

[Styria]

There is no doubt that these Speedy rims weigh the proverbial 'ton' - honestly, picking one up and fitting it is quite an exercise. Chances are that the suspension has to work harder on account of the greatly increased unsprung weight, but I am prepared to accept that for the sake of, dare I say it, better looks and, in my view, superior road handling and road feel. Regards Styria

[SEL_69L]

Superior handling and road feel is, in my humble opinion, a resultant combination of the ability of the suspension to do it's work and the result skill of the tyre manufacturer to make his procuct perform to the customer's required expectations.