Space Saver Spare

I know how a diff works but for any given distance travelled, the smaller wheel will be turning faster than the standard wheel to keep up with it. Why would that not trigger a traction control fault - the car won't know it's got a smaller wheel on it, it will think one wheel is losing traction surely?

(13-04-2016 04:45pm)Steve D Wrote:  I know how a diff works but for any given distance travelled, the smaller wheel will be turning faster than the standard wheel to keep up with it. Why would that not trigger a traction control fault - the car won't know it's got a smaller wheel on it, it will think one wheel is losing traction surely?

steve i can see your reconing but i think that the actual dif and axle would bee turning at the same speed.
but dont ask me to explain this.

One would assume that the differential transverse axle speeds caused by the different circumferences taken care of by the diff (therefore one axle will turn faster than the other... not that different to turning a corner) will be allowed for in the computer software, The Traction control will be looking for a marked increase of speed on one wheel, probably in excess of the differential caused by the differing wheel sizes.

The differential has to be working harder though, if only slightly, so I woudn't drive more than you actually need to, not that you'd want to given your increased stopping distance, but not get into a fret about it.

Has anyone confirmed their Space saver is also smaller like the op's? (I don't have one to check!)

I've just measured, as accurately as I can, my factory supplied (2013) Space Saver 155/85 R18 and it is 71cm diameter. My 20" wheels are approx 72cm dia.

Hope this is of help.

Mine's slightly larger - 155/90/18

I thought that regardless of the tyre (space saver or anything between 17" and 20" as is standard on the Evoque) LR recommends wheel/tyre combinations that have as near as dammit the same rolling radius, the difference between the tyres is taken up by the differing sidewall height. IIRC a space saver is speed limited to 50mph and should not be driven on for more than 50 miles, which is why they generally have either a solid steel wheel or a fluorescent patterned marking, so everyone knows that you are on a space saver.

You guys are within 1 inch circumference, the op's are 7 inches out... seems wrong to me.

(13-04-2016 09:42pm)PhilSkill Wrote:  You guys are within 1 inch circumference, the op's are 7 inches out... seems wrong to me.

The guys above are quoting differences in diameter. I'm not sure where your figure of 7 inches has come from.

This has turned into a bit of a Simon Mayo Homework Question, but I think the consensus is that the differential can deal with the slight change in diameter but you wouldn't want to push it too far (i.e. 50mph max for a short duration)

(13-04-2016 04:50pm)bee Wrote:  steve i can see your reconing but i think that the actual dif and axle would bee turning at the same speed.
but dont ask me to explain this.

Definately not bee. The whole point of a differential is to allow each axle to turn at a different speed when cornering. The clue is in the name!

steve i stand corrected, its my age u know.