Rear ARB removal / 6" rims at the back

[Julian H]

Hi,

I have a "Superlight spec" Roadsport 140. Currently with 13" dia 6" fronts and 8" rears. Rear ARB is set one up from lowest setting.

It has been suggested, to improve feel and balance that I remove the rear ARB by one who has done the same, and another suggested I put 6" wheels on the back to match the front.

What do POBC think please?

Cheers.

 

 

[Mankee]

Road or track driving? If mainly road driving, especially bumpy stuff, try removing the rear ARB. I tried removing the rear ARB for track and it was RUBBISH. In my opinion of course. I also tried 6" wheels on the back (with the ARB back on the softest setting and ~200bhp K-series) and it felt very light on its toes and fun. Needed careful throttle control though. I prefer the 7" wheels I now use as I can back get on the power a bit earlier. Not tested different ARB settings yet though.

[Julian H]

Hi Mankee

Your experience seem to fairly closely reinforce the advice I have been given which is good. My Seven is mostly for road use. I intend doing occasional track days but the road set up will take precedence.

Thanks, Julian

[6speedmanual]

I too agree with the above discussion.

I am very happy driving my R400, 220hp on 185/60x13 all round, 6" rims ARB disconnected.

I wrote quite a bit about this before.  Maybe you could even find it using the "marvellous" search function...

Main thing is that armed with a choice of rear rims and tyres you can try all different settings and find out what you like and feel comfortable with.  Fatter tyres do not always give more grip! Especially if a very stiff ARB is effectively negating the grip from the inner wheel on a turn.  Go play and enjoy!

 

Peter

[ballisticsupersport]

Hi Julian,

Do you have a limited slip diff? I was on track with a similar spec seven (ChrisC) behind me except I have 6" rears with a limited slip diff and he had 8" rears and no limited slip diff - I had better traction out of the tighter corners, sadly he had the talent everywhere else!

Also I like the feel of the car moving around a bit! with only 140 horses my view is that 8" are a bit overkill, but that said they fill the arches nicely, the 2 other supersport owners I know run 8" rears so each to their own I guess. I haven't experimented with adjusting the ARB...yet

Tom

[Pendennis]

The rear ARB resists body roll so in effect increases the spring rate of your suspension. It is impossible to state that your ARB should or should not be removed. As has been said you should experiment, that way your establish whether your springs and dampening offer enough roll resistance.

[Julian H]

Hi, thanks for your comments.

Tom, I do indeed have an LSD. Interesting to hear with "your limited talent" you still had an advantage that you put down to the smaller tyres and LSD. 

Naturally I will try the options and make up my own mind but, as a reletively recent Seven owner, it is good to hear the advice given is solid and worth pursuing. Next decision seems to be should I buy a whole new set of four wheels and tyres or just the two smaller rears? Having the two sets will give me the opportunity of running different tyres too...

I think I may slacken off the rear ARB to the minimum for a while and see how I get on with that first though. Little steps...

Cheers, Julian

[Julian H]

Hi

As a follow up to my previous posting, please can someone let me know what wheels I have to buy? If I am getting 4no 6" rims do I just order the wheels that normally go on the front for the rears? Do I need to worry about specifying offsets etc..? Sorry for such a numpty question.

Thanks, Julian

[6speedmanual]

Yes, "offset" is important. Actually the rims should be inset.

(carr ret) ET (actually German for inset) should be 23 or 24 mm on a 6" rim.

(carr ret) Peter

[Pendennis]

Before you go off buying new wheels you should know that an 8" tyre will always offer more grip than a 6".

Its contact patch is larger so it will always offer more grip. However, if the car or driver is not benefiting from that and the 8" tyre is sliding across the surface it could be many reasons, but it won't be because it's too wide. The tyre compound could be too hard for your power or style of driving or more likely the tyre is just plain and simple to damn old and lost its adhesion.

If you go and buy new wheels with new tyres your never know what the cause was because the new tyres will work immediately better than an old hard one.

Your would be far better off buying a new set of decent tyres to suit your 6/8 rim combo and working from there.

[Jonathan Kay]

Its contact patch is larger... 

... only if the pressure is lower. If the pressure is the same the contact area will be the same, but a different shape. (Ignoring the extra weight of bigger wheels and tyres.)

Jonathan

[Pendennis]

When stationary or driving straight. But when the car corners the tyre scrubs and rolls with the ground.

If the camber is correct the full width of contact patch available would be used, but if its all wrong then yes you will not be able to scrub and roll the tyre, but that's another subject altogether and the point I am making

[ChrisC]

Hi Julian

i 100% agree with Tom's comments about low speed corner exit speed, my non LSD, 8" wheeled car could not match his LSD, 6" exit grip.   The other comments I am not so sure of, but thanks Tom the check is in the post ;-)

My car has a rear arb, and it's one of the areas I have been playing with.  I have found if I increase the stiffness, by moving the connections closer to the bar, I get better slow speed corner exit traction, but at a cost, and that's high speed corner understeer.  So I soften it, which reduces the understeer but looses exit grip.  No sure what is faster, cos it's only on track days.

Chris

 

[Jonathan Kay]

When stationary or driving straight. But when the car corners the tyre scrubs and rolls with the ground.

Thanks. So what's the difference between the tyre contact areas of narrow and wide wheels when cornering or rolling?

Jonathan

[Pendennis]

No problem Jonathan.

How could anyone possibly answer that without knowing the type of tyre, profile and actual width let alone the camber setting of a car.

But if you really want to know, the manufacturer could advise. The tyre manufacturer will always provide a recommended camber setting and tyre pressure of their tyre

 

[Julian H]

Hi Chris. Thanks for the corroboration. 

Peter, thanks for the technical info. 

Pendennis, I have no problem with rear end grip. Note I am not dashing out to buy anything, just want to know what I am talking about for now.

Thanks

[6speedmanual]

But if the RARB is lifting the fat 8" tyre off the road it gives zero grip.

 

[Pendennis]

Peter, if your lifting an inside wheel the width of the tyre is irrelevant.

And to be fair, if your lifting a tyre off the ground you have serious issues, I couldn't see the RARB causing that either, in fact more likely caused by the lack of one, springs too soft or too much front end causing excessive body roll. 

FWIW, I do not use a RARB. I've also had turn in oversteer, which was a result of the inside wheel trying to lift. All caused by too much body roll, I didn't want to reinstate the RARB to reduce roll so increased the dampening, problem solved.

Julian, apologies but I was reading into your comments that you have oversteer. So if you have no problem with rear end grip, why mess around with it and why consider going from 8" to 6"

If you want smaller rims thats up to you, of course it is, all was I trying to do is challenge the claim that narrower tyres are better than fatter ones.   

 

[6speedmanual]

I don't. I'm just making the point that just fitting "fatter" rear tyres will not necessarily give more (or optimum) cornering force if other aspects of the chassis are not right.

It is quite easy to demonstrate that the RARB can transfer load to the outside tyre and off the inside tyre to the point where the inner wheel is doing virtually nowt. At this point a non lsd car starts to spin off any torque surplus. An lsd can mask the issue and provoke power oversteer.

Over stiffening the RARB is a nasty place. I can quite understand running without one as I do myself for skinnies all round.

I didn't claim narrower tyres to be better but give a different feel and dynamic to the car from running fatties on the rear.

If OP or anyone wants to experiment there is a world of variation and different pleasures to be experienced.

Just don't expect just to change tyres/rims in isolatuon and expect to be getting the best from the change....

[Pendennis]

Morning Peter. I am not trying to be critical but you state

"It is quite easy to demonstrate that the RARB can transfer load to the outside tyre and off the inside tyre to the point where the inner wheel is doing virtually nowt"

The RARB or the FARB actually do the opposite, the outside tyre being under load will try and force the inside tyre down, not lift. It is meant to prevent the forces of the inside tyre transfering to the outside tyre thus resist body roll.

In trying to understand your comment, I think your stating that if the RARB wasn't connected it would allow the suspension of the inside wheel to travel further ? Is that what you mean? 

If you are then that's not the fault of your RARB, but you should look at your spring rate or dampening.

[6speedmanual]

What can I say?

Not wishing to be argumentative, but simply factual. The ARB does the opposite of what you describe.

Picture this:

Right hand bend. Car rolls to the left compressing LH suspension spring.  Lever arm on LH end of ARB is pushed upwards.  By the torsional stiffness of the ARB the RH lever arm of the ARB is also pushed upwards.  This "lifts" weight off the inside (RH in case case) wheel.  The outside (LH) tyre has to do more of the work.  Load is transferred to the outer (LH) tyre.

For the car to get round the corner a certain amount of cornering force has to be developed by the tyres.  If the cornering force is shared between the tyres they both do some of the work (not equal amounts due to weight transfer).  As the ARB is made stiffer, the load transfered to the outer tyre reduces the ability of the inner tyre to do its share of the work is reduced due to lower ground contact force.  If the ARB is so stiff as to be able to cause the inner wheel to lift, this reduces the ability of the inner tyre to work to zero.  At this point the outer wheel is doing ALL the cornering work.

To illustrate this at an extreme, consider an ARB so stiff that is does not twist with cornering loads. In this instance as soon as the car enters a corner the load transfer to the outside wheel is instant and 100%  The corning force available is that from 1 tyre, not the two working together sharing the load and both developing some cornering force.

A fat tyre putting down more rubber in its footprint does in theory have more mechanical grip, all other things being equal.  A car with stiff ARBs becomes eventually dependent on just the outside tyres for the grip (assuming the CofG is above the roll centre of the car and that the tyre has not yet started to slide at the point where the inner wheels are still on the ground).  So Fat tyres should help here.  But... as most cornering is not done on just the two outside wheels, the inner tyres do some work also.  Now... it is entirely possible to create a situation where two narrow rear tyres both in contact with the road could create availability of more total cornering force than one fat tyre.  

Would never say narrow is better that fat.  Just that narrow tyres on a set up with ARB stiffness more suited to Fat tyres will not really work.  Narrow tyres with a softer ARB stiffness can work very well indeed.

There is plenty of evidence for this - Rally Mk1/11 Escorts. Low and soft at the back, hard and high at the front.  Gives goot turn in at front and nice even load distribution across the back axle to keep both tyres down on the tarmac/gravel.  Optimising cornering force and traction.

Prove for yourself -  Run a test like this:  I run my RARB on second softest setting for CR500 205/50s on rear.  When I change to 185/60 CR28s* the outer tyre gets totally overwhelmed in hard cornering.  The grip level at the rear and early transition into oversteer becomes the limiting factor.  Disconnect the RARB and the cornering force is shared between both the rear tyres (no load transfer to the outer wheel from the ARB).  Greater grip is available than with the ARB connected. Balance is restored and higher cornering force before transition into oversteer.  In fact makes the 7 back into the natural understeerer that it is.  Throttle balanced yaw control and oversteer available on demand.

Pendennis, you must have a pair of spare 175 fronts you could lob on the back to try this....?

* the CR28 is a much less grippy tyre than a CR500.

Ones choice of tyre width depends on many things.  The biggest discussion point on this thread is between ultimate cornering grip of fat tyres and relative playfulness of narrow tyres.  Whilst a 7 on 175s allround "may" not be as fast as one with 175F and 205R around a race track ARBs set to suit), a narrow shod car can still offer huge and ample levels of grip and greater driver adjustability and playfulness.  Recent press on 7s have had a good share of cars shod narrow all round - 160 and the 175 supersport.  All thr write ups and videos relish the tactile pleasure and adjustability of the chassis.

Going back to the very top,... Julian H,  I would suggest you try different ARB settings with The 205s to learn about what changes it makes.  then kit yourself with some 6" and 175s for the rear and go play again. The changes in behaviour from ARB adjustment are more obvious to identify when running narrow tyres.  The great joy of a 7 is that the settings can be changed in a jiffy making experimentation play very accessible.

Go enjoy!

Peter

[Jonathan Kay]

I'll leave you to discuss anti-roll bars.

"A fat tyre putting down more rubber in its footprint does in theory have more mechanical grip, all other things being equal." But, other things (weight, tyre pressure) being equal a wider tyre doesn't have a bigger contact patch, it's just a different shape. (The illusion may occur because it's easier to see the width of the tyre than the length in contact.) This is true across the sum of the tyres at all times (disregarding hills and squat) but the distribution of contact area between the tyres might be affected, depending on camber, compliance etc. (This isn't anything to do with the relation between contact area and grip, which is genuine and not adequately described by classical friction.)

Jonathan

[6speedmanual]

Thanks for keeping out of the ARB discussion   You're very wise, Jonathan! 

The concept that the footprint is proportional to the load carried and the air pressure in the tyre is ok up to the point where the carcass stiffness changes everything.

I intend to do a suspension overhaul this winter. What I will do is take prints of the tyre foot prints of 215 and 185 ZZRs carrying the same load (corner weight).  I can also do this at a series of pressures (I expect that there will have to be BIG steps to make a reasonably measurable difference, so I'm thinking 5, 15, 25, 35 psi) to see how much the amount of rubber going down varies.

It is my belief that between 2 psi increments which are commonly discussed on BC, the biggest changes are due to tyre dynamic feel rather than the absolute level of grip obtainable.  Also the "cold" pressures everyone likes to work to are often radically different from the warmed up running pressure.

Please be patient, I don't expect to be pulling my 7 in bits until after January!

Peter

[Jonathan Kay]

:-)

"The concept that the footprint is proportional to the load carried and the air pressure in the tyre is ok up to the point where the carcass stiffness changes everything." It's always true if there's no vertical acceleration. It's only the shape of the contact area and the distribution between the tyres that changes.(It's inversely proportional for the air pressure.)

Look forward to seeing the results... measurements in Newtons, perhaps? How are you going to measure the areas: fine-squared paper and count, cut out and weigh paper or a camera and image-analysis software? I'd do the last with some pieces of toughened glass as the platforms.

Jonathan

[6speedmanual]

Yes, not having access to a glass workshop floor, I will use the graph paper method.  Probably rub some engineers' blue on the tyre and then lower it down onto the paper.

Yeah, just like potato prints at primary school! :)