Tyre sizing - "Aspect ratio" - going up a size

[Dominic TVRetto]

I am running 18's on the back wheels, currently with Eagle "Assymetric 3" 235/40/18 tyres on.

I am looking at Rainsport to give a little bit of extra compliance through the softer compound, but am wondering whether a little extra sidewall would bolster this compliance further...

Comparing against the OE fitment (225/50/16), the current sizing 235/40/18 will be doing 72mph when the OE would be doing 70mph - but moving up to 235/45/18 would be doing nigh-on 76mph when OE would be doing 70...

Speedo may overread (not guaranteed) towards 70, which the larger tyre radius may mitigate, but means under-reading at slower speeds etc

Does anyone have any experience of running oversize tyres? Especially with regards to the rainsport softer compound and tyre compliance...

Just fishing for thoughts from those who have done it - is it best to keep the difference smaller than the ~8% increase proposed, or in day-to-day driving does the extra increase not amount to much, etc..?

Any thoughts welcome...

Thanks!

Dom

[Kawasicki]

Dominic TVRetto said:

I am running 18's on the back wheels, currently with Eagle "Assymetric 3" 235/40/18 tyres on.

I am looking at Rainsport to give a little bit of extra compliance through the softer compound, but am wondering whether a little extra sidewall would bolster this compliance further...

Comparing against the OE fitment (225/50/16), the current sizing 235/40/18 will be doing 72mph when the OE would be doing 70mph - but moving up to 235/45/18 would be doing nigh-on 76mph when OE would be doing 70...

Speedo may overread (not guaranteed) towards 70, which the larger tyre radius may mitigate, but means under-reading at slower speeds etc

Does anyone have any experience of running oversize tyres? Especially with regards to the rainsport softer compound and tyre compliance...

Just fishing for thoughts from those who have done it - is it best to keep the difference smaller than the ~8% increase proposed, or in day-to-day driving does the extra increase not amount to much, etc..?

Any thoughts welcome...

Thanks!

Dom

What makes you think the rainsports have a softer compound?

[magpies]

Hi
what is the width of your wheels 7" or 7.5" ?
Also bear in mind that it is illegal for a speedo to under read the speed by any amount (not sure how they would find out though)

this website allows you to change and compare wheels and tyres
https://www.wheel-size.com/calc/

[Dominic TVRetto]

Kawasicki said:

What makes you think the rainsports have a softer compound?

From everything I have read it seems they have a soft compound to aid wet grip (the trade-off being shorter life) and softer sidewalls than the Eagles which are designed for a mid-weight car and have stiffer sidewalls from what I can gather.

Without wishing to sound like a d1ck, what makes you think they don't when compared to the Eagles? Genuine question...

Thanks.

[Dominic TVRetto]

Thanks Magpies,

Believe the wheel to be 7.5j..? Unsure about this measurement...

I have been using calculators to get all the stats - I just wanted some qualitative input from experience readers as there is more to it than just the measurements.

Eg speedo implications as noted, "gearing" effect on acceleration, wheel arch rubbing, etc

[QBee]

Rainsport 3 wear out test results might interest you, bearing in mind that 6 years is the normal max safe life for tyres, and the average TVR driver covers just over 880.................yards a year.

My Saab 9-5 was shod on them, front wheel drive, 260 bhp if you wound the clockwork right up to the top.
Fronts lasted 21,000 miles, rears 28,000 miles.

And by God the handling was fantastic - I mean hitting a 45 degree transverse river at night on the A1 in heavy traffic and not diverting a millimetre fantastic.

I have a set now on my Chimaera and love them. The great things about them is that h=they can handle any amount of rain, and they grip from stone cold.
Ride seems nicely compliant, but I am running 17 inch wheels all round and 45 ratio tyres. Frankly, I just adjust my dampers to suit my tyres anyway.

[aka_kerrly]

Perhaps it's worth reading some up to date tyre tests as well. The Rainsport 3 (much like the Toyo t1r) has been surpassed by quite a lot of tyres in both wet and dry performance, there is a rainsport 5 available now.

I also run the Rainsport 3 XL rated on one of my 1200kg/200hp cars and honestly can't say the sidewalls make the car any more confortable than when ive ran it on GY Eagles, Falken 914, Bridgestone S001s.

[HiAsAKite]

Hi - can't talk about different tyre types - (I've only used Toyo's and Dunlop RTMax on my chim) - but I've been running 245/40/18 on the rear of my chim for the past 12 years without issue..

So same aspect ratio, but wider overall diameter on account of the tyre width (245 vs 235)

[Kawasicki]

Dominic TVRetto said:

Kawasicki said:

What makes you think the rainsports have a softer compound?

From everything I have read it seems they have a soft compound to aid wet grip (the trade-off being shorter life) and softer sidewalls than the Eagles which are designed for a mid-weight car and have stiffer sidewalls from what I can gather.

Without wishing to sound like a d1ck, what makes you think they don't when compared to the Eagles? Genuine question...

Thanks.

You really don’t sound like a d!ck, though you have me worried that my question was d!ckish now.

Anyway, I tested tyres in the past. I had to measure the hardness of the tread compound before each test. I can’t say I noticed a big difference between normal tyres. I also didn’t notice that softer tyres has more wet grip.

I‘m not sure that there are such thing as tyres designed for light and mid weight cars.

Obviously some tyres are stiffer in certain directions than others. You can tell a bit by feeling the stiffness of the sidewall, before the Tyre is fitted. Obviously a low stiffness sidewall will be more round when inflated than a higher stiffness sidewall.

What do you want to achieve with your tyre change?

[Olivera]

Just be aware that you can only increase the overall tyre diameter and width by so far until the tyre hits the wheel arch liner, or outer wheelarch, under compression.

On the front you have all of the above problems plus hitting the inside wheel arch on full lock.

[Dominic TVRetto]

Kawasicki said:

What do you want to achieve with your tyre change?

I have to change the tyres anyway for MOT, so looking at options...

While much improved over years of tweaking, the rear of the car still suffered from occasional loss of contact with the ground - a combination of little weight over the rear, the classic Chimaera short-travel suspension, 18s and low profile tyres with mid-stiffness sidewalls...

A softer compound, softer sidewall and "higher" sidewall would all add up to give me a little more flex in the tyre, hopefully more like the flex on 16's tyres again, but without actually having to change back to 16's...

Add to this the much-lauded grip in wet conditions, and it seems like a win-win scenario.

I just want to hear people's experiences with running oversized tyres...

Thanks!

[magpies]

EDITED

https://www.wheel-size.com/calc/?wheel1=215-60-15X...

comparison shows side wall is shallower by 5mm on new tyres

new tyres will be 10mm wider (5mm inover and also outover)

new tyres makes the speedo OVER read by 5% so true 60mph shows 63 on speedo.

so all seems ok

Edited by magpies on Friday 22 May 13:12

Edited by magpies on Friday 22 May 13:15

[Dominic TVRetto]

I'm sorry Magpies, what tyre sizes are they, that you are referring to..?

[magpies]

Dominic TVRetto said:

I'm sorry Magpies, what tyre sizes are they, that you are referring to..?

if you click on the link in my post above, it shows the comparison between the two sizes as requested 215/60/15 vs 235/45/18

EDITED

Edited by magpies on Friday 22 May 13:14

[Kawasicki]

Dominic TVRetto said:

Kawasicki said:

What do you want to achieve with your tyre change?

I have to change the tyres anyway for MOT, so looking at options...

While much improved over years of tweaking, the rear of the car still suffered from occasional loss of contact with the ground - a combination of little weight over the rear, the classic Chimaera short-travel suspension, 18s and low profile tyres with mid-stiffness sidewalls...

A softer compound, softer sidewall and "higher" sidewall would all add up to give me a little more flex in the tyre, hopefully more like the flex on 16's tyres again, but without actually having to change back to 16's...

Add to this the much-lauded grip in wet conditions, and it seems like a win-win scenario.

I just want to hear people's experiences with running oversized tyres...

Thanks!

A few not very structured thoughts...

Goodyear have a great reputation for wet grip. At least as good as Uniroyal.

Your TVR should have fairly even weight distribution, so the car shouldn’t be hampered by that.

Suspension travel is bloody important though. If there is low travel then you might need a stiffer, shorter spring and more damping. How much extension travel is there from the normal ride height. How much bump travel? Is there a spring aid/bump stop?

Using a tyre to get more rebound travel is searching in the wrong area. A tyre is more like a spring than a damper. A less stiff tyre might even make the issue of losing ground contact worse.