Scrub Radius- How much of it?

[ripp]

Hi all,
Just went recently to my first track day, and found the wheel bolts undone after only a 20 min session (they were torqued up one day before) and decided to convert to studs after that..
Could only find 100mm long ones and since I am running no spacers they stick out quite a lot - looks quite funny - and was thinking of maybe getting some 16mm spacers (no more than that as I'm gonna have inner arch liner scrub for sure when turning)
The thing is, I know this will alter the scrub radius (beside other things like bearing load, etc) and after reading something about this I didn't came to a certain conclusion regarding how this will affect the handling of a clio (182 cup suspension with 16" ET 37 wheels I should add) so I decided to ask instead
I know there are people running 20-25mm spacers on the front, and others that say you will alter (in a bad way) the handling even with something like 5-10mm.
The last thing I'd like to do is getting it worse for the visual aspect only
Suspension guru's feel free to speak!
Cheers!

 

[imprezaworks]

I had 5mm front and 10mm rear for looks. There was an odd vibration and not wheel balance. Removed spacers, drives faultless, no vibration etc. Personally wouldn't have spacers again

 

[ripp]

wouldn't blame the spacers for your problem (well not mostly). I guess they just amplified some issue you had which wouldn't show up without them on

 

[imprezaworks]

Maybe but the car drove better without them

 

[JamesBryan]

I use 5mm spacers on the front just for clearance peace of mind (and it looks a bit better) when running -1.5-2deg camber.

I'm using Speedline 2118's though which are ET43, so my offset is pretty much inline with oem 16's now.

 

[ripp]

Well that is what I am trying to find out here. It looks like a large number of people are running spacers, are they all doing it for the looks? Have seen proper track cars spaced out, what's the deal then? I'd prefer not to either but because of the fat federals rsr's I have and the 1,5 deg camber the sidewall is quite close to the coilover spring adjusters, so a little as 5mm extra space would help

 

[ripp]

I use 5mm spacers on the front just for clearance peace of mind (and it looks a bit better) when running -1.5-2deg camber.

I'm using Speedline 2118's though which are ET43, so my offset is pretty much inline with oem 16's now.

If you are referring to the ET38 172 cup offset then yeah you are pretty much the same as OEM
What tyres are you using? the 205/45 r16 federals that I have are wider than the stock michelins so they sit closer to the strut, and that is with ET 37 ultras on

 

[JamesBryan]

If you are referring to the ET38 172 cup offset then yeah you are pretty much the same as OEM
What tyres are you using? the 205/45 r16 federals that I have are wider than the stock michelins so they sit closer to the strut, and that is with ET 37 ultras on

Sorry i meant i'm close to the ET37 of the 182 16" wheels. Mine is a 182 also.

I use 195/50/15 AD08R's which are probably equivalent to a 205 width.

172 Cup wheels are ET35/36 i think, so that's probably the most you'd want to go.

 

[JamesBryan]

@NorthloopCup is an expert on all this :tongueout:

 

[ripp]

clio 182 std wheels are ET45, 172 cup wheels are ET38

 

[NorthloopCup]

It can have quite a dramatic effect on steering feel and stability mate if you alter the scrub radius too much.

Have a read on the link below for the full sp.

http://www.hrsprings.com/technical/scrub_radius

 

[ripp]

ok but how much is too much then? I would think that 20-25mm of spacers is quite a lot, but I never intend on going that far, maybe just 10 or 15-16mm maximum on the front
I already read that article Thanks anyway!
On some cars 5mm of extra positive scrub can be nasty, maybe others cope or even benefit from extra 20mm, I just wanted to know what people think as being a good setup for the clio 1*2 range

 

[NorthloopCup]

Well I ran 15mm front spacers on my cup at one point, and it was awful to drive. Personally I'd go no more than 5mm, but that's just me I guess.

 

[ripp]

Well thanks for your opinion! Any idea if the clio has positive or negative scrub straight from the factory?

 

[NorthloopCup]

I'll have a look on mine at some point this week mate as it'll be in bits.

 

[JamesBryan]

I'm guessing they are negative from factory.

 

[Sonic Boom]

I'm going to buck the trend, I run 25mm spacers up front with 2.5 neg and 8mm spacers at the rear also with 2.5 neg
Toyo R888 SG Compound 205/50/15 888's fitted all round.
And to me it handles great, I do have roll centre hubs, full coilovers front and rear with large poundage springs (setup properly) full black series bushes all round, PMS rear beam kit and a White Line ARB.
So I suspect the whole package does have an effect thou.
Rich

 

[con1981]

I ran 16mm spacers upfront with 10mm at the back up until last month. A bit of hard driving and one side came loose now I have terrible vibration, got the wheel that came loose 're balanced and even with the spacers off in still getting vibration st 70mph. I'm presuming the wheel coming loose has distorted the face of the wheel. I will be going for stud conversion in the future once I sort this dam vibration , New wheels on at the weekend. Aesthetically the car does look better with spacers. With 16mm I didn't notice a massive difference in response

Sent from my HTC One M8s using Tapatalk

 

[fixedgear]

I am also keen for someone to advise on this issue on the 1*2 platform.

I have seen a Roll centre kit exists with ball joints to correct any bump steer.

Has anyone explored Ackerman angle benefits/tweeking or fully understood the desired scrub radius on the 1*2. Perhaps some of the race boys have but won't divulge such information on the web.

 

[NorthloopCup]

I am also keen for someone to advise on this issue on the 1*2 platform.

I have seen a Roll centre kit exists with ball joints to correct any bump steer.

Has anyone explored Ackerman angle benefits/tweeking or fully understood the desired scrub radius on the 1*2. Perhaps some of the race boys have but won't divulge such information on the web.

I'm the one who did/does the roll centre kit mate. Correction of roll centres is just one of the major items that need to be addressed.

The Ackerman angle was altered by Renault on the 60mm spacing hubs compared to the 54mm items, and it's noticeable. How much improvement you get is unclear though, and I've never driven one to see if there's any differences.

The only way you could get things bang on would be to have a set of custom made hubs made for a Clio out of billet. Unsprung weight is reduced and the Ackerman + scrub geo could be dialled in to where it needs to be.

On the cup racers, they extended the track width by 20mm per wheel. Then, to get the scrub geo back somewhere near, they ran ET43 wheels.

 

[fixedgear]

On the cup racers, they extended the track width by 20mm per wheel. Then, to get the scrub geo back somewhere near, they ran ET43 wheels.

By leaving the TCA's alone, moving the hub/drive flange out and increasing offset?

Or

Extended TCA's and increasing offset alone?

Please explain

 

[NorthloopCup]

By leaving the TCA's alone, moving the hub/drive flange out and increasing offset?

Or

Extended TCA's and increasing offset alone?

Please explain

They lengthened the tca's mate.

 

[fixedgear]

Correct me if I'm wrong but wouldn't that have very little effect on scrub radius? Only when the wheel centre line and bottom pin relationship changes does scrub radius alter.

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[NorthloopCup]

Correct me if I'm wrong but wouldn't that have very little effect on scrub radius? Only when the wheel centre line and bottom pin relationship changes does scrub radius alter.

Sent from my SM-T230 using Tapatalk

Well by extending the track width, you end up with massive negative camber. This in turn means that the hub angle needs altering to correct the geo, so this in turn alters the scrub radius.

 

[MG cup]

In lemans terms what you will feel on a machpherson strut setup is the car snatching at the wheel when you hit uneven roads, like holding tight to a country lane which is rough on the inside, it wants to rattle the wheel/drag you either way depending on negative/positive scrub IME.

Appoligies if I'm telling you to suck eggs but I wouldn't worry too much about it unless you have found the ultimate setup for you and are happy... As mark said by simply altering camber on these you throw it out so (if you're staying with fairly off the shelf parts) you want to dial in as much caster as you can first of all (AST top mounts) then just enough camber to suit (again ast top mounts and camber bolts to balance both camber and caster each side) then you can play with toe in/out on front an the back to get a nice balance for you. Then there is pressures to get a heat spread across the width. Then depend how you work you need it corner weighted at some point through that too, a genuine black art and nothing but experience will say what's good or not.

Change tyre's and you're back to square one lol, if you're going that deep into it you'll need To start with the building blocks first.

Ignore the spelling, punctuation. On my phone and arsed

 

[ripp]

Very interesting thoughts here
I am thinking of going like this:
FIrst of all I will assume that the std geometry is quite good, hell, that's why we love these cars right?
And based on the things I know about the std suspension setup(ride height, toe, camber, caster etc) I will try to determine what was my initial scrub radius with the OE cup suspension and wheels.
I will then calculate what the current scrub is and see if it changed and by how much.
Will post the results here and from then on experiments will happen
Does it sound like a decent aproach or am I missing something?
p.s: would be interesting to have the complete cup racer suspension changes ..

 

[CamMoreRon]

I have a reasonably good model of 172 Cup suspension in my kinematics software and it shows kingpin offset at ground level (scrub radius) to be -5mm.

Without going into confusing detail: negative scrub is generally considered to be a good thing on road cars (especially FWD) as the way the wheel steers due to accelerating / braking forces counteracts the effect those have on steering feedback & the car's stability in a straight line as you start breaking traction. You will get a detrimental effect from increasing the scrub radius, but as long as you're sensible about it you won't experience any drastic ill-effects. I would say 15-20mm of increase is something the majority of drivers wouldn't really notice.. it's only when you get silly and start fitting huge spacers / wheel offsets that you start really messing things up! I've worked on OEM suspension systems for 6 years and while negative scrub is desirable, you don't get crucified by Vehicle Dynamics if you wander into +ve a little bit.

TL;DR - you'll be fine with 16mm spacers IMO.

 

[ripp]

Interesting that the 172 cup has ET38 wheels while the 182 cup/cup pack has ET45, and despite this the official figures don't spec different track width between them (as I can remember)

 

[fixedgear]

I have a reasonably good model of 172 Cup suspension in my kinematics software and it shows kingpin offset at ground level (scrub radius) to be -5mm.

Without going into confusing detail: negative scrub is generally considered to be a good thing on road cars (especially FWD) as the way the wheel steers due to accelerating / braking forces counteracts the effect those have on steering feedback & the car's stability in a straight line as you start breaking traction. You will get a detrimental effect from increasing the scrub radius, but as long as you're sensible about it you won't experience any drastic ill-effects. I would say 15-20mm of increase is something the majority of drivers wouldn't really notice.. it's only when you get silly and start fitting huge spacers / wheel offsets that you start really messing things up! I've worked on OEM suspension systems for 6 years and while negative scrub is desirable, you don't get crucified by Vehicle Dynamics if you wander into +ve a little bit.

TL;DR - you'll be fine with 16mm spacers IMO.

Thanks for your input here. Do you have any drawings that you could post with some dimensions to use as reference? I found a piece on (vw) mk2 drivers with that info, so something similar would be great to see.

Tapatalk

 

[ripp]

+1

 

[fixedgear]

I've just ordered some 16mm hubcentric spacers for the front.
http://www.ebay.co.uk/itm/371033285011 £26!

Trackday at Bedford on 24th then a sprint with MLR at Cadwell on the 29th October.

Car hasn't moved since April 2015!

 

[ripp]

feedback after fitment!

 

[STEVE.M]

I have 20mm spacers with std et 48 ph2 rims.Since fiiting 205/45r16 I get a scuffing if I`m forced into a pothole or if I really lean on it.This did not occur on 195`s,so if you are running et38,5 mm spacers is probably the max & not really a lot of point to it.

 

[fixedgear]

feedback after fitment!

I think I may run with and without at Bedford to compare.

Tapatalk

 

[CamMoreRon]

Thanks for your input here. Do you have any drawings that you could post with some dimensions to use as reference? I found a piece on (vw) mk2 drivers with that info, so something similar would be great to see.

Tapatalk

I don't have any drawings to hand, but I can sort something out for you! It will take me a little time to prepare, so bear with me.

 

[fixedgear]

I worked on a World Rallycross project up until a couple of years ago, many long days were spent measuring and fine tuning then drawing up and modelling.

Attached was a revised bottom pin that was brought in for roll centre correction. It was really interesting working among some talented designers and engineers including Reiger.

 

[JackJack]

Suspension geo is so interesting, good thread :up: