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Thread: Wheel Offset vs Scrub Radius

  1. #1
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    Wheel Offset vs Scrub Radius

    Firstly let me say straight off the bat that I'm not worried about "Stance" or "Filling out the guards" as they are looks based and not handling/performance based things.

    I've been reading things about offset and scrub radius like the following:

    http://www.miata.net/garage/offset.htm

    http://en.wikipedia.org/wiki/Scrub_radius

    If I'm going to up the wheels from 14" to 15" and the tyres from 175/??/14 to 195/50/15 then I know that wider rubber will make it feel different, but if I want good precise handling and the lowest possible tyre wear shouldn't I be looking at factory offset wheels (ET45 in my case)?

    If I get wider wheels then they'll fill out the guards better (y0) anyway.

    If I went to ET30-40 on an early model MX5 then how much can/does wheel alignment need to be changed to compensate for a different front track? What will it give me in terms of stability with a wider track? Can someone describe to me what they mean by 'tramlining' when they talk about changing things? People talk about 'tramlining' being bad but to me it sounds a little like 'handling like it's on rails' which to me sounds good.

    If I want it to drive nicely and be well behaved at parking speeds and I'm not worried about handling on a racetrack should I just get some ET45 wheels and make sure the wheel alignment is inside tolerances before putting new rubber on?

    I want the setup of the car (alignment, wheel diameter, offset etc.) to give the tyres their best shot at lasting a long time no matter how it's driven.

  2. #2
    Registered User stb's Avatar
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    Depends if you also change your wheel width. If you increase width by 25mm, then your offset will need to change by 12.5mm to keep the same theoretical scrub radius - but changing caster and camber will also affect the steering weight too.

    Tramlining is very different to handling 'on rails'. In fact it is the exact opposite in my eyes. Car wants to go somewhere you don't want it to. My MX5 gets very tramliney particularly under braking when there is any sort of road irregularity because I've got a reasonable amount of caster and camber and no PS and a small steering wheel.

    I don't see the link between offset and tyre wear though - wear will be most linked to alignment, particularly toe and camber.

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    Quote Originally Posted by stb View Post
    Depends if you also change your wheel width. If you increase width by 25mm, then your offset will need to change by 12.5mm to keep the same theoretical scrub radius - but changing caster and camber will also affect the steering weight too.

    I don't see the link between offset and tyre wear though - wear will be most linked to alignment, particularly toe and camber.
    If I'm reading things correctly then the ET method of measuring offset is relative to the middle of the wheel no matter how wide it is? So if you have an ET45 wheel and you go from 5.5 inch to 6.5 inch then the wheel will both "fill out the guards" and protrude inboard by an extra half an inch.

    So if I use the factory offset and a wider wheel it won't change the scrub radius which is measured at the centre of the wheel:



    As a follow up question, I read on some page or other that the inside wheel turns slightly sharper than the outside wheel in a turn (because it's closer to the inside of the turn), is this correct, and does it work with both steering boxes and steering racks? How does this work in theory/practice?

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    >Resident Deflamer< 260DET's Avatar
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    "As a follow up question, I read on some page or other that the inside wheel turns slightly sharper than the outside wheel in a turn (because it's closer to the inside of the turn), is this correct, and does it work with both steering boxes and steering racks? How does this work in theory/practice?"

    That's Ackermann which can be designed into the steering, I have no idea if MX5's have it.

  5. #5
    Meh Billzilla's Avatar
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    Quote Originally Posted by FUBAR View Post
    Stuff
    Keep the same scrub radius as what the car came with, no more. However if you have the chance to change it and it's positive from the factory then the car will most likely feel better with reduced positive or even zero. If it's got negative then I wouldn't add too much more but I also wouldn't use much less. Generally the steering will be nicer and the car behave better dynamically with close to zero scrub or a touch of negative.





    Quote Originally Posted by 260DET View Post
    That's Ackermann which can be designed into the steering, I have no idea if MX5's have it.
    Good luck finding a car, any car, that's been made in about the last hundred years that doesn't have it.
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    Quote Originally Posted by Billzilla View Post
    Keep the same scrub radius as what the car came with, no more. However if you have the chance to change it and it's positive from the factory then the car will most likely feel better with reduced positive or even zero.
    So fitting lower offset wheels will not improve the handling/driveability? It just makes it look more fully sick?

    Will any change to scrub radius cause noticeable tyre wear or just change the characteristics of the handling (make it steer heavier)?

  7. #7
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    Quote Originally Posted by FUBAR View Post
    So fitting lower offset wheels will not improve the handling/driveability? It just makes it look more fully sick?

    Will any change to scrub radius cause noticeable tyre wear or just change the characteristics of the handling (make it steer heavier)?
    Offset =/= scrub radius.

    Changing the scrub radius probably won't change the tyre wear patten, but it can have a pretty substantial change on how the steering feels.
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    Do you mean offset <> scrub radius? But I change the scrub radius by changing where the wheel sits (by changing offset) if I don't change the hub at all right?

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    Meh Billzilla's Avatar
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    Quote Originally Posted by FUBAR View Post
    Do you mean offset <> scrub radius? But I change the scrub radius by changing where the wheel sits (by changing offset) if I don't change the hub at all right?
    I mean offset and SR are not the same thing, though they often do affect each other when you make changes. For example if you use spacers on a wheel you change both the offset and the SR. If you get a wide wheel on the front, depending on the offset if may make no change to the SR or it may change it in or out.
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    >Resident Deflamer< 260DET's Avatar
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    Quote Originally Posted by Billzilla View Post
    ....................................

    Good luck finding a car, any car, that's been made in about the last hundred years that doesn't have it.
    Umm, seeing that the early Datsuns, including the 260Z I had, did not have it I'd guess quite a few don't and I'm not about to guess about something I don't know about.

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    Quote Originally Posted by 260DET View Post
    Umm, seeing that the early Datsuns, including the 260Z I had, did not have it I'd guess quite a few don't and I'm not about to guess about something I don't know about.
    I'm prepared to be wrong, but I seriously doubt that is correct.
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    i have never seen a car that was built in my lifetime without it.
    Originally Posted by Tut
    THEY SHOULD ANALYY RAPE THEMSELVES WITH A FUCING CACTUS UPSIDE DOWN WITH COCONIUTS AND PINEABPPLES AS CHRISTMAS BALLS THE FUCKNIG THIEVENG CUNTS ID RATHER DIE FROM HAVING A RAINBOW UNICORN URETHRALLY RAPE ME THAN TO FUCKNIG DO BUSINESS WITH THEM CUTNST EVER AGAN

  13. #13
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    Quote Originally Posted by bp18t View Post
    i have never seen a car that was built in my lifetime without it.
    It's not impossible but very unlikely.
    BTW, zero Ackerman means zero Ackerman - Even if it's got 10% Ackerman then it still has it though that amount isn't much good. My thinking (and quite a few others) is that something more like 110% - 125% Ackerman is likely to be best.
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  14. #14
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    Often the best way to find out about wheel offset changes and their scrub radius effects is to try them.
    I succesfully raced an IP RX3 with an enormous scrub radius, probably around 70 mm.The steering action was akin to a billy carts and quite heavy. I felt the increase in stability at the front with the wider track was worth the trade off. Tyre wear wasn't a factor but this set up depended on having a very stiff and strong front hub/bearing/stub axle assembly. I played around with Ackerman a lot on this car from quite extreme pro Ackerman to as close to zero Ackerman as I could get given the constraints of the steering gear. There was no real differences observed so the std setting was the default.
    The hub assembly on an MX5 is not overly robust and I have a feeling that they are probablya bit flexible and can contribute to handling issues.

    I currently look after a Sports Car, a Cobra, with quite a hard to achieve zero to 10 mm scrub radius. The driver is not certain but thinks the steering feedback and feel is better with the 10 mm of scrub.

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    So getting back to the basics, on a road car that's driven around the street how much increased tyre wear will there be if I go from 0 to 10mm scrub radius?

  16. #16
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    You won't notice any extra wear.
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  17. #17
    Registered User hrd's Avatar
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    the main effect you will feel of too much positive scrub is instability under brakes, particularly if the road is uneven/bumpy. There was a point at Morgan Park where my old Datto would try to rip the wheel out of my hands while braking if the left front touched the concrete curb at the end of the esses on the old long track with the 13x7 -7 wheels. I changed to 14x7 +6 and that trait was completely gone. The slightly larger diameter tyres (195/60x14 vs 205/60x13) would have marginally reduced the scrub radius also.

    Mac Strut front ends (as my example above had) hate + scrub. Double wishbone or multilink front ends will take alot more + scrub (on RWDs anyway). FWDs hate + scrub radius.

  18. #18
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    Quote Originally Posted by hrd View Post
    the main effect you will feel of too much positive scrub is instability under brakes, particularly if the road is uneven/bumpy. There was a point at Morgan Park where my old Datto would try to rip the wheel out of my hands while braking if the left front touched the concrete curb at the end of the esses on the old long track with the 13x7 -7 wheels. I changed to 14x7 +6 and that trait was completely gone. The slightly larger diameter tyres (195/60x14 vs 205/60x13) would have marginally reduced the scrub radius also.
    Yup - Positive scrub is dynamically unstable, negative scrub is dynamically stable (within reasonable limits of course for both) hence why a negative SR often feels a bit better, etc.
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  19. #19
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    Big positive scrub radius is downright f*cked up. I've spent ages on the liberty trying to reduce it whilst still keeping a 9inch front tyre on.

    Aim for at the most only a very small amount or stock. As part of getting it a little better I've tried to bring as much antidive into the front as possible, which unfortunately has introduced a little more bump steer. But the big benefit of such a big track increase and having big camber adjustment at my lower clevis is the ability to bring that scrub radius back in...which means I now dont have arms like chris atko from a run to the shops.

  20. #20
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    Double joint Strut kinematics

    double%20joint%20strut.jpg Here is a PIC of a double jointed lower control arm , the intersection of the Arm pivots this gives a virtual lower pivot point from which your scrub radius is measured . when the steering wheel is turned scrub radius kinematics changes to a Slight postive offset . BMW consider the slightly positive off set on turns to give the driver a better feel and feed back through the steering wheel .
    So MOMUS your feed back sounds correct on your quote " I currently look after a Sports Car, a Cobra, with quite a hard to achieve zero to 10 mm scrub radius. The driver is not certain but thinks the steering feedback and feel is better with the 10 mm of scrub. "
    Last edited by Cameron_Datto; 16-10-11 at 09:36 AM.

  21. #21
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    That diagram won't tell you the actual scrub radius though, it's on at the level of the lower control arms You need to see the upper mount pivot and look at it vertically.
    But yes, you can see how it does move the SR around a little as the wheel turns left/right.
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