Scrub radius, and offset importance

Discussion in 'Chassis' started by Brian.G, Jan 6, 2011.

  1. Brian.G

    Brian.G Forum Member

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    This came up discussing something else so Ill share:thumbup:

    As I have found through careful measuring and full scale drawings, a Mk2 golf with a wheel fitted with an offset of 35mm, has a scrub radius of exactly 0.

    This 0 scrub radius gives it neutral feel through braking and accelerating, the KPI(king pin inclination) axis lies exactly in the centre of the tyre contact patch when viewed from the front.

    This is clearly seen below.

    The vertical dark blue line is the tyre centre line.

    The horizontal green line is the ground.

    The Inclined blue line is the front elevation of the KPI axis(Line through top mount and bottom balljoint(BBJ).)
    (Ignore highlight lines)

    [​IMG]

    Fitting a wheel with an in-correct offset throws off these two intersection points, that is the tyre centre line, and the KPI axis point at the contact patch.

    Keep in mind fitting a wider tyre is not a problem if when fitted, both measurements either side of the KPI axis intersect point with the ground are the same width. This is not the case though with many 'modified' cars.

    Most cars that have bigger/cooler wheels fitted have the tyre centre line lie outside the KPI intersection point with the ground.

    This is bad for many reasons, but good for just one, and that one point is to do with when the car is barely moving!

    Ill try explain why,

    Imagine these two beams below are the tyre contact patch(left to right) viewed from the top and the KPI axis intersection point to ground(black pivot pin)
    The other beam that Im holding is the force transmitted through the contact patch.

    The wheel we are imagining will be the drivers wheel, and we are standing on the drivers seat, looking down at the contact patch from overhead.

    I have it setup just when the car is floored from a standing start.

    With the beams setup as the factory setup(0 scrub radius, pivot in centre) You can see that when I apply force, as the driving torque would, both elastic bands are happy out, equally loaded, equal distance from centre, the beam stays straight, and the car goes forwards in a straight line in the direction of the floor board its pinned onto.

    [​IMG]

    Now....lets set it up with an incorrect offset wheel fitted. The wheel centre no longer lies on the pivot centre(KPI axis point) but a few mm outside it.
    Lever forces are now unequal around the KPI axis and these forces get transmitted un-equally from the contact patch causing the tyre to steer off its intended course. Not good.

    [​IMG]

    You can see the wheel trying to steer to the right under acceleration.
    The same thing also happens under braking, but the forces are reversed. The wheel tries to steer inwards.
    This causes the car to become twitchy, or indecisive as to which way it actually wants to go under acceleration and braking.
    Some people often think this is torque steer after fitting bigger, heavier, and wider wheels, but its more often not. Its what I have shown above.

    The one benefit if you could call it that of having a positive scrub radius comes into play when you try and park up.

    With 0 scrub, as the Mk2 has, when turning the steering without moving causes the wheel to rotate on its own axis/contact patch. This makes it heavy unless you move a bit too.

    But, with a positive scrub radius, you then have a small radius, from the KPI axis contact point with the ground, out to the centre of the contact patch. The wheel then pivots around this radius when you turn the steering, and is not scrubbing on its own axis as much. If you have your foot off the brake when steering, the wheel will actually roll a bit when following this radius out from the KPI axis, and doesn't scrub as much.

    But this is just not worth it when you think what an Incorrect scrub radius will do to your handling at speed/braking.

    BG
     
    Nige and Rayhoop like this.
  2. mec82 Forum Member

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    Good info that man!

    Don't suppose you fancy sharing your suspension measurements? I've got a solid works model of a mcpherson strut setup and I'd like to plug the golf numbers into. Its only a 2-d sketch really but you can move it into bump and rebound and take measurements. I'm going to make it 3D at some point to add in castor.

    Mike
     
  3. Brian.G

    Brian.G Forum Member

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    Yeah I can do, Ill be posting it all up in a while, I thought Id get it posted last yr but that didnt happen as always!

    Id safely say itll be another 2 weeks before I get it done, no pictures are in photobucket yet, and I wanna watermark the main ones cos they took a while:thumbup:
     
  4. danster Forum Addict

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    :thumbup: For the the man that laughs at folk fitting the wrong ET wheels. :lol:

    Bump steer. What is this phenomenon you talk of? [:s]

    I ain't got no bump steer over my full range of suspension travel. I feel left out. [:^(]
     
  5. Brian.G

    Brian.G Forum Member

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    Bump steer=wheel steer as wheel goes over a bump, normally brought on by poor lowering, throwing the relationship between the wishbone angle, and the tie rod out of sync.

    Thats briefly now Dan:thumbup::lol:
     
  6. A.N. Other Banned after significant club disruption Dec 5th 2

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    Jet fighter, overhead :lol: !
     
  7. danster Forum Addict

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    Yep, that is why I designed out all bump steer when fitting the mk3 5 stud hubs on to my Rocco.

    It can't be done you know. ;)
     
  8. Brian.G

    Brian.G Forum Member

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    :lol:
    I was going to try write it simpler but I thought you actually show up at my house with said tie rod:lol:

    You obviously put the word cant out of your head when undertaking such a project

    Thats very important at the start:thumbup:

    Peace out Danjo
     
  9. strimmer Forum Member

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    Very interested in this right now, I've tried briefly to bash up a spreadsheet where variables can be input to calculate scrub radius, the motivation is to calculate offset for wheel choice after widetracking and lowering my Corrado.

    Fantastic work, and such a clear explanation. :clap:

    That you are imbued with such wonderous illustrative technique is a blessing to us all. :lol:
     
  10. DJUPLIFT Forum Member

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    been telling the "scene" boys this for ages... i didnt know the technical terms or the true science behind it but i knew moving the center line using spacers or using massively different offsets would have a negative effect on geometry.

    great read and should be stickied!
     
  11. m1keh Forum Member

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    How does this work out on the rear beam?

    I'm thinking minimal effect due to the fact the wheels can't actually move much in toe. But then I question this as the fronts would have equal and opposite turning effects on each other through the rack. Meaning driver feel would be affected but real world grip would not change so much?
     
  12. Brian.G

    Brian.G Forum Member

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    Indeed driver feel is the effect, but that effect can be daunting enough for you to react and in turn, effect the handling.

    yrs ago on a car that I dont remember, the scrub radius was around +30mm, any hump or unevenness under the outside or inside of just one wheel would concentrate this 'force leverage' to that area, and the car would try and turn violently in that direction unless you had a white knuckle grip on the steering wheel.

    As for the rears, less of an effect, but certain deflection gets amplified the farther the wheel is from the trailing arm flange face.
     
  13. A.N. Other Banned after significant club disruption Dec 5th 2

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    I normally regard the rears as having nil effect on geometry, notwithstanding vw_singh's notes on the handling characteristics here post 8 onwards.

    The only effect of ET or wheel spacers is surely to change the angle of forces on the wheel bearing races? Spacers behind the stub axle remove this altogether?


    Whilst it's on the other thread, I'll post this here:

    [​IMG]
     
  14. Brian.G

    Brian.G Forum Member

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    I have test details that Ill post soon Chris(see above) on the rear beam, and its deflection across the beam when a bump is encountered on one wheel. Depending on the distance the wheel centre lies from the flange face, the more this twisting would get amplified in respect to the contact patch of the other wheel.
    Twisting forces do exist in the trailing arms, and way more than you would think in respect to camber change.

    But of course, this is all just bench testing, and not real world(The rear beam tests) Sometimes, stuff thats looks plain bad on the bench, can actually turn out to be a major benifit on the road, even though it looks like it would be the exact opposite. Every part becomes active once you bolt it on there.
     
  15. mickey marrows Forum Member

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    The variable you haven't mentioned Brian is tyre diameter.. changing the rolling diameter also effects the scrub.
    A couple of years ago I was doing some similar calcs to work out the roll centre on a mk1 and found that when 8 x 13 et0 wheels with 19x8 R13 tyres (as per gp H cars) are fitted the scrub is almost identical to when the same car is fitted with stock GTI 13 wheels and tyres.. i.e the change in offset counteracts the effect of the lower rolling radius...

    Also the ride hight effects scrub.. so if you've lowered your car, you altered the scrub.

    I've got the sketches in Pro/E if anyone wants them (they're mk1 geomerty with stock wishbones)

    The killer was that using dropped ball joints gave the benefit of a higher roll centre but screws up the scrub.. (due to the vertical nature of the balljoint spindle, only on mk1's!)
     
    Last edited: Jan 14, 2011
  16. chopperoli Forum Member

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    I'm sure I read somewhere that on a Mk1 you shouldn't go lower than ET31 for this very reason. I can't for the life of me remember where I read it though, and for what width rims that was for so I can't really back it up right now :lol:
     
  17. strimmer Forum Member

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    I've put together a spreadsheet to calculate scrub radius using a number of measurements from the car, but I am missing the thickness of the brake disc between the hub and wheel.

    Anyone please :thumbup:
     
  18. A.N. Other Banned after significant club disruption Dec 5th 2

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    So just the thickness of the brake disc steel which sits over the centre over the hub? The non-frictional bit?
     
  19. strimmer Forum Member

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    Yes please.
     
  20. danster Forum Addict

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    As I am sad enough to have a vernier and brake disc on my dining room table it would be lazy not to post up the dimension you crave. :thumbup:

    6.35mm
     
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