labrat

Discovered a puncture this morning. A roofing nail had left its mark in the tread. Installed the dorky emergency tyre - checked the pressure - way too low. Noted it said inflate to 80psi (420kPa). Hauled out trusty emergency pump. Found runs out of puff at about 340kPa (49psi). Anyway, it seemed to be OK at 70kph on the way to the tyre repair guy. I think I need a compressor with more grunt, but on the upside, you can't accidentally over-inflate a normal tyre.

labrat

Hmm, I just googled 420kPa in psi and it came out to 61psi. Someone needs to check out their math!

LittleJohn

u can uy a slime kit for about $60 from repco which has some slime and a compressor, which is much more capable.
While you're at repco, you may as well but buy some plugs (tyre repair kit) to fix that puncture...

labrat

[QUOTEWhile you're at repco, you may as well but buy some plugs (tyre repair kit) to fix that puncture...[/QUOTE]

Good tip, but I don't have the capability to take of the tyre at home (let alone re-balance the wheel as well)

Gomez

...or someone needs to check their glasses prescription.... . Maybe Qld psi is different to ordinary psi??!!

Gomez

Hmmmm, digging the spare out and looking at it for the first time in 18 months made me notice something. The space saver isn't balanced. I assume no manufacturer balances them. Why would you bother??!!

auzoom

Isnt the tyre rated to something like only 50kph? if so then you wouldnt need to balance it.

Andrew

Gomez

....without going back out into the shed, I think it is speed limited to 80Kph..?

How did you go finding another wheel, Andrew?

RXP33D

OH! Another fact!

They're enkei's!

auzoom

Good thanks Gomez. Rang around an dthe price was identical. Stock was in melbourne so it was ordered and picked up in 4 hrs. I am just heading off to have it all looked at.

Andrew

skc

Somewhere I picked up that you should not use the spare tire as a spare for the rear. Aparently it can have an harmful effect on the diff. This may be due to the different rim size.

skc

LittleJohn

Good tip, but I don't have the capability to take of the tyre at home (let alone re-balance the wheel as well)[/QUOTE]

u don't need to take the tyre off the rim for this sort of repair.. nor would you need to rebalance it.. the additional weight of these things are negligable..
Check out the kit and you may surprise yourself ;-)

LittleJohn

yeah.. but it's only in emrgencies.. to get u to the nearest service sation.. not to be running around town for a week or two..

xxup

And people wonder why I travel with a full size spare... No way I am going to try to drive 200Km in the coolest car at 80km/hr running a poofy yellow rim and risk a blown diff...

BVD

I've never really understood the diff thing. Is this just a forum rumour based on somebody's nervous question, or is there really an issue here??


The difference in effective diameter would have the same effect as going round a corner - one wheel would do slightly more rotations than the other. Now this is exactly what diffs are there for - to allow this to happen - so what's the problem?

Our cars can spend the day going round a circuit all day - in one direction - without blowing up (well, usually anyway! ). They can even do 24 hour endurances on circuits. So what's the big deal about driving home for 40 or 50 ks?? I certainly had no problem when I did it.

I guess with a "limited slip" style of diff you are asking it to work slightly more in one direction than the other. But on a normal drive, there will be corners in either direction to help avoid any "winding up" situation, if that's considered to be the danger.

I would have thought that if you stick to the 80kph recommended speed the only problem would be the boredom and the feeling that it looks pretty stupid (as xxup says). That's all that happened to me anyway.

Why is diff damage or 'wind up' considered to be even a possibility? Is it based on any real feature of our diff, or something that's been brought up, based on potential difficulties with equipment or situations that don't really apply here?? Anybody know?

Cheers, Chris

timbo

I agree. Crikey, when you look at the handbook (and the wheel!) there are enough warnings about what not to do. I'm sure if there was a problem using it on the rear I am sure there would be some large red lettering somewhere.

(actually, I'd be more worried about using it on the front!)

MattRX

Actually there is a big warning (hard to spot amongst the other warnings) about using the spare on the rear!!

With old friction type LSD's a different size wheel on the rear would kill it in no time.

I believe the RX8 has a torsion LSD so it shouldn't fail, but you would be subjecting it to increased wear and heating. If it is a five minute drive I would live with it, but if you have to limp back from a jaunt in the country I would do a front rear swap.

Matt

BVD

The only warning I've seen in the manual suggests that the spare should be taken off the back as soon as possible (which is what we'd all do anyway). It does not say that it should never be used on the back. Given the generally paranoid nature and frequency of warnings in the manual, I'm sure that Mazda would have had no hesitation in telling us not to ever to use it on the back if they thought there really was a major problem.

I believe that these warnings are there to cover every eventuality, including some sort of bizarre situation caused by an exceptionally dumb driver with an outstandingly good lawyer. I.e. some bozo who leaves it on for weeks and also ignores the 80kph usage limit.

I had no qualms about using my spare on the back, and I still don't. I subject the diff to "wear and heating" every time I drive the car, and every time I go round a corner (which I do quite a lot. ). It can handle it, that's its job. It can handle going round corners for more than five minutes at a time, and it can also handle a small difference in wheel diameter for more than five minutes. A lot more in fact, as I have proved to my own satisfaction by actually doing it.

If somebody has a genuine technical explanation of the alleged problem, rather than just generalised opinions then I'd be interested to read it. I don't mean that sarcastically, I'm genuinely interested. Don't worry about being technical, I've been qualified as a motor engineer for 35 years, so I should understand the details. And if I don't, I'm not afraid to keep asking questions.

I think that the potential for wear and/or failures has been rather blown out of proportion. If you drive slowly and with care (as you tend to with a spare like that on, believe me!) then I think you'll be just fine, and so will your diff. But, hey, if people prefer being very conservative with their vehicles that's fine too.

Cheers, Chris