Scrub radius and offset - any real experience?

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]

 

English would help...

 

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.

 

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.

 

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?

 

Less then 2 degrees in the rear

 

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.



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

 

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.

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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.

 

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.

 

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.

 

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.

 

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..

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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....

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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.

 

cars forums in general..try a honda forum ,if you want all kinds of conflicting information!

 

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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.

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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

 

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.

 

Different strokes for different folks as they say. Perhaps not in Russia, but they say it here!

 

I have et50 wheels on my R26,and as said..there is possibly a bit more tram lining..but nothing silly.