Underslung spare wheels

[turbotommy]

A while ago i thought that i saw a post about 31x10.5R15's not fitting as an underslung spare wheel. I did the 'experiment' last night, and it does seem to fit. It does take up a bit more room than the 215/70/15 (on very rusty steel wheel). Apart from this does anyone else keep a spare alloy wheel (standard type) mounted under the back ? Mine have just been re-fubished and i'm concerned that the spare will corrode rapidly...

Would i be correct in assuming that you can't use a smaller spare wheel at all (diff windup ?)?

[Slugsie]

I have an underslung spare (although it's a !!!!ty steel rim ), with standard size tyre.

The guy who I bought my Surf off had fitted a vinyl tyre cover (as you would normally fit over a rear hanging spare) over the spare to protect it from road crap etc. This seems to work well, with one exception. When going through deep water it picks up and carries quite a substantial amount of the wet stuff. I've cured this with the modification of the cover (I cut a small cross shape in the middle of the cover).

[codlord]

I think the thing about the 31x10.5 not fitting is when you have a tow bar fitted - it may not fit then (my standard sized wheel sits right against the tow bar) although you may be able to modify or jam one in there (I have yet to try).

I assumed fitting a smaller (standard size) wheel in an emergency would be o.k. (at least for a short/slowish drive) - be interested to know if this assumption is wrong though (before I get a flat and have to try it!).

[turbotommy]

I think the thing about the 31x10.5 not fitting is when you have a tow bar fitted - it may not fit then (my standard sized wheel sits right against the tow bar) although you may be able to modify or jam one in there (I have yet to try).

I have a tow bar (a witter) and it isn't a problem. There's some kind of sheet metal panel below the tailgate (before the bumper) that limits the size.

I assumed fitting a smaller (standard size) wheel in an emergency would be o.k. (at least for a short/slowish drive) - be interested to know if this assumption is wrong though (before I get a flat and have to try it!).

'Skinny' spare wheels all appear to have the same outer diameter as regular wheels, i take this as some evidence that smaller outer diameter is not a good idea...

[Smokin]

dont know if the same applies but i had larger lwb wheels on my lightweight landy plus a swb spare mounted on the rear door - had to put this on when i sold it - it did drive okay - no transmission wind up but the steering was awkward as it was a front wheel that it was on

hope this helps?!?

Chris

[Linda]

Would i be correct in assuming that you can't use a smaller spare wheel at all (diff windup ?)?

Using a smaller spare to get out of trouble is ok, you wont wind up the transmission if you are in 2wd. It will look a little odd but will be fine to get home.
Only problem will be if you have a rear puncture and you have an LSD (Limited Slip Diff) in the back (generally only on Limited edition 3.0l although not exclusively). In that case you'd have to put the spare on the front and a front wheel on the back.

[silvtr1000]

Here's my tuppence worth...

If the standard spare is a 215/75 on a 15" rim, a little calculation means the outer diameter of that tyre/wheel will be...

1 inch = 25.4mm

15" wheel diameter = 381mm (15" x 25.4mm)
Tyre height = 161.25mm (215mm x 75%)

Wheel Diameter (381mm) + Height of Tyre x 2 (322.5mm) = Outer Diameter of 703.5mm (27.69").


If the current tyre is a 265/75 on a 15" rim (as mine are), the same calculation means the outer diameter of that setup will be...

15" wheel diameter = 381mm (15" x 25.4mm)
Tyre height = 198.75mm (265mm x 75%)

Wheel Diameter (381mm) + Height of Tyre x 2 (397.5mm) = Outer Diameter of 778.5mm (30.64").


So yes, there would be a difference of outer diameter (in this example, of 75mm (2.95")) if the standard spare was used with larger WIDTH tyres. Because the HEIGHT of a tyre is proportionately related to the WIDTH, any increase in width will affect height.

Whether this will cause transmission wind-up, I'm not sure. One of the primary functions of an axle diff is to allow the two wheels on the same axle to rotate at different speeds. My understanding of this is that there would be no diff wind-up between wheels on the same axle - eg, rear right/rear left.

I think that because there's a diff in each axle, the propshafts will not be affected by a different sized spare.

I need to have a lie down...

Mark

[turbotommy]

Here's my tuppence worth...


I think that because there's a diff in each axle, the propshafts will not be affected by a different sized spare.

I need to have a lie down...

Mark

Mark, lie down at once. That much maths is BAD for you. Despite a degree, a PhD, and being recognised as a world authority on my subject (an arcane bit of biology) i managed to work out that one was bigger than the other (and had a larger circumference). Cunningly, i stood them next to each other ;-)

OK folks, it seems that i can use a smaller spare. But i'm not going to. 'Cos it's a rusty lump of poo that would make the truck look silly.

[Guest]

Here's my tuppence worth...

If the standard spare is a 215/75 on a 15" rim, a little calculation means the outer diameter of that tyre/wheel will be...

1 inch = 25.4mm

15" wheel diameter = 381mm (15" x 25.4mm)
Tyre height = 161.25mm (215mm x 75%)

Wheel Diameter (381mm) + Height of Tyre x 2 (322.5mm) = Outer Diameter of 703.5mm (27.69").


If the current tyre is a 265/75 on a 15" rim (as mine are), the same calculation means the outer diameter of that setup will be...

15" wheel diameter = 381mm (15" x 25.4mm)
Tyre height = 198.75mm (265mm x 75%)

Wheel Diameter (381mm) + Height of Tyre x 2 (397.5mm) = Outer Diameter of 778.5mm (30.64").


So yes, there would be a difference of outer diameter (in this example, of 75mm (2.95")) if the standard spare was used with larger WIDTH tyres. Because the HEIGHT of a tyre is proportionately related to the WIDTH, any increase in width will affect height.

Whether this will cause transmission wind-up, I'm not sure. One of the primary functions of an axle diff is to allow the two wheels on the same axle to rotate at different speeds. My understanding of this is that there would be no diff wind-up between wheels on the same axle - eg, rear right/rear left.

I think that because there's a diff in each axle, the propshafts will not be affected by a different sized spare.

I need to have a lie down...

Mark

On mine I have 31 inch tyres all round but a standard 265/70/15 spare. 265/70/15 is 29.6 inches diameter and 31s are now not quite 31 inches as they are part worn and the 31 inch diameter is round the outside of the tyre. My spare is nearly new so hopefully if I had to put the spare on it would make next to no difference. Also on a second gen with no LSD driveability would not be compromised.

Once in the US I drove for about 200 miles on a space saver wheel which was not only narrower but also smaller diameter, this driving was all on main highway at 60mph or greater (it was 3am in the morning and I wanted to get to my hotel before I fell asleep at the wheel).

To be honest, I didn't notice any difference in drivability even though the spare was on the front.

Cheers

[Gamedawn]

I've got a Witter towbar fitted and the 31x10.5 spare fits in quite nicely (pity I'm running 32x11.5's on the road wheels). I think that's about as big as you could go though.

[TonyN]

Its all OK as long as you don't do it for to long on the rear axle if you have a LSD. (assuming it still works)

And a 31 will fit underneath, I've not had any problems with the towbar, but it has been mentioned.

I think a 33 will fit with no towbar, but not 35's.