Scrub radius, and offset importance

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

  1. LeftcoastTigger Paid Member Paid Member

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    MkII steering geometry

    Kudo's to BrianG, the original contributors, and Daved for reviving this thread:)

    Come Spring I hope to repeat my original on-car measurements, this time on a pukka level setup platform at a local race car build & preparation shop, with security guard at hand:o

    Regarding the mounting face thickness of front brake discs:

    1993 Passat VR6 5 stud 280mm 22mm 6.15mm

    1994 Golf MkIII 4 stud 256mm 20mm 5.10mm

    1986 Jetta MkII 4 stud 239mm 20mm 5.25mm

    All discs appear oe, the last dimension being the mounting face:clap:
     
  2. mec82 Forum Member

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    top work Brian :thumbup:

    Please don't give up on this though after all the work you've put in though, I think there's a lot to gain from drawing up all this geometry.

    It doesn't matter if its it's not spot on because as you mentioned; every car will be different anyway. after 20 years of abuse there will be bent wishbones, uprights and shells. Not to mention after-market parts that are out of tolerance.

    What's more important I think is to be able to tweak parameters and assess the effects. As long as you know what the margin of error is then you can allow for that and make a judgement. I think the result you got from moving the upright/strut holes is interesting in itself, as that's something that can be varied with camber bolts and slotted struts.

    would you mind sharing more dimensions such as the relative ball joint positions etc? I'd love to get this drawn up in 3D and have a play around with it. :)
     
  3. kyky New Member

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    Hi there!

    A lot of nice info here! I don't know how accurate this data is - it is taken from the brembo catalog. Here are the drawings of various discs - if you need another one - let me know.

    239x10mm 4x100
    Brembo part number: 08.2631.34
    VAG: 811 615 301, 823 615 301
    [​IMG]

    239x12mm 4x100
    Brembo part number: 08.4177.10
    VAG: 321 615 301, 839 615 301
    [​IMG]

    239x20mm 4x100
    Brembo part number: 09.4765.10
    VAG: 321 615 301 C, 841 615 301
    [​IMG]

    256x20mm 4x100
    Brembo part number: 09.5166.10
    VAG: 321 615 301 D
    [​IMG]

    280x22mm 4x100 (G60)
    Brembo part number: 09.6728.10
    VAG: 535 615 301
    [​IMG]

    280x22mm 5x100
    Brembo part number: 09.5570.10
    VAG: 1H0 615 301 A
    [​IMG]

    288x25mm 5x100
    Brembo part number: 09.6747.10
    VAG: 3A0 615 301, 3A0 615 301 A
    [​IMG]

    The dimension that you were looking for is always 5.2mm except on the 288mm where it is 6.2mm. But I think it varies from manufacturer to manufacturer. I have a lot of parts laying around (Mk1/Mk2/Mk3) so if I could help with something - let me know :thumbup:
     
  4. mk2sp Forum Member

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    Nice info here!

    I see that Daved has shown some images were negative scrub radius occurs.

    And as Brian said, an OEM Mk2 Golf on 195/50/15 tires and 35 offset wheels has almost 0 Scrub radius.

    Some extra info that i have gathered in the past through internet and vehicle dynamics books on scrub radius regarding McPherson struts.

    If the scrub radius is zero, the scrubbing action of the contact patch is equal on either side of the pivot point causing the tire to act like a car with a welded differential, inducing a condition called 'squirm'. In a straight line the tire tends to be stable and tracks well. As you turn though, the portion of the contact patch on the outside of the pivot point moves faster than the portion on the inside of the contact patch. Since the scrubbing area is equal on each side of the pivot point, yet the forces are different, the tire tends to fight itself and it becomes 'grabby' causing tire wear to increase and the steering to become unstable.

    In racing conditions, a McPherson strut assembly typically performs well with a lot of Kingpin inclination and caster values, a system where negative scrub radius works well in. A system that i have chosen for my wide tracked Mk2 long time ago. Because both KPI and caster increase the amount of camber on the outside wheel when steering, the fulcrum pivot point is at a point that has more leverage, requiring less steering effort.
    Negative scrub radius moves the outside tire forward, extending the wheelbase slightly and decreasing it for the inside front tire. That can enhance stability especially with cars that have close to square wheelbase to tread ratio.
    Negative scrub radius also helps reduce torque steer in strong front wheel drive cars and compensates for split braking.
    Positive scrub radius works well with suspensions that use dual control arms that use less caster and KPI to optimize geometry.

    As with anything else, a little of a good thing is great, but lot of a good thing is not necessarily better. Values of 10-15mm negative scrub radius is a good compromise for sport-trackday vehicles. Values of 30-35mm are observed in circuit racing cars like SEAT LEON SuperCopa Mk1.

    Finally, something really important, if we plan on doing some personal garage work on modifying scrub radius, we must take into account the amount of sidewall flex the tire will encounter under hard cornering. When the sidewall flexes, the contact patch moves in relation to the KPI and can make a slightly negative scrub radius pass zero and become positive. For this reason it is critical for us to choose the tires the car will be driven/raced so that scrub radius can be correctly selected and then tested.
     
  5. Dave

    Dave *Very Smart* Pedantic Old Fart Paid Member

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    I am still re-measuring everything to sort out why I can't match the VW data for track and drive flange/wheel bearing offsets. I'll post shortly.

    I will check using ET35 wheels. The 3mm difference from ET38 will bring my original drawings very close to 'zero' scrub radius.

    I have been trying to find a reference to the actual scrub radius that VW built in, but have found no definative data as yet. Having said that, my poor memory informs me that VW built some negative scrub radius in? Also, as the suspension ball joint moves in an arc, the scrub radius will change as the suspension moves up and down. So, it is important to know the loading condition when scrub is to be measured.

    I am not a suspension guru by any means. In fact, I never thought much about it at all. I am looking at this purely as an exercise in drawing up the MkII GTI front suspension geometry because I have a very presise CAD system and just wanted to see what the numbers are, as no definative information appears to be available hand.

    My real interest is the first asembly I drew up using Audi 80 hubs as this is the arrangement I shall run when I can afford to get my home made VWMS type uprights bored out for Passat wheel bearings.
     
    Last edited: Feb 11, 2012
  6. LeftcoastTigger Paid Member Paid Member

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    Eliminate dodgey? datum points first, then ta da - -

    Mmm, what a can of worms, and we haven't even considering tyre squirm and suspension flex "yet"![:^(]

    I can provide dimension "X" for a 1986 MkII Jetta Carat (RD 8v engine, 9A trans, discs all round, electric windows, air conditioned - - the Nth American "luxury" version), or a 1990 MkII Jetta GTX (9A 16v engine, 2Y trans, discs all round, electric windows and Recaros - - the Nth American "sports sedan" version)[:D]

    But:clap:

    I note the lower datum point on the drawing is at the upper edge of the spring pad protective lip rather than where the spring actually seats, and can report that the pad/lip on the three pairs of oe shocks I've examined are both offset and angled relative to the shock tube axis, & therefore unlikely to provide a secure datum point[:x]

    Challenging as it may be, I suggest we first establish - - the distance from one of the two bearing carrier/strut mounting holes, to the spring/spring plate seating surface, that we can confirm all springs are seated the same distance above the axles' imaginary axis:)

    Now find dimension "X", formerly 171.5mm on the Feb 2nd drawing, by measuring the spring length where it seats on the upper and lower spring seats (this assumes oe spring upper seating cups and bearing assy's)[:s]

    So far as the Carat and GTX are concerned, "X" will vary due to completely different springs, shocks, and mass over the front wheels - - presumably the GTX specification would be more typical of members' vehicles?[:*:]
     
    Last edited: Feb 11, 2012
  7. Dave

    Dave *Very Smart* Pedantic Old Fart Paid Member

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    I take your point LCT, and you are quite right to point out my lack of common sense![:$]

    The reason I used that measurement is that all my MkII shocks are identical Sachs/Boge Turbo units. So, it is easy to measure and compare units both on and off the car. Never thought that anyone would used other than Sachs/Boge items!:lol:

    It is sensble to establish the spring length as you suggest.

    I have re-measured everything again, and found a few small errors in my original measurements and will re-draw as time permits. Having said that, the errors that I have found make me think the re-drawn set-up may be even further adrift?[8(]

    We shall see!
     
  8. LeftcoastTigger Paid Member Paid Member

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    A task for warmer weather & a pukka suspension set up pad

    Lack of common sense, hardly![:s]

    Ta for the gracious response - - will check individual components as can, then disassemble & remeasure the entire front suspension including A arms, bearing housings, and strut upper bearing assys. including spring pads when ambient temps hit >8C

    ps sadly, this www.faro.com/edge/us/applications no longer available to me, had plans to map entire inner fender too, that tyre/fender interference could be predicted, ah well[:^(]
     
    Last edited: Feb 14, 2012
  9. 1.8T_mk2

    1.8T_mk2 Paid Member Paid Member

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    Seemed to work ok for me :thumbup:

    [​IMG]
    [​IMG]
     
  10. Dave

    Dave *Very Smart* Pedantic Old Fart Paid Member

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    Very interesting!:thumbup:

    How did you establish the datum points? Measurements from your car? Or do you have factory data?

    Is your set up so arranged to allow for the wishbone pivot axis being at an angle reltive to the centre-line of the chassis?

    I shall post up some revised data for my own measurements shortly, and would appreciate your comments comparing my own findings with yours.

    These are the only data I can find at present. There is so much tolerance in the camber that any of my measurements would probably fit!

    [​IMG]

    Does anyone have a body jigging measurement for the strut top mount turrets?
     
  11. jamesa Forum Junkie

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    Any good Dave ?

    [​IMG]
     
    Last edited: Feb 20, 2012
  12. Dave

    Dave *Very Smart* Pedantic Old Fart Paid Member

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    Blooming perfect Andrew!:clap:

    Thanks.:thumbup:
     
  13. Admin Guest

    Chassis setup up is something that has been the focus of my attentions recently, scrub radius is an interesting and often overlooked measurement I imagine most people add spacers to wheels or lower wheel ET to achieve greater track resulting in an undesired positive scrub. There was a nice informative post by mk2sp, recalling info from H&Rs website on scrub earlier, he also mentioned seats supercopa scrub settings, well this manual has plenty more settings in it and shows the effects of scrub, caster, camber, bump steering etc when turning. It's a good chunk of info that can really help people who are wanting to set up a car for track work.

    Negative 30mm Scrub radius might not be achievable with most wheel and shock combinations but it would be interesting to know what the highest ET people have run a MK2 on with coil overs?
     
    Last edited by a moderator: Feb 20, 2012
  14. 1.8T_mk2

    1.8T_mk2 Paid Member Paid Member

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    I'll do my best :thumbup:

    I used the measurements of my own car to make the string computer. It's not mm perfect by any means but gave roughly the same results as what I had initially drawn in 10th scale.

    The set up is to be able to quickly and easily change pivot points too see what effect this had on roll centre movement. The wishbone angle is a side effect of the changes I made while moving the bottom ball joint location.
     
  15. sparrow Paid Member Paid Member

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    Dave, if you ever have some spare time, would you be able to redraw the Mk2 suspension as you have, but with the wide track Passat suspension arms?

    This is what I (and others) have done to my track car, but I've never been able to measure the effect this has on the scrub radius, and I've not been able to find any discussion on this either.

    Obviously the longer suspension arm is going to move the centre point of the contact patch out by 30mm. This will also move the ball joint further out, so increase the inclination of the strut, which will move the point at which the line drawn through the strut top and ball joint out where it passes through the contact patch.
    This is where my brain melts and falls out my ears (although that could just be the ear wax), and shuts down.

    You could just use the diagram of your car. This wouldn't need to be accurate for any particular car, as I'm only interested in the relative movement when going from standard arms to wide track arms.

    Thanks

    Alex
     
  16. Dave

    Dave *Very Smart* Pedantic Old Fart Paid Member

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    Alex.

    Later today, I'll post up some drawings of the wishbones (suspension arms) as I measure them from my 1987 PB. Can you then indicate the relatively different dimensions from the Passat arms?

    Dave
     
  17. sparrow Paid Member Paid Member

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    Yes, no problem.
     
  18. jamesa Forum Junkie

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    Highest numerical or actual width / track ?

    For me ... 7J x 15 ET 37 with Eibach coilovers (195/55 tyres)
     
  19. jamesa Forum Junkie

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    You`re welcome ... Syncro any use ?

    [​IMG]
     
  20. vw_singh Events Team Paid Member

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    Making the wishbone longer does indeed increase KPI/SAI which inturn does lead to a larger scrub radius. So for a fixed ET wheel, a wider track via wishbones give greater negative scrub. My basic and overly exagerated sketch shows this.

    [​IMG]

    Gurds
     

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