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Thread: Any negative effects of increasing scrub radius?

  1. #1
    Join Date
    Mar 2018
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    Any negative effects of increasing scrub radius?

    Hi all,

    Apologies if this has been addressed previously, had a search but couldn't find the answer.

    I was after advice/opinion on whether an increase of ~5mm of scrub radius will have a noticeable effect on my soon to be delivered Mk7.5 Golf R?

    I'm looking to change the stock 19x8 ET50 wheels to a set of 19x8.5's with ET45 (transferring the same 235/35R19 rubber). From my research I think they should fit fine but will increase the scrub radius by ~5mm. I have tried to get my head around the effect of making the scrub radius more positive vs more negative but as I am hoping to get the tires transferred onto the new rims won't be able to run them back to back to see.

    Anyone have any thoughts on whether there would be any noticeable difference at this level?

    Cheers!
    Current: Mk7.5 Golf R Grid Manual, Mk6 118TSI DSG
    Past: Mk4 2.0 Sport Manual, Mk5 2.0 FSI Manual, Jetta 2.0 TDI DSG
    Family: Polo 77TSI DSG, Polo 81TSI DSG

  2. #2
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    It will have a softening affect on the front end as you move the wheels out more and it can also lead to tram lining in some cases. The steering loads will also increase but with power steering the effect should be minimal. Whether 10mm total will have a noticeable effect is to be seen as it isn't much. The standard wheels are stupidly heavy why would you want to go heavier? I would like to drive a Golf with 18's that are really light, I reckon it would run rings around the factory wheels.

  3. #3
    Join Date
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    Thanks Ozsko. The VMR 718s I’m looking at are pretty much spot on the best estimate of the 19” Spielberg’s (26lbs/11.8kgs) floating around another forum so increased weight shouldn’t be an issue. Don’t really want to downsize in diamater and had settled for the 718s in 8.5 because I haven’t found anything I love in 8. Might have another look though

  4. #4
    Join Date
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    Traditionally most vw's have negative steering offset/scrub radius which keeps a car going in a straight line if you have to brake the car with one wheels brake not working. If you have positive scrub in that instance the car will be more likely to slew sideways. I think that was the theory as to why audi/VW marketed it. Whether that's 5mm neg or more like 10mm neg I don't know. Steer offset apparently can be checked by jacking the front of the car on a clean surface, chalking the tyre contact patch, and then dropping the car on the jack till the tyre juuuusst contacts the ground. Mark the tyre centre line in this position in a permanent marker. Then go side to side on the steering wheel so that the tyre leaves a witness mark on the surface of the ground. Get the car out of the way and then look to see where the centre of the scrub pattern is relative to where the centreline of the tyre was. If the patterns centre is inboard of the tyre centre line then you have negative scrub and hopefully you can measure by how much.
    My 9N3 Polo Gti had ET38-43's with negative scrub (read it in an obscure VW paper somewhere). On the track I run ET40's with 15mm spacers giving me an ET25. At that kind of rim offset it'd have to be out through centrepin steering into positive scrub territory and it certainly feels it. There's a lot more torque steer and tyre kick and the LSD really makes you need to wrestle with it. I've run at stock offsets/stock scrub with the same tyres and diff and its nowhere near as physical in slight neg scrub . That sort of gels with the theory that I've read that says the more offset you go either way (pos or neg scrub) that steering wheel fight increases. But obviously on a track car you want to widen your track as much as possible so moving towards positive scrub is the more usual. I'm pretty sure I stumbled across centre pin steer offset when I was trying lots of different spacers and it was awful. It felt like I had under inflated tyres and lots of toe out, not palnted at all. I actually thought I may not have done up the wheel bolts and went home to check them! So if you space the rim out and it feels weird it could be that you've taken it from neg scrub and made it centre pin. You could then keep going to get the same scrub radius but in positive scrub territory or it may not bother you. Just something to be mindful of. In general driving you are not going to notice heavier steering with positive scrub when you have power steering, its just when you are really pushing the tyres communicate a lot more to you through the wheel.

  5. #5
    Join Date
    Mar 2018
    Location
    Sydney, NSW
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    Sorry for the delayed response Sam, thanks for the detailed reply!

    I am only planning to track it very occasionally and have zero track experience so probably will never notice. Given all of the I've convinced myself that I need to go all out and get a set of BBS CH-R 19x8.5s in ET48 to minimise any issues and save some weight (BBS 24lbs vs VMR 26lbs) while maintaining the 19inch look. Now just to save the funds and convince the misso! Can't believe I can get the same wheels shipped from the UK for ~$500 cheaper including GST than I can through Jax Tyres
    Current: Mk7.5 Golf R Grid Manual, Mk6 118TSI DSG
    Past: Mk4 2.0 Sport Manual, Mk5 2.0 FSI Manual, Jetta 2.0 TDI DSG
    Family: Polo 77TSI DSG, Polo 81TSI DSG

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