ever heard of this?

Honest John Back Room » Technical matters
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Sun 25 May 2003 21:43
ever heard of this? - wemyss
Interesting story from the Green Flag man who came to get my Vectra started the other morning.
He was an experienced mechanic and at a garage he worked at a few years ago they had a problem with a car with an automatic gearbox which kept sticking in one gear and wouldn?t change up.
He remembers it was either a BMW or Mercedes but wasn?t sure which.
When everything failed they rang the manufacturers technical service and asked for help.
The surprising answer knocked them askew. ?Has the customer recently had a puncture and fitted the spare and has he checked the pressure?.
Almost thinking it was April fools day they asked the customer who said yes he had changed the wheel and no he hadn?t checked the pressure.
Reporting this back to the man on the phone he said that this would be the problem and explained that on these cars the gearbox is designed not to change gear when for instance going round an island as it could upset the handling.
How this is done is that it detects that the outside wheel is going slower than the inside and locks up the gearbox.
If one tyre has a difference of 4mm to the other this also happens and even though the car may be going in a straight line there is a speed difference between the wheels and the autobox can lock in the same gear.
They put the repaired tyre back on and all was well.
As Mr Green Flag said ?you?re never too old to learn something new?
alvin

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Sun 25 May 2003 21:53
ever heard of this? - henry k
Does this rule out using a space saver spare on many automatics? They often appear to be a smaller diameter than the regular wheels
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Mon 26 May 2003 08:09
ever heard of this? - Andrew Moorey (Tune-Up)
I forget who told me this but I was told never to put a space saver onto a driven wheel. If you have a puncture on a driven wheel replace it with the unpunctured tyre from an undriven wheel and replace that with the space saver. Makes sense if you apply it to this instance and also if you consider that most cars are fwd I dont relish the idea of a dinky toy tyre doing my braking and steering!
Andrew




Simplicate and add lightness!
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Mon 26 May 2003 21:39
ever heard of this? - BodgeJob
Are space savers really a smaller circumference? I know they are narrower section but not aware they are smaller diameter?

Interestingly just read in the Telegraph motoring supplement that the new BMW 5 series will use a change in wheel rpm (presumeably sustained and relative to the other wheel on the same axle) to detect a puncture and warn the driver.
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Mon 26 May 2003 22:21
ever heard of this? - P.Mason {P}
The spacesaver spare on my wife's G reg. Honda civic was about 2" smaller in diameter than a normal tyre. I assumed this was so that the obvious tilt caused by this difference would remind the driver that a spacesaver was fitted.
P.
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Tue 27 May 2003 00:49
ever heard of this? - henry k
All the vehicles I have seen with a space saver fitted seemed to tilt. I will measure the Focus spare.
A tilt did not stop a ****** in a Boxter with a space saver on the rear pass me at about 90MPH on the M40 - Brain dead as 50MPH is clearly indicated on my spare and I understand all others. Some space saver wheels I have seen were bright colours to possible remind some with small brains to get it back in the boot asap.
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Tue 27 May 2003 08:13
ever heard of this? - leatherpatches
My space saver on Civic is smaller and can physically only be fitted to rear (fwd car). Which means if I get a puncture on the front, I have to swap a rear tyre to the front and put spare on rear. With locking wheel nuts this could be awkward. Let's hope I don't have to!
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Thu 29 May 2003 23:30
ever heard of this? - kithmo
The spacesaver spare on my wife's G reg. Honda civic was
about 2" smaller in diameter than a normal tyre. I assumed
this was so that the obvious tilt caused by this difference
would remind the driver that a spacesaver was fitted.
P.

>>
The rim may be 2" smaller and the tyre may be a lot thinner, but the outside diameter (rolling radius) should be the same. If it wasn't I'd be very concerned about fitting it and would rather buy a full size spare that occupied some boot space rather than drive virtually balancing on three wheels, even at low speed.
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