Scrub radius, and offset importance

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

  1. strimmer Forum Member

    Joined:
    Mar 6, 2010
    Likes Received:
    3
    Location:
    On the limits of adhesion
    I put together a spreadsheet a while back that auto calculates scrub radius if anyone thinks it would be useful I'll email it over. Changeable inputs are wheel and tyre sizes and offset. I drafted it up when I was going wide track so the chassis dimensions between top mounts and ball joints are changeable, you can try out wide track/narrow track or any others and see affect on scrub. It also has changeable ride height, on the spreadsheet a 10mm change in height makes for 1mm change in scrub.
    [​IMG]
    Diagram here to help explain were it all calculates from with apologies if a little short on summary, trying to tip tap keyboard quietly with one hand typing and sleeping 5 month old in arms
    [​IMG]
     
    Last edited: Feb 23, 2012
  2. Admin Guest

    I want to make a sweep on the suspension travel and measure the effect this has on the scrub and put it into a graph, I know that there when altering the ride height it is never a fixed figure for a given change, the wishbone pivot point and arc it traces effects the srub at certain points.
     
  3. strimmer Forum Member

    Joined:
    Mar 6, 2010
    Likes Received:
    3
    Location:
    On the limits of adhesion
    Graphing yields clearer results, I hope this spreadsheet could get pretty accurate. Mmmmm, I think I could calculate the changing length of the wishbone as it arcs and affect the changes to the pivot point measurement as the ride height changes.
     
  4. Dave

    Dave *Very Smart* Pedantic Old Fart Paid Member

    Joined:
    Feb 26, 2007
    Likes Received:
    473
    Location:
    Sligo, Eire
    Mike.

    I have massive e-mail problems at the moment. O2 have failed to maintain their system to such an extent that some users, and I am one of them, have almost nil service![:x]
    So, to cure the problem they have decided to scrap the e-mail service completely, so I need to find another provider.:o

    I say this only because you would be very welcome to a .MI file, or a file converted from .MI to AutoDesk. (DWF or DXF or whatever crappy AutoCad files are named:lol:)

    So, until I get a new e-mail service provider, the best I can do is produce a drawing with the data which you require.

    Please post exact details, sketch if possible, of what you need.

    Dave.

    ps. Any other CGTI members who have set me e-mails to which I have not replied. You now know why. Either I did not recieve them, or my reponse was lost in the ether!
     
  5. sparrow Paid Member Paid Member

    Joined:
    Jan 2, 2006
    Likes Received:
    741
    Location:
    Leamington Spa
    Dave

    Get a gmail account. www.gmail.com. It will never change, get lost, go down, etc. You can access it anywhere. You can even use your favourite email program to get the emails from it.
    If you need some help setting it up, let me know.
     
  6. LeftcoastTigger Paid Member Paid Member

    Joined:
    Jan 5, 2010
    Likes Received:
    3
    Toe when braking - -

    On this lovely schematic, a line bisecting the brake disc friction surfaces would fall outside the tire contact patch center

    Presumably toe in under braking is less important than correct scrub - - [:s]
     
    Last edited: Feb 24, 2012
  7. strimmer Forum Member

    Joined:
    Mar 6, 2010
    Likes Received:
    3
    Location:
    On the limits of adhesion
    Err, not quite sure what you mean.... The red line cutting thru the brake disc perhaps, that line is part of a calculation that determines how far the tyre contact patch is from the pivot point for any given tyre size ...

    I have no variables yet to determine changes dynamically.
     
  8. LeftcoastTigger Paid Member Paid Member

    Joined:
    Jan 5, 2010
    Likes Received:
    3
    Steering geometry and effect on braking - - ?

    Oops, my late night brain fade - - leading to an incorrect conclusion[:$]

    To clarify my original suggestion:

    If an imaginary line "1", parallel to and centered between the brake disc friction surfaces, is extended to the road surface - -

    - - and the yellow line representing steering axis inclination is likewise extended to the road surface - -

    - - then the distance between them at the road surface indicates the leverage acting on the tie rod ends & steering rack during braking

    WRONG ASSUMPTION![8(]

    On reflection, I suspect another imaginary line "2" needs to be drawn - - at right angles to the wheel hub and centered in the outboard wheel bearing race (of the two in each bearing assy) - - and extended to the road surface - -

    - - now measure the distance between "1" and "2" where they intersect the road surface - -

    - - to find the leverage acting on tie rod ends and steering rack when braking

    PHEW!

    Must look into purchasing a primitive CAD programme to create diagrams rather than awkward wording

    Apologies to all, and please sing out if my visualisation is again incorrect![:s]
     
  9. mec82 Forum Member

    Joined:
    Jul 23, 2008
    Likes Received:
    9
    unless I'm mistaken the force from braking doesn't act on the steering directly, the only force felt would be that from the scrub offset acting at the ground.

    the hub/disc/wheel assembly is constrained in the bearing/upright, the caliper is also secured to the upright. So there's no resultant force on the steering from braking. the force you feel pulling the steering wheel when braking is caused by the leverage of the scrub offset.

    Dave, thanks for that I'll mark up with dimensions I'm after. If you could export a copy in .dxf format that would be awesome. :thumbup: I can second gmail as well, google are taking over the world so might as well get on side!
     
  10. mec82 Forum Member

    Joined:
    Jul 23, 2008
    Likes Received:
    9
    I've started on a 3D model, it doesn't look like a mk2 but all moves realistically :)

    [​IMG]

    Need some help with dimensions if can Dave or anyone else could oblige?

    Firstly I need the front to rear distance (from plan view) from the strut tops to a subframe reference point, i.e. wishbone mount, not easy one to measure [:s]

    Second set of dims marked on this screenshot of the hub carrier,

    [​IMG]

    I'll also need some steering joint positions as well to constrain the steering arms at some point.

    cheers
     
    Last edited: Mar 1, 2012
  11. Admin Guest

    I could give you some dimensions, but am unsure of there accuracy, therefore I am loathed to post them in the public domain incase they are wrong and people use them to find the results are not what they expected.
     
  12. LeftcoastTigger Paid Member Paid Member

    Joined:
    Jan 5, 2010
    Likes Received:
    3
    Simple, ideally free CAD

    Mec, can you or other generous members suggest a simple 2D cad programme for idiots?[xx(]

    I may have relevant specifications to contribute but lack the word skills to accurately describe their datum points (ahem, refer. previous posts)[8(]

    And, having inherited "tritanopia", the rarest of 3 types afflicting the "colour blind", I'd prefer programmes relying on myriad versions of lines rather than colour:thumbup:

    - - here's an excellent colour test for the three different types http://www.bbc.co.uk/news/technology-13054691
     
    Last edited: Feb 29, 2012
  13. Dave

    Dave *Very Smart* Pedantic Old Fart Paid Member

    Joined:
    Feb 26, 2007
    Likes Received:
    473
    Location:
    Sligo, Eire
    Mike (mec82).

    I have re-drawn most of the lost work. I am now detailing the individual parts in Plan & Elevations + Sections. The upright, top mount, ball joint and wishbone are complete. I am still working on strut side elevation and steering gear.

    Please let me know what you need? Short term, I'll try to post the details on here. Long term, I'll e-mail you dxfs.

    I have resurected an old e-mail account, on Eircom. Trouble is, it is very limited as I don't subscribe to Eircom services any more. So, it may limit what I can send?

    I tried gmail, but was not able to choose a sensible user name.[:x] So I am looking elsewhere.

    LCT.

    I am not sure that there is a simple 2D CAD system worth its salt. ('SALT' is a great book by the way. I have mislaid my copy, which is a shame as I want to reference the chemistry of Saurkraut.:lol:)

    I run Hewlet Packard ME10, which is very intuitive, compared with AutoDesk products. If you can find a copy it is fairly easy to use but you would definitely need to get hold of manuals. Mine uses a network card which can hold the password, as it's a version for server/workstation setups. A stand alone system with dongle should be out there somewhere, as I have seen them. I use a PC running Windows 2000 Pro, as the software I have was designed for NT. I don't know if it works on later versions of windows?

    Dave.
     
  14. LeftcoastTigger Paid Member Paid Member

    Joined:
    Jan 5, 2010
    Likes Received:
    3
    ME10 et al

    Lawdy, there's hope for me yet :clap:

    And, maybe it's formulae will prove as elusive as Belgian beer's?:thumbup:

    Thanks for the ME10 tip, will check online - - regarding Mr Gate's outstanding OS's I have Win 98SE, ME, 2000, XP Pro, and 7, the latter forced upon me recently aargh! - - will hope the latter suits ME10 or equivalent as cannot dedicate one lappy to it:o
     
  15. mec82 Forum Member

    Joined:
    Jul 23, 2008
    Likes Received:
    9
    LCT...might be worth a look at FreeCAD or LibreCAD. Older versions of Autocad are good but as dave said they need training really to use effectively.

    Dave, glad to see you're up and running again :thumbup:
    The main thing I need is the hub carrier geometry, something like the drawing below but I need the dimensions taken from a common reference point ideally like the ball joint centre

    [​IMG]
     
  16. samfish

    samfish Forum Member

    Joined:
    Jul 30, 2009
    Likes Received:
    46
    I am trying to understand all this., so bear with me....

    By scrub, do you mean induced rolling resistance? Scrubbing the tyres?
    Is all scrub bad when racing? Some tracking setting will tend to produce some 'scrub' (rear toe), but could also make you faster around a corner....

    By calculating scrub radius, is that to ideally reduce the amount of rolling resistance for certain sitiations (hard cornering, straight acceleration etc)?
     
  17. Truckinduc Forum Member

    Joined:
    Sep 16, 2011
    Likes Received:
    26
    I have a quick and hopefully easy easy question. I made my strut tops adjustable and made 2.5 inch weld in ball joint spacers. If I push the top of the strut all the way towards the center this will help my scrub radius for a wider wheel correct? I can still set my camber at the strut bottom with the spindle. Ill be running 15 x 8.25" ET0 wheels up front on 195 45 15"s. Here are some pictures.

    [​IMG]

    [​IMG]
     
  18. Truckinduc Forum Member

    Joined:
    Sep 16, 2011
    Likes Received:
    26
    Oh ya. I can take any measurments that we need. I have the whole front of my car apart at the moment.
     
  19. Admin Guest

    The amount of adjustment you have at the top will not make much of a dent to the positive Scrub the extreme 0ET will create.
     
    Last edited by a moderator: Mar 1, 2012
  20. vw_singh Events Team Paid Member

    Joined:
    Oct 29, 2003
    Likes Received:
    793
    Location:
    United Kingdom
    Hey Sam.

    Not that type of scrub mate. This is to do with where the front steering turning pivots act on the floor compared to the actual centre of the tyre across it's width.

    Gurds
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.
    Dismiss Notice