Stiff ARBs on a road car - good or bad??
Discussion
Specifically on an E36 BMW, what are the downsides??
From my limited knowledge I would have thought they offered an ideal solution. Less need to use stiff springs / dampers so the ride stays acceptable whilst minimising roll.
Recently I've read cars are using stiffer springs and not relying on the ARB so much - who's right??
From my limited knowledge I would have thought they offered an ideal solution. Less need to use stiff springs / dampers so the ride stays acceptable whilst minimising roll.
Recently I've read cars are using stiffer springs and not relying on the ARB so much - who's right??
As a general rule, ARB's are bad and should be kept as light as possible.
Without getting too deeply into what is a very complex subject, the biggest and most obvious reason for this is that a single wheel bump at one wheel will try to lift the wheel on the other side of the car, hence they reduce mechanical grip.
Without getting too deeply into what is a very complex subject, the biggest and most obvious reason for this is that a single wheel bump at one wheel will try to lift the wheel on the other side of the car, hence they reduce mechanical grip.
I understand there is more to this than a simply adding bigger bars = better handling, which is why I posted in this section. When certain manufacturers release a Cup, RS or HGTE model they usually uprate the ARBs as part of the package. Is there a certain 'window' of stiffness to work within for a particular application before the disadvantages outweigh any gains?
Is this mainly advantageous for track orientated cars on smooth circuits?
Is this mainly advantageous for track orientated cars on smooth circuits?
Suspension on road cars is always a compromise in that it has to cope with a wide range of loads, speeds, road and weather conditions etc, therefore will be softer than if sheer speed were the only consideration. Stiffening then will have a beneficial effect - trouble is people sometimes go too far and end up so stiff that it starts reducing grip.
Mark34bn said:
Is there a certain 'window' of stiffness to work within for a particular application before the disadvantages outweigh any gains?
No.To repeat what I've already said: ARB's should be kept as light as possible.
Any ARB has negative side-effects, and the stiffer it is, the worse those side-effects become. But there's no line in the sand that you can draw to say 'this stiff is tolerable, but that stiff isn't'.
So why do manufacturers fit them at all?
Two reasons:
- To control body roll.
So there's a compromise to be made between ride quality, acceptable body roll and mechanical grip in terms of main spring stiffness vs. ARB stiffness vs. roll centre location.
- To balance diagonal weight transfer, and thereby the basic handling balance of the car.
You can also manage diagonal weight transfer by varying the relative stiffness of the front and rear main springs (but apart from the obvious ride quality issues mentioned above, there's also pitching response to worry about when you so this), by varying front and rear tyre sections or tyre pressures, or by varying front and rear geometric roll centre heights (as these affect the amount of weight being transferred through the sprung mass; but there is the jacking effect mentioned above to consider, as well).
...So the bottom line is that you should make the ARB as light as possible, commensurate with achieving an acceptable compromise on all the other factors involved.
Sorry, there's no magic formula: it's a matter of juggling lots of different factors (we've only touched on a few of them so far) and every car, and every engineer, will be different.
But to respond to your original comment; ARB's used to be regarded as a bit of a 'magic fix', with people not being fully conscious of their side effects were. Conversely, as dampers have improved it's become apparent that stiffening the main springs doesn't have quite as big a negative effect on ride quality as you might imagine, provided the damping is correctly matched to the springs. So, yes, there has been a trend toward controlling more of the body roll and weight transfer by means of stiffer main springs instead of the old expedient of bolting on a beam axle conversion kit.
Sam_68 said:
... it's a matter of juggling lots of different factors ...
+1Although I think it is true that a large number of car companies put soft ARB's on to reduce to corning performance of lower spec vehicles in a range.
VAG for example put woefully soft ARB's onto their cars as standard, even on the higher specced 'performance' models.
snowmuncher said:
...I think it is true that a large number of car companies put soft ARB's on to reduce to cornering performance of lower spec vehicles in a range.
Just to be clear, though, in and of itself, a stiffer ARB does not increase cornering performance (grip/cornering power). On the contrary: it reduces it.snowmuncher said:
There does come a point though, when the ARB's are so soft, and body-roll and lurch created cause the wheels on the 'inside' to loose traction.
I think you're misinterpreting cause and effect.'Body roll' does not cause the inside wheels to loses traction. That is caused by weight transfer, and the total weight transfer is nothing whatsoever to do with body roll... it's dictated by the cornering force, vehicle track and height of centre of gravity only.
It can, certainly (though can, not necessarily 'does'; and my experience of Skodas and other VAG products leads me to have doubts).
But the ARB is only one contributory factor in the overall roll resistance, so it is not fair to say simply that inadequate ARB stiffness is at fault... you might just as easily (and just as incorrectly) say that roll centre height was too low or main spring stiffness was inadequate.
But the ARB is only one contributory factor in the overall roll resistance, so it is not fair to say simply that inadequate ARB stiffness is at fault... you might just as easily (and just as incorrectly) say that roll centre height was too low or main spring stiffness was inadequate.
ARBs are one part of getting a chassis working best for its main use.
The suspension rate needed for resisting roll is far greater than the other three modes of suspension. Although damper technology has moved on quite a bit in the last fifteen years and is used extensively to slow down roll, ARBs are still a necessary evil, both in road cars as well as race cars.
For most people, who modify their road cars for extra grip, traction and balance, and who do not have huge budgets, ARBs are a cheap way to change the chassis balance as well as sometimes creating more grip and/or traction (and sometimes creating less grip and /or traction). On some cars, where an ESP system is used as an integral part of chassis design, the results are even more blurry. Quite often on these cars the geometry is more designed for perception of balance and grip, rather than reality. Fitting larger ARBs without other modifications to these cars can quite often have opposite effects to what is expected.
On an E36 with more or less standard geometry, a slightly larger front ARB will create a bit more grip and traction if used on dry tarmac and with decent tyres. In the wet it will probably have reduced grip. Before fitting larger ARBs, you might want to determine exactly what you would like the chassis to do different from your current set-up and what you would like the outcome to be. This might lead you to different solutions like stiffer bushings, more caster, roll centre modification or different dampers etc.
The suspension rate needed for resisting roll is far greater than the other three modes of suspension. Although damper technology has moved on quite a bit in the last fifteen years and is used extensively to slow down roll, ARBs are still a necessary evil, both in road cars as well as race cars.
For most people, who modify their road cars for extra grip, traction and balance, and who do not have huge budgets, ARBs are a cheap way to change the chassis balance as well as sometimes creating more grip and/or traction (and sometimes creating less grip and /or traction). On some cars, where an ESP system is used as an integral part of chassis design, the results are even more blurry. Quite often on these cars the geometry is more designed for perception of balance and grip, rather than reality. Fitting larger ARBs without other modifications to these cars can quite often have opposite effects to what is expected.
On an E36 with more or less standard geometry, a slightly larger front ARB will create a bit more grip and traction if used on dry tarmac and with decent tyres. In the wet it will probably have reduced grip. Before fitting larger ARBs, you might want to determine exactly what you would like the chassis to do different from your current set-up and what you would like the outcome to be. This might lead you to different solutions like stiffer bushings, more caster, roll centre modification or different dampers etc.
^^^ Good post.
The only observation I'd make is that the suspension rate necessary to resist roll is not always far greater than for the other modes. For cars with a low CoG, it's perfectly possible to run without ARB's altogether (for example Sylva and Westfield kit cars and the Gordon Murray designed LCC Rocket do with out them, amongst many others).
But yes, as a general rule they're a necessary evil. The problem is that many people rely on them as a primary tool, instead of a means of last resort/fine tuning.
The only observation I'd make is that the suspension rate necessary to resist roll is not always far greater than for the other modes. For cars with a low CoG, it's perfectly possible to run without ARB's altogether (for example Sylva and Westfield kit cars and the Gordon Murray designed LCC Rocket do with out them, amongst many others).
But yes, as a general rule they're a necessary evil. The problem is that many people rely on them as a primary tool, instead of a means of last resort/fine tuning.
Cloggie said:
ARBs are one part of getting a chassis working best for its main use.
On an E36 with more or less standard geometry, a slightly larger front ARB will create a bit more grip and traction if used on dry tarmac and with decent tyres. In the wet it will probably have reduced grip. Before fitting larger ARBs, you might want to determine exactly what you would like the chassis to do different from your current set-up and what you would like the outcome to be. This might lead you to different solutions like stiffer bushings, more caster, roll centre modification or different dampers etc.
Thanks for your post. The car in question is a 328 which has Eibach springs and Bilstein dampers giving a 30mm(ish) drop. This has enhanced aesthetics (IMO), cornering feel and response without giving up ride quality. I have the staggered M3 wheel setup so grip isn't an issue. I don't really drive on the limit of lateral grip, I don't use the car on track and compliance was a major consideration when selecting components. I have tried poly bushes and didn't like the extra harshness so they went in favour of new std items.On an E36 with more or less standard geometry, a slightly larger front ARB will create a bit more grip and traction if used on dry tarmac and with decent tyres. In the wet it will probably have reduced grip. Before fitting larger ARBs, you might want to determine exactly what you would like the chassis to do different from your current set-up and what you would like the outcome to be. This might lead you to different solutions like stiffer bushings, more caster, roll centre modification or different dampers etc.
I'm not really looking for more grip, more to eradicate the initial 'lean' on turn in to give a feel of greater agility and response.
Sam_68 said:
^^^ Good post.
The only observation I'd make is that the suspension rate necessary to resist roll is not always far greater than for the other modes. For cars with a low CoG, it's perfectly possible to run without ARB's altogether (for example Sylva and Westfield kit cars and the Gordon Murray designed LCC Rocket do with out them, amongst many others).
But yes, as a general rule they're a necessary evil. The problem is that many people rely on them as a primary tool, instead of a means of last resort/fine tuning.
You're absolutely right that there are some cars with low CoG and relatively high RC that do without ARBs. Mind you, quite a lot of these do suffer from high speed roll oversteer (also due to lacking rear geometry)and could do with some additional roll control, especially for track use.The only observation I'd make is that the suspension rate necessary to resist roll is not always far greater than for the other modes. For cars with a low CoG, it's perfectly possible to run without ARB's altogether (for example Sylva and Westfield kit cars and the Gordon Murray designed LCC Rocket do with out them, amongst many others).
But yes, as a general rule they're a necessary evil. The problem is that many people rely on them as a primary tool, instead of a means of last resort/fine tuning.
I must say that I sometimes use finely adjustable ARBs as a trimming tool for balance and traction :-)
Mark34bn said:
Thanks for your post. The car in question is a 328 which has Eibach springs and Bilstein dampers giving a 30mm(ish) drop. This has enhanced aesthetics (IMO), cornering feel and response without giving up ride quality. I have the staggered M3 wheel setup so grip isn't an issue. I don't really drive on the limit of lateral grip, I don't use the car on track and compliance was a major consideration when selecting components. I have tried poly bushes and didn't like the extra harshness so they went in favour of new std items.
I'm not really looking for more grip, more to eradicate the initial 'lean' on turn in to give a feel of greater agility and response.
I would be very careful with selecting aftermarket ARBs. Some are not made accurately enough and have large differences in leverarm height, resulting in too much pre-tension. This effectively adds spring rate on one side and takes it away on the other side. I'm not really looking for more grip, more to eradicate the initial 'lean' on turn in to give a feel of greater agility and response.
Fitting a larger rear ARB on your car will almost certainly reduce traction and without a LSD will lose you a lot of speed through a spinning inside rear wheel. While fitting a larger front ARB will increase the traction from the rear wheels, it will, most likely, reduce front end grip in the wet.
With regards to polyurethane bushes, I would say that if you tried British or American PU, the NVH would be pretty bad. But not all PU is created equal. In Australia, where PU is used for normal replacement bushes, very high quality PU is made that has almost the same NVH properties as rubber.
If I were in your position, before I would fit uprated ARBs, I would fit off-set PU bushes (made by an Australian manufacturer like Super Pro) to the front wishbones to increase caster somewhat. This will increase the negative camber on the outside front wheel during turns as well as increase the weight on the inside front wheel. I suspect that this will give the change in feeling during turn-in that you are after, without suffering more NVH.
Mark34bn said:
I'm not really looking for more grip, more to eradicate the initial 'lean' on turn in to give a feel of greater agility and response.
We're getting into the realms of detail that I'm not really happy commenting on without having driven the car and had a detailed face-to-face conversation about your needs and expectations, but from the above comment I wouldn't immediately be thinking 'bigger ARB'.Anti-roll bars have a basically linear response; the more the car rolls, the more they try to resist it.
Obviously, what this means is that on initial turn-in (when the car hasn't got much by way of roll angle), they're basically doing bugger all - they won't be giving maximum effort until the mid-corner point. And you're not worried about traction at turn in, since you shouldn't be standing on the loud pedal at that point so you certainly shouldn't be tryng to take grip away from the front wheels to give it to the rears if initial turn-in is what's worrying you...
Setting aside the possible benefits of an overall increase in spring and damper rates, my immediate response to providing sharper turn-in without degrading traction on the corner exit (and without affecting anything else too much) would be to think in terms of providing more low speed bump resistance on the dampers, so that they 'prop up' (stiffen) the car during the transitional entry to the corner.
In case you're not familiar with damper terminology, I should probably explain that 'low speed' and 'high speed' has nothing to do with vehicle speed, when we're talking about dampers, incidentally; 'low speed damping' tends to control the stuff that happens slowly -cornering roll, brake dive and aceeleration squat, whereas 'high speed damping' tends to control the response to surface imperfections that 'happen' quickly.
Probably the best advice that I can give is that you take the car to a specialist (somebody like Centre Gravity or Steve Guglielmi) and explain to them what you want see as its current deficuiencies and how you would like to see it improved.
Cloggie said:
I would be very careful with selecting aftermarket ARBs. Some are not made accurately enough and have large differences in leverarm height, resulting in too much pre-tension.
...While fitting a larger front ARB will increase the traction from the rear wheels, it will, most likely, reduce front end grip in the wet.
Any decent quality aftermarket ARB (that's not to say all of them - as Cloggie says, there's a lot of badly designed and overpriced 'bling' on the market) will have adjustable length drop links, with slide clamps so you can infinitely adjust the lever arm length....While fitting a larger front ARB will increase the traction from the rear wheels, it will, most likely, reduce front end grip in the wet.
You should always set the corner weights/ride height with the ARB's disconnected, then adjust the drop links so that there is no pre-tension when you re-fit them (and obviously ensure that the sliders are set with equal lever-arm lengths either side).
All other things being equal, fitting a larger front ARB will always reduce the grip at the front, regardless of whether it's wet or dry - that's just basic physics - there's no such thing as a free lunch.
Edited by Sam_68 on Tuesday 30th August 21:10
Gassing Station | Suspension, Brakes & Tyres | Top of Page | What's New | My Stuff