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Premium Member
2005 R53, 2005 R53 lightweight, 2008 R55S, 2012 R58FJCW, 2014 R60SD All4, 1996 Mini Cooper 35SE.
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6,439 Posts
didn't you mention it squirrelling about under braking before with spacers on? It's one example of an effect of negative scrub radius
 

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Premium Member
2005 R53, 2005 R53 lightweight, 2008 R55S, 2012 R58FJCW, 2014 R60SD All4, 1996 Mini Cooper 35SE.
Joined
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6,439 Posts
But same front and rear would be better?


I did indeed. Seems better since lowering front end slightly, though not completely.
Perhaps I should stop being a tight arse and widen track the proper way with lower offset wheels.
Currently on ET48.
If I went to ET35 it would have same effect as 13mm spacers without affecting scrub angle/radius?

well my understanding is it'd want wider rims with adjusted offset, not just a 7j with lower offset

fwiw I'm actually more in favour of spacers than lower offset wheels tbh, one can adjust spacing with those for little outlay, the other way means new rims.... £££'s,

so actually no, imo you're still pushing the tyre centre line outward,

scrub 'radius'....

my interpretation is essentially whether the wheel centre line rotates in an arc outside the outer ball joint pin/centre line, where '-' is the ball pin, for example right side, pos = '-)' which reflects a lower offset or spacers,

or if it rotates in an arc inside it, for example right side, neg = '(-', which reflects a higher offset, or no spacers under JCW 18" alloys (et52) for instance,

'zero' scrub rotates the centre line of tyre tread under the pin, or 'about' the pin's centre line, poor example if '-' is the pin (-)

a wider rim typically places width either side of the hub face, so with relative offset compensating for additional width, potential exists to maintain a zero scrub radius with enhanced overall track, these don;t really need it though, most are into it for stance/image, and with it comes compromise,

'negative scrub' you would find will lighten steering and sharpen response, note the JCW 18's are ET52 and accomplish this imo. Most 'positive scrub' will weight steering and can increase or exacerbate bump steer or tram lining, as you encountered, also under braking, as well as load the hub bearing to a degree, it also at the front, will alter motion ratio.

add stickier tyres into the mix, and it all becomes enhanced feedback potentially,

motion ratio in simplest terms is leverage,

inner lower arm pivot, so inner ball joint, vs outer ball joint if zero scrub radius, or tyre centre line if not (or just anyway), is the length of your lever, compared with the centre line of the damper where it would intersect the distance between the first two, almost like a fulcrum at that point I guess,

you will find the damper centre line is nearer the inner pivot than the tyre centre line, this forms a ratio, which represents leverage against the damper, or 'motion ratio'. It affects the performance of the damper's valving but in particular the spring rate, essentially lowering the spring rate in the instance of spacing out or lower rim offset, hence one should ideally decide on wheels and spacers including ARB's before choosing coilovers, if the coilovers cater for such variances. Some talk about rates they want, not realising the spec change they're about to make will affect them. Minor effects they may be, they are still relevant. As is vehicle weight in a similar way.

hopefully that makes some sort of sense 😂
 
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