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Different Tyre Size on same axle

Crispin

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Folks,

As I am about to purchase 4 265 70 17s for my 120, I was wondering what to do should I puncture one of them this weekend at Salisbury plain.
Until the pocket money man comes again, I will have to live with the 120's standard spare which is the 265 65 17.

As the profile is different between the two but by a small margin, would it be ok to drive home (100 miles or so) with a different tyre size on the same axle or not?

I see my options as:
Deflate the other tyre slightly - this will bring them closer together.
Borrow someone else's spare so I have two of the same on an axle. (Could the CD then take some punishment?)
Just drive as the difference is so small it would not matter.

If I could remember anything about pie, I would work it out. :oops:

Cheers,
Crispin
p.s.
I know it's PI... ;)
 

Andrew Prince

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Ok, just asked a related question on your other thread!

The different sizes shouldn't cause too many issues provided it's for moderate distances. Will the traction control on the 120 pick up the different speeds of the wheels? I wouldn't have thought so with a speed differential of <5% but you might want to check this out :mrgreen:

Cheers,
 

Crispin

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Andrew Prince said:
Will the traction control on the 120 pick up the different speeds of the wheels?

That would cause some interesting situations.
 

Jon Wildsmith

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The diff will be fine, the pinion gears will just have to do a little more work than normal but the ABS and ATRAC / VSC could make things interesting so I would go slow rather than risk one of those suddenly misbehaving at high speed or pull the fuse ;)
 
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Rob

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Crispin IIFC you said that you have a rear LSD. If this is the case and then it may change things, im not entirely sure as the how the LSD works, but could cause increased wear.
 

Crispin

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I think Jon's idea of pulling the fuse would be best if I do have to drive home like that.
Imagine at 60-70mph on the hiway and VSC decide you are skidding and locks one of the front wheels. It'll then deside it's over-corrected... :shock: :shock:

Rob - I thought[i/] the 120 had a LSD, I was quoting someone else but in honestly, I'm probably imagining I heard it...
Seems there is some argument: http://www.exploroz.com/Forum/Topic/160 ... o_LSD.aspx
One chap says it has, another says if it has traction control it does not.

I'll dig out the book :|
 

Paul

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Most systems will accept a slight change.
It is likely that you could have one worn and one new tyre on same axle which is quite a difference. Its the reason most speedos are inaccurate, to account for the wear in the tyres.
 

Crispin

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Thinking about it, there has to be some way the computer knows it's not a real skid. Sense lateral acceleration?

If you have a puncture, either slow or blowout at speed, the last thing you want is the VSC trying to fix things.
 

Jon Wildsmith

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The VSC uses the steering angle sensor and wheel ABS sensors as well as the deceleration sensor. Going round a bend the signal from your smaller tyre may get a reaction. ABS in a hard stop may be similarly confused. Sometimes the electronics are too clever for their own good - with the 36" Simex on I can confuse the traction control, foot to the floor up a steep twisty tarmac hill (SE end of Strata for those that know it) it kept backing off the power and flashing the slippy light because presumably the deceleration sensor (it's a 2 DOF G sensor not just a decel) says something doesn't compute even though I know the traction is there. Never had that happen with the smaller 33's, will see what happens next time I go that way with Simex on since I've regeared.
 

Dave 2000

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The difference works out at about 22 revolutions per mile difference using this calculator:
http://www.miata.net/garage/tirecalcold.html so IMHO it is not an issue. ABS if it did detect what it percieves (is that spelt right) a fault it does not slam on the brakes it assumes a wheel slip sensor has gone awol and disables the ABS part of the system which will reset when the ignition is switched off and then 10 seconds later switched back on.

Lowering tyre pressures does not reduce tyre ROLLING diameter, put a mark on a tyre with chalk and then drive one complete revolution, now lower the tyre pressure by 15 psi for example and do another revolution, now measure the distance between the start and end marks and you will find they are the same.

regards

Dave
 

Crispin

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Nice tyre calculator :)


Dave 2000 said:
T
Lowering tyre pressures does not reduce tyre ROLLING diameter, put a mark on a tyre with chalk and then drive one complete revolution, now lower the tyre pressure by 15 psi for example and do another revolution, now measure the distance between the start and end marks and you will find they are the same.
regards
Dave

Doh, it would not would it. Deflating the tyre does not shrink it, it only removes the air from the lower half of it :lol:
 
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