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 Post subject: Re: 18" wheel fitment
PostPosted: Mon Jul 03, 2017 12:32 pm 
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Vehicle: MY08 SH XT Premium

Posts: 13
Location: Victoria
craig747400 wrote:
BotBot wrote:
craig747400 wrote:
Add to that, they're track-width change is illegal too. Scrubbing is illegal, and very bad for handling too (instant infinite spring rate).

Page 19 onwards, is where most the good gen is: ... .pdf?la=en



I'm fairly cluey but you've got me stumped

Track width is measured from the centres of the tyres' contact patch (this confuses many people). Narrowing a vehicle's track-width leads towards a more unstable platform. Going from a Forester rim offset of +48mm to a WRX offset of +55mm reduces the track-width of the vehicle (14mm overall) ... this also confuses a lot of people, due to the +/- nature of the use of offset's units of measurement. It also reduces the scrub radius, which reduces steering feedback and feel (but also reduces the stress / moment arm). Going too wide has the opposite effect.

Some people think putting wider tyres on their car, increases the track-width of their car: it does not ... even though the outside of the tyre sits wider, the centre of the rim / contact patch is unchanged, hence the track-width is unchanged. The tyre is also 'wider' towards the centre of the car an equal amount. More grip from a bigger contact patch, but no change to the track-width, unless the offset is different, or other radical suspension changes have been made.

Scrubbing occurs, any time during the suspension's travel, if the tyre comes into contact with any part of the vehicle. There's good reason it's illegal: it's dangerous, for more than one reason.

Thanks for the clear and concise explanation.
These are things I already have thought of....
Just that I'm not aware of the terms you use.....
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