Tyre Size Compatibility

Life Cars & Motorcycles
1

I’ve got this Cheng shin 831 155SR13 spare, which seems to be in good condition, and (IF I’m interpreting its date code correctly) was apparently made toward the end of 2007.

If I understand the size spec correctly, its a different nominal width to my Bridgstone 175/70R13 SF-226 road tyres, but the same rim. Since its aspect ratio isn’t stated, its presumably the default 80, so that’s also different to the 70 of the road tyres, but I guess its good enough for short term emergency use as a spare, being less different than one of those “space savers”, for example.

EDIT: If I’m doing the sum correctly the road tyres are 122.5 mm high and the spare 124mm, so within the limits of treadwear variation.

I’m told that this spreadsheet (from Cheng shin) is saying the spare is interchangeable with 175/70R13’s. I don’t get this. My understanding was that any size difference (apart from 80 v 83) on the same axle was, for example, a reason for a fail on the UK MOT.


spare_COMPATIBILITY by ed_lithgow, on Flickr

I’m assuming its a misunderstanding and these are the tyres they make that are compatible with 13 rims, or compatible with the Skywing, rather than with each other on the same axle.

Can anyone confirm this?

It would make sense to put this spare into service and demote one of the road tyres to a spare, but I assume if I do that I’ll have to get another 155SR13 (ideally another Cheng shin 831) and avoid mixing on the same axle in future (except perhaps for short term emergency use, when using the spare).

2

I have information which states that for European metric tyres, the aspect ration for that 155SR13 tyre is 82, which in this case, would put its height at 131.75mm and quite a way off from your other tyre which is 122.5mm.
Also the width is quite a long way off too. You are no doubt going to have some steering and/or other performance issues if running these tyres at the same end.
This will most likely only do as a spare and used for only a short distance.

3

[quote=“sulavaca”]I have information which states that for European metric tyres, the aspect ration for that 155SR13 tyre is 82, which in this case, would put its height at 131.75mm and quite a way off from your other tyre which is 122.5mm.
Also the width is quite a long way off too. You are no doubt going to have some steering and/or other performance issues if running these tyres at the same end.
This will most likely only do as a spare and used for only a short distance.[/quote]

Thanks Mr S. That’s what I thought, only more so.

4

I’ve run wider and/or taller tires on cars before and they don’t present a safety issue. They may make your speedo off by a little, but hey, is that such a big deal?

Even if the Chengshin 155 R13 are 82 aspect ratio (instead of the 80 as per your screenshot there), a 10 mm difference isn’t going to cause rubbing issues - and that’s the issue you need to be worried about here. The Chengshin is also skinnier than your current tire, which means there certainly won’t be any fitment issues (horizontal fitment, rubbing up on shock, etc.).

All that said, it’s possible to fit a wider variety of tire on your scooter or car than you think; you just have to watch out for rubbing issues AND rim fitment issues.

I wouldn’t use your Chengshin anyway… a five-year old tire is not going to be as good as a new tire (even if it was never used). Why risk the most important safety aspect of a two-wheeler for $1,500!!!

5

[quote=“shawn_c”]I’ve run wider and/or taller tires on cars before and they don’t present a safety issue. They may make your speedo off by a little, but hey, is that such a big deal?

Even if the Chengshin 155 R13 are 82 aspect ratio (instead of the 80 as per your screenshot there), a 10 mm difference isn’t going to cause rubbing issues - and that’s the issue you need to be worried about here. The Chengshin is also skinnier than your current tire, which means there certainly won’t be any fitment issues (horizontal fitment, rubbing up on shock, etc.).

All that said, it’s possible to fit a wider variety of tire on your scooter or car than you think; you just have to watch out for rubbing issues AND rim fitment issues.

I wouldn’t use your Chengshin anyway… a five-year old tire is not going to be as good as a new tire (even if it was never used). Why risk the most important safety aspect of a two-wheeler for $1,500!!![/quote]

Thanks for your comments. I thought they would probably fit OK. The issue I was concerned with was mixing them, which I’d been told was OK.

I think it should be OK if I get another one and put them both on the front, otherwise only OK for short term emergency use, as Mr S confirmed.

Putting them on the front presumably means I’ll have a smaller footprint on the front, which I understand is where you’re supposed to put the lower grip tyres.

I accept that a five-year old tyre isn’t as good as a 1 or 2 year old “new” tyre, especially as its been kept inflated while I’ve had it

If I can’t get access to inert gas fill I may keep my spare deflated in future to prolong its life. I carry a pump.

Though I think its good enough for my car and driving style, it would be less trouble just to chuck it and get some more Bridgestones and, although I don’t like waste, I may just do that.

I know a couple of my tyres are a bit on the dodgy side, which is why I’ll be changing them soon, but that doesn’t make my car a two-wheeler, at least not yet. These tyres wouldn’t work well with a 2-wheeler, unless it was fitted with a side car.

6

Mixing tire sizes on an axle is absolutely a safety issue! Actually, in the States I believe you even have to have the same tread pattern on an axle. Of course, regulations are overkill in the US, but the purpose is that you will get different performance from different tread styles, and significantly more so from different tire sizes. Braking is probably the only real concern. Think of it this way: a wider tire has more contact area with the ground than a narrower one. When you hit the brakes hard on the highway, the two tires won’t hold the road the same.

Now, if you’re only doing city driving, I doubt you’ll experience a situation where you are experiencing something that significant, so it may not matter at that point. I still wouldn’t recommend it though.

As for deflating the spare, you would need to seal the spare externally in a bag or something as well if you’re concerned about deterioration. However, If you just use your spare as a spare (I know, it’s a waste!), you only need it to last for a relatively short distance. I had a spare on my truck that had probably been there for 15 years without being touched, and it was still good to go when I got a flat. Sitting under the truck, exposed to moisture, snow, salt, sand, etc.

7

Yeh, I know. That’s why I say I’ll have to get another one the same if I put the spare in service. I’d then have a slightly dodgy, but (as you seem to agree below) probably good enough Bridgestone as a spare. I’m not buying more Bridgestones so if I dont do this I’ll probably get Michelins (max pressure 51 psi) for the front.

[quote=“thirdstring”]
As for deflating the spare, you would need to seal the spare externally in a bag or something as well if you’re concerned about deterioration. .[/quote]

Well, that doesn’t seem too taxing, especially if I’m not rotating it, though I could rotate the rear threesome. However, I think, even uncovered, that there would be some advantage to deflation, since it seems, both from published research, and intuitively, that the pressurisation speeds oxygen permeation into the casing.

[quote=“thirdstring”]
However, If you just use your spare as a spare (I know, it’s a waste!), you only need it to last for a relatively short distance. I had a spare on my truck that had probably been there for 15 years without being touched, and it was still good to go when I got a flat. Sitting under the truck, exposed to exposed to moisture, snow, salt, sand, etc…[/quote]

Don’t think exposure to moisture, snow, salt, sand, is likely to be much of a problem. The problems seem to be ozone, oxygen, and heat, although UV has some destructive effect as well.

I believe vegetable oil teatment may be particularly effective against ozone (which apparently causes crack propagation), by acting as a physical barrier, and , more importantly, as a sacrificial target for ozone attack. Its a complex system, however, and I can’t rule out negative effects, especially if the oil is absorbed by the tyre, as it seems to be.

8

Here’s some good info from Goodyear. It’s about RV tires, but I think it probably still applies in general. 10 psi for storage.
goodyearrvtires.com/tire-storage.aspx

Also, sounds like oiling them is not a good idea. The bag would be the best way to go.

9

[quote=“thirdstring”]Here’s some good info from Goodyear. It’s about RV tires, but I think it probably still applies in general. 10 psi for storage.
goodyearrvtires.com/tire-storage.aspx

Also, sounds like oiling them is not a good idea. The bag would be the best way to go.[/quote]

Oh dear, another unspecified variable: “Ensure that the rim manufacturer’s inflation capacity is not exceeded”

I’ve never, ever heard of a quoted “rim manufacturers inflation capacity” and I bet, if I could be bothered to look for it, that it’d be hard to find.

I’d expect this to say “Ensure that the [strike]tyre[/strike] tire manufacturer’s inflation capacity is not exceeded”, though, if my tyres have an inflation capacity, it seems to be a Bridgestone company secret.

Another direction in which to pass the buck?

I suppose the 10psi is so they keep shape if stacked (probably not an issue with a spare), and maintain rim adhesion. Inert gas fill should negate most of the downside of keeping them inflated.

I think the cautions against oily environments probably refer to mineral oil, which is known to attack rubber and would make them slippery long term.

10

I suspect a minor benefit of SFO, for a spare, might be that it would stick the tyre bead to the rim. I can’t rule out negative effects, though