Scrub radius and offset - any real experience?

Discussion in 'Suspension, Brakes, Wheels & Tyres section' started by Gtigor, Aug 11, 2017.

  1. Ok, the wheel for the M3RS have been always a debatable topic. But most of the threads are about clearence/rubbing/lowering (I get a little nervous on things like lowering for looks and all that boy-racer stuff). But I have not seen any fruitful discussion on offset effect on scrub radius and the handling.

    So the M3RS has -14mm scrub radius (google), fitting ET50 rim will make it +1mm which theoretically should drastically influence in a bad way on turn-in, steering balance (add understeer), steering feel, torque steer.

    But what the real life experience is? I know a lot of folks says its fine on ET45 wheels, but how competent drivers theese guys are? Did any one make back to back tests?

    I really would like to preserve the handling of the Megane, it's the best part of the car (I am a fan of oversteering french hatches since 205gti and SaxoVTS), why ruin it?
    So any real life experience with that?
     
    trickmansa likes this.
  2. mie

    mie

    Игорь, судя по нику, тебя зовут...[emoji6]
    На моем мегане, 17х9 вылет 42, еду BTG Nurburgring, 7:42 , так что все ок [emoji1305], не заморачивайся [emoji23]
     
  3. English would help...
     
  4. racers like `jamie and oddison` have used various offsets, on there track orientated mk 3`s to great effect.
    never heard them complain about wayward,or unpredictable handling.

    I have et50 wheels on my R26,and havnt noticed any difference in on the limit handling.
     
  5. -Jamie-

    -Jamie- RSM Moderator

    There is a definite difference with lower offset wheels, More torque steer/understeer are the most noticeable ones.

    I wouldn't say its that bad to not go lower offset though, My road wheels are ET55 9j but i run a 255 on the front so that effects how its drives as well.


    New track wheels are 17x9 ET45, be interested to see how they work
     
    Gtigor likes this.
  6. -Jamie-

    -Jamie- RSM Moderator

    The majority of those that run low offset wheels are also on decent suspension setups that you can adjust to balance any ill effects, Id be interested to see how a lower offset rim works on stock suspension mind you
     
    Gtigor likes this.
  7. He says he's got a Megane with 17x9 wheels with ET42 and has done a 7:42 at the Ring. Then tells the OP so it's all OK and not worry or confuse himself about it.
     
    ultralight likes this.
  8. Thanks Dooook.
    Was more of a statement that it's an English speaking forum (which isn't my mother tongue either...).

    17x9 ET42?
    ATS GTR?

    That suggests that the car runs with a completely different setup and at least -2.5 degree camber on the front and a fair bit on the rear (as the wheels wouldn't fit otherwise). So hardly comparable?
     
    DOOOOK likes this.
  9. -Jamie-

    -Jamie- RSM Moderator

    Mie runs on AST 3 ways, not sure about rear camber
     
  10. mie

    mie

    Less then 2 degrees in the rear
     
  11. The basic purpose of the Renault Perfohub strut design (similar to the Ford Revoknuckle design used in their high power FWDs) is to decrease the scrub radius, by having a negative scrub radius, stability and steering feel are improved as well as a reduction in torque steer. Decreasing offset will increase scrub radius, the more scrub radius you lose, the lighter your steering will feel and the less feedback you will get from the road, not to mention an increase in torque steer.

    There is more to it as well, the system isolates the king pin axis angle from the angle of the suspension strut, reducing the kingpin offset and thereby controlling unwanted camber change during turning.

    car_photo_338014.jpg

    See how the angle of the steering axis is different from the angle of the strut. The scrub radius is the point at which the wheel centreline, and that kingpin axis angle intersect. By shifting the wheel outward, you shift that centreline and change the scrub radius. All bad things.


    Plagiarised from FuRyZ 25 April 2015
     
  12. So a camber adjustable top mount, like most of us who track Meganes frequently have, should also impact on scrub radius? As it will angle the whole hub assembly... Could that not then also have a "negative" impact?

    Having used 9j et42 wheels on my Megane both on road on a 235 and 255 and on track on a 255 and not just theorised, I would recommend it.

    Do we also no longer consider a wider track and larger wheelbase a benefit when you consider it effectively lowers the centre of mass......

    It's apples and oranges, we all work within the basic structure Renault gave us, and all changes have both positive and negative impacts, usually in equal and therefore a balanced nature.

    If you want lower offset wheels, buy some, put them on... If you don't, then don't.

    Sent from my FRD-L09 using Tapatalk
     
    odinson and Jimbob 2705 like this.
  13. NJH

    NJH

    Adding -ve camber at the top mount can only increase the scrub radius a small amount.

    ISTR back in the midsts of time SAAB put -ve scrub radius on their cars (900) for safety reasons, it meant the car was controllable if a tyre failed or if there was an imbalance in the breaking. The reason why is because the resulting forces on the steering tend to lead the driver to correctly compensate whereas with +ve scrub the wrong input would be given potentially leading to an accident. I believe that later on companies realised the same force control loop could be engineered to reduce torque steer and here we are.

    Set your toe in to 0 to help limit torque steer, my mechanic recommended it, I was very surprised how much of a difference it made to my R26. It used to tug over bumps and broken heavily cambered bits of road.
     
    dale.groves likes this.
  14. Yes, but only a small amount.

    I've also used the OEM tyre and wheel on a Trophy R and an OZ 18x9" ET55 with 255 R888Rs on the track and therefore not a theoretical experience but certainly anecdotal. Scrub radius is not a theory, it's an engineering principle.

    I can't speak for you but you can get a wider wheel to increase track but keep the same offset and therefore not ruin the handling characteristics. You can't increase the wheelbase unless you move the rear axle relative to the front wheels no matter how wide you go.

    Whether it's apples, oranges or a pumpkin, it all adheres to the laws of physics. I disagree, not everybody modifies in an equal or balanced nature. If you have two choices with one increasing track without negatively impacting on scrub radius and the other one with increasing track but negatively impacting scrub radius, I know which one I would choose. But each to their own.

    Sure, if you want to make informed choices then research to make smarter choices. If you don't, then don't.
     
    Gtigor and ultralight like this.
  15. -Jamie-

    -Jamie- RSM Moderator

    The reality it though, 99% of people won't notice much of a difference or be bothered by it and those that will/do will either drive around it run weak wheels or adjust setups to suit.

    I know I'm in the latter, my wheel specs ain't ideal but they are a damn sight better than oem and allow me to run a 255
     
    massivewangers likes this.
  16. I think this is the crucial thing in reality. While there are potential negative affects, most normal people are unlikely to really notice them. I had some involvement with the K&C design of a small sports car in my last job and we essentially used a table with all of the values (camber, toe, KPI, scrub radius etc etc), based on the proposed geometry. Each one had a "window" in which it would be acceptable, so the suspension hard points were adjusted in order to try and keep each parameter within the given window, while meeting other requirements such as packaging. One or two could even be slightly outside their respective window and it wouldn't necessarily be dire in terms of the way the car drove. Of course, you could argue that Renault will have made the car as good as possible but, the reality is, if you change it slightly, it probably isn't going to make that much difference. I know a lot of us like to think that we're Lewis Hamilton, but we're not. We're not going to notice a tiny change in geometry. Sorry.
     
    ewant81 and engine killer like this.
  17. After switching to aftermarket wheels (et50), the car moves around more on the road while under full acceleration. Like torquesteer but not that jerky if that makes sense.
     
    Gtigor likes this.
  18. The body, where the suspension pickup points remain unchanged in an rs, is built to receive a 1500cc diesel engine that wouldn't pull the skin off a rice pudding.

    The car is mass produced ffs... If you want wider wheels, put them on, set the stopwatch and show me the difference. I argue I get more time back for having a 255 front tyre, than I lose for a few incremental scrub radius changes.

    This forum drives me insane..

    Sent from my FRD-L09 using Tapatalk
     
    ianplymouth likes this.
  19. To clarify, when I say "drives me insane" believe it or not, most people go on forums because they want to modify their car. There are other reasons, but that is the Biggie.... And every other post I open there's someone else telling people not to do this and not to do that... I wondered, when I first got renaults, why so many are standard or lighly modified at best....

    Sent from my FRD-L09 using Tapatalk
     
  20. whys that then pal?
    If you know better than the masses then please explain.
    How would you expect someone with limited mechanical knowledge to know any better?

    I am a roofer,and people spend hundreds trying to repair a roof, i could sort out for a fraction of the cost,because they dont know any better.
     
  21. cars forums in general..try a honda forum ,if you want all kinds of conflicting information!
     
  22. Because rather than telling someone things just won't work. Tell them the possible ways it could.

    I would sooner the next discussion about wider wheels, which we have every week, be about helping someone get what they want, onto their car.

    That's all.

    Sent from my FRD-L09 using Tapatalk
     
  23. With 17 sets of different wheels on my Megane in the last 2 years with a different in width from 8-9j and offsets form et30-55 I can tell you that nothing changed the feel unless it was the 8.5j et30 which ultimately didn't feel great, the car felt baggy but overall you won't notice much at all tbh. Feel wise I was a fan of 17x9 et43 with 255 front but it still felt pretty much the same as an 18" wheels ona. 235 tyre. Moral is don't get too caught up on it just stick with a wheel that doesn't rub and give you any drivjng issues
     
    R3LMV, Gtigor, ting_tong and 3 others like this.
  24. In my opinion, having a forum where people add their own technical insight is ideal. It's up to you to decide what you want to do, and who you listen to. Why else would you post on a forum? If it's just get to someone to give you positive feedback, then just ask for positive feedback and ignore the negative.
     
    Gtigor and DOOOOK like this.
  25. I am on the other side of the fence. I am surprised that sooooo many people want to modify their cars for some uncler sake... Mostly for the sake of modifying itself or even worse for looks (arch gap, wider track and so on). It is plain stupid. Especially fitting coilovers for those reasons.

    Guys who track/race seriously is another story.

    Roads in Russia are not so bad as they used to be, but we have lots of lenghwise ruts where even standart car behaves terribly under acceleration. And if it's wet...:fearscream: It literally can jump fron side to side. So this is a big point...

    To summ it up I will probably be looking for some 50-55 et for track/summer use.

    Thanks everyone for input!

    PS Here's a pic from my last track fun.:grinning:
     
    DOOOOK likes this.
  26. Thanks, that was exactly what I was interested in!
     
  27. Different strokes for different folks as they say. Perhaps not in Russia, but they say it here!
     
    Gtigor likes this.
  28. I have et50 wheels on my R26,and as said..there is possibly a bit more tram lining..but nothing silly.
     

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