Anti-roll bars

[igosling]

Are the anti-roll bars an absolute must? Did anyone out there do a build without them and what the driving issues with or without them. Is there significant difference in the way a Hawk HF will perform.
Front and Back required or have owners found only one is needed?

Second point is if I go for a build without anti-roll bars are they easily added after. Looked at a thread today, with pictures, that looked like it would not be a problem to retro fit these items at a later date

Thanks

Iain

[Martin K]

Should I answer this?

[Bob]

Yes please Martin as I don't have any on my car either!.

Bob,

[chris.richard]

Are the anti-roll bars an absolute must? Did anyone out there do a build without them and what the driving issues with or without them. Is there significant difference in the way a Hawk HF will perform.
Front and Back required or have owners found only one is needed?

Second point is if I go for a build without anti-roll bars are they easily added after. Looked at a thread today, with pictures, that looked like it would not be a problem to retro fit these items at a later date

Thanks

Iain
Not an absolute must, no. Different philosophical attitudes to roll control to be found in threads on suspension. Well worth reading.
Easily retrofitted themselves on Hawk. More complicated by the fact that you'll probably want to change to softer springs when you do!
I can feel a Martin K techno-post coming on....

[igosling]

Thanks so far. I am waiting expectently for the Martin K techno-post

Iain

[Martin K]

I don't really think the subject is philosophical - it's more a matter of facts!

Here follow some facts, you can think about them and apply them, or ignore them. But they are facts!

1) Roll stiffness without anti roll bars is determined by the suspension geometry, which dictates the height of the roll centre, and the stiffness of the springs, and the wheel track.

2) If the roll centre is at the same height as the centre of gravity then there will be zero roll, so the springs and any anti roll bars will contribute no anti-roll effect. The further (generally lower) the roll centre is from the centre of gravity the more roll will be induced.

3) High roll centres have many negative side effects on handling and are generally not found in 'sporty' cars, never mind super cars.

3) If the correct springs are fitted to give reasonable wheel rates and frequencies then in most cases they will be too soft to provide much roll resistance. Conversely, if the spring rates are increased to give the desired roll resistance then in most cases the resulting wheel rates and frequencies will be far too high for good tyre grip.

4) Generally one end of the car will have greater stiffeness than the other. Usually the rear is stiffer than the front, particularly if most of the weight is in the rear.

5) Generally the rollcentre height is different front to rear. The rear roll centre is generally placed higher so that the roll axis slopes down towards the front of the car as this gives a better feedback to the driver when the car is in roll. If the roll axis is inclined the opposite way the car feels less stable in a corner.

7) The track of a car is often different front to rear, particularly in cars which run different tyre widths front and rear.

6) If the rear roll centre is higher than the front, and the rear roll rates are higher than the front and the rear track is wider than the front, the car will have higher rear roll resistance than the front. The increased rear weight transfer when cornering will make the handling tend towards oversteer.

8) Adding a front anti-roll bar - common on even the most simple shopping car for 40 years or more - will address this weight transfer imbalance so the handling can be made more neutral or, more commonly in shopping cars, shift the balance towards understeer. This is because even the least skilled driver will instictively come off the throttle if a car starts to go straight on in a corner. The same driver in a car that begun to oversteer would more than likely leave the road.

In conclusion, it is unlikely that without an anti-roll bar on at least one end of the car that a car could make use of all the grip available from the two outside, loaded tyres in a corner because one will begin to lose grip while the other still has some un-used cornering power going to waste. In other words, the car that uses the available grip from the front and rear tyres most effectively will be able to go round a corner quicker than a car which wastes grip at one end or the other. Anti-roll bars enable the correction, or re-distribution, of cornering loads and can tune the mid corner (constant radius, constant speed) undesteer/oversteer characteristics of a car to suit the driver's preferences.

Anti-roll bars always have my vote for the above reasons.

[catswhiskers]

Thanks for that excellent description Martin.

I've always found this subject much deeper and complicated than I had the ability to take in, but I think I actually understood most of that in the way that you explained it. :)
Well done

Mick

[Marmott]

I do not use any roll bars,but have a lsd, not saying they would not improve matters but its a most controllable car as is,
guess a properly set up roll bar is agood but how bad is a badly set up one!

[igosling]

Martin,

Thanks for the very detailled response.

Iain

[Martin K]

I do not use any roll bars,but have a lsd, not saying they would not improve matters but its a most controllable car as is,
guess a properly set up roll bar is agood but how bad is a badly set up one!

I've got a two cats and a dog. They have about as much impact on weight transfer as your LSD!

Of course, a badly adjusted anti-roll bar can have negative effects. On the Hawk the one supplied by Gerry is adjustable, so you have a range of adjustment to experiment with - from disconnected, then connected but full soft through to connected and full stiff.

The Hawk does need a front anti roll bar - particularly if running the 325lb front springs and 300lb rear springs that Gerry supplies (or used to) as 'standard issue' because the rear roll stiffness is significantly higher than the front with this setup. So really you could only improve matters with a front anti roll bar - even on full soft. A test session, making small adjustments between runs, should enable the correct setup to be arrived at fairly quickly, so there should be no excuse for running a badly setup one.

A badly setup one would be apparent when if instead of having oversteer you ended up with drastic understeer - easily identifiable and quickly corrected by adjustment I would have thought.

Adding a rear anti roll bar does add to the compexity of setting the car up, so best avoided unless camber changes in roll need to be managed. This might become necessary if running very high grip tyres - slicks for example.

[John]

Martin,
thanks for that post. I do find your contributions very interesting and like Mick it's all a bit over my head normally but that was quite clear.
As someone keen to learn more I wonder if you can recommend any books?
No doubt you have learned from a lifetimes experience but you may know of something other than Alan Staniforths Race and Rallycar book. I have read Herb Adams book too, which is straightforward and explains some things that Mr Staniforth presumes you already know.
Do you use any software too?

[Martin K]

I'll do a list of books tonight - be good if I could post it somewhere that could be refered to more generally - rather than imbedded in a thread somewhere. Any suggestions?

I'm afraid that my experience is that for every book you read you have to do a load more study to get to the bottom of the theory. As you say, Saniforth assumes you know x amount, Milliken assumes you know x^2.

You need a basic ability and confidence with maths, including geometry, trigonometry and algebra - but nothing much beyond school stuff (well 1960s school stuff anyway!)

When you have read enough you get to the stage where you can find errors - as I have in Staniforth's Competition Car Suspension Design, Construction, Tuning. It's only an error in a formula - probably a type setting arror - but if you followed the book blindly it would all go horribly wrong! Interestingly the error is in Chapter 8 - the weight transfer section!

I have written loads of little spreadsheets to help me quickly understand a setup. I have 4 roll bar calculators in one spreadsheet which use the formulae of Mike Pilbeam, David Gould, Fred Puhn and Martin Kift to calculate bar rates. I have incorporated these (well mine actually) into a full chassis analysis spreadsheet based on David Gould's one. This can predict cornering G, wheel lift, roll angle and weight transfer to individual wheels and can allow the selection of the correct anti roll bar size and the correct lever arm length.

I wrote another spreadsheet which was simply for wheel rates and wheel frequencies (including lever arms etc) based on the cars weight front and rear. I did this for our car originally to select the spring rates to input into the above formula, but I have adapted extensively since to quickly answer questions posed here! I have a Hawk and Corse setup on the same spreadsheet, allowing for the different front and rear suspension geometries.

This spreadsheet has helped several replica owners here, I am happy to say. I have now extended it to help with damper settings. When I have proved that this works on our car I should be able to advise on damper settings too, based on car weight and spring rates.

I had to do the damping stuff from scratch myself because, despite years of searching, this is one area of research that appears to end in a black hole as deep as any in the universe, from which no imformation can escape. A mathematics book called Statics and Dynamics borrowed from a mate of mine who learned how to do build tanks, rocket launchers and other such horrors while sponsored by Vickers on his degree course, finally revealed enough in just a few pages of equations to get me started. As I said, if my mathematical model works (and I am sure it will with a few basic assumptions corrected after empirical research) I will happily share the knowledge I find here. A background in electronics and experience of tuned circuits, and the damping of them, also came in useful with this spreadsheet.

One day maybe I'll find time to write a book, tidy up the spreadsheets and sell it all for a couple of quid on CD-ROM. One day........

[mudhut]

One day maybe I'll find time to write a book, tidy up the spreadsheets and sell it all for a couple of quid on CD-ROM. One day........

Please do it soon, there's a whole host of people who would pay rather more than a couple of quid I suspect. :)

I'm nearly too old to get into the car to try out any changes. :(

[John]

I'm nearly too old to get into the car to try out any changes. :(

You and me both!

Martin,
that is all most interesting. Perhaps further posts in a new thread with a suitable title to attract attention from anyone searching in the future? Suspension Design?
Thankyou very much.

[Sando]

Good stuff Martin thanks.
Separate thread on books would be fine.
cheers
Rob

[PANOS]

I always new this guy Martin is one of the good guys...!!! RESPECT.

CIAO PANOS

[Arthur]

Weight transfer - a simple start.

This is a very simple overview, and hopefully explains what springs and bars do in a way you can keep in your head.

Cornering generates a lateral acceleration. That acceleration mimics gravity, and effectively moves your Centre of Gravity outboard in a corner. This therefore moves weight from your inside wheels to your outside wheels. The effect of that weight transfer is to generate roll as the springs compress or expand, and the amount of roll depends on your specific geometry and spring rates.

Any acceleration moves your CG either in a lateral plane, a fore/aft plane (braking and accelerating) or a mixture of the two - diagonally, depending what's happening at the time.

Weight therefore shifts about all the time, and the whole point of setup and tuning is to modify this to best suit your car and your driving requirements.

The amount of weight transferred laterally is indepent of everything except the height of the CG, the track dimension, and the amount of acceleration ("G") developed.
For fore/aft weight transfer, substitute wheelbase for track.
(Note that most books assume 1G to simplify the maths).

The amount of roll developed depnds on your specific suspension layout - in particular your "roll moment". This is the G developed multiplied by the "roll lever", which is the vertical height between the CG and the roll centre.

Already we can see that the TOTAL weight transfer is independent of the roll centre height, but dependent on the CG height.

So the amount of roll can be modified by geometrically moving the roll centre height to suit you.

So what are we aiming for, and why?

First, we want minimum weight transfer.
Why? - because tyres grip harder with increasing download, but the increase is less that 1:1. Therefore the gain in grip on the loaded tyre in a corner, is less than the loss of grip of the the unloaded tyre. Put another way, the two tyres more lightly loaded will generate more total grip than one tyre heavily loaded, and one waving around unloaded.

How do we get that?
Two ways. Wider track (or longer wheelbase), and lower CofG. And that is that. Everything else is concernerd with dealing with the weight transfer you have.

Now on to roll.
So what's wrong with a vehicle rolling like a jelly, provided we can keep the wheels on the ground? Essentially nothing - provided you can keep the tyre treads in best grip with the road at the same time. And there is the problem.
As the car rolls, the geometry of your suspension pulls the wheels all ways; therefore the whole of the problem is to maintain tyre tread in best touch with the road in any given circumstance.

No roll
Looks easy on paper - design your suspension to put the roll centre at the CofG height. No roll lever; no roll. Does it work? I've never tried, but since no-one does it, I'd say no. Why? Swing axle jacking.
What's that then? OK, when cornering, the tyre force is reacted at an angle, through the roll centre. But the cornering force required is flat lateral parallel to the road. This means we have one of those nasty force vectors. Imagine the triangle - from the tyre contact patch through the roll centre (arrow in direction of roll centre). From the tyre contact patch at gorund level and parallel to it, arrow towards the centre of the vehicle; now in your mind drop a vertical line from the roll centre to the ground. The vertical height of that line is loss of weight. It acts upwards. It is generated by every suspension, and is a function of roll centre height alone.
(I'm given to understand that the Formula boys set this slightly undergound, not so much to GAIN extra load geometrically, but just to be sure they don't lose any).

So what does that mean, then?
It means that while a high roll centre will minimise roll, you don't want that particularly, becuase it geometrically buggers the load on the tyres, and the grip they can generate. On the other hand, the lower you place the roll centre, the more roll the vehicle will develop, and the more problem you will have controlling the wheel angles relative to the road.

How critical are the wheel angles? Pretty much everything. Doesn't matter how good the rubber is, if you are on an outside shoulder in a corner, you won't get much grip. Add to that the deformation of rubber under load, and you start to see the size of the problem.

So how do we control wheel angles, then?
By limiting roll.
How?
By resisting roll with springs, and with roll bars.

So - pros and cons.
Springs first.
Light springs, much roll, wheels waving around. BUT - clever geometry can limit the damage quite well, such that the angles generated by bump and droop can be countered by the angles generated by roll, the result being pretty good.
Heavy springs, limited roll, wheels better controlled. BUT - also the ride becomes a liver-crippler, and on rough surfaces the tyres will spend more time skipping across the surface, rather than in contact with the road and gripping. But this is still far and away the best way to go on smooth tracks.

So this is where bars come in.
They serve two functions. They act as extra springs, stiffening the car in roll only, and have no effect otherwise.
They are also tuning devices. They can move weight diagonally across the vehicle, and do it in a way that is relatively quickly tunable.
Downside? - they do it by moving weight directly from the inside wheels to the outside, loaded wheels, which is something we've so far been trying to avoid. On the other hand, they can allow excellent roll stiffness, while maintaining a reasonable ride quality for the driver.

That last bit - diagonal weight transfer - do I want that? Maybe you do, maybe you don't. Your vehicle will almost undoubtedly transfer weight diagonally. Weight will transfer, whether you like it or not, from the wheel with least roll resistance, to the wheel with the most.
Note that I didn't mention springs there - just because a spring is heavier, it doesn't mean that end of the car is stiffer. That depends on the geometry and the weight at that end, so you need to work that out. But it will happen one way or the other.

(Beware spreadsheet calculations here - they all (mine included) imply steady state cornering, which never happens. They are OK for illustrations of what's going on, but need interpretation rather than belief. For instance, I have been unable to model what happens when you left-foot brake a car into a corner, gradually easing the brake as the steering lock goes on. This shifts bump travel in a controlled fashion, trades it for roll generated travel, and in doing so moves weight I can't find to places I can't calculate. And that's all done by the driver; it's deliberate, and it's variable.)

So, we get to under- and over-steer.
Understeer - more grip at the back than the front.
Oversteer - more grip at the front that the back.
WE can affect this with springs or bars. Let's say you believe your understeer to be caused by too-little load on the front tyre on turn-in. Putting a stiffer spring on the front will increase the stiffness on that corner, and cause weight to transfer to it as the car rolls. The extra weight will come from the inside rear (mainly).
Of course, it may be that you're overheating the front tyre on corner entry, cos its rolling onto the shoulder and frying under brakes and cornering. In this case softening the front may well helps; or indeed stiffening the rear.

Now putting a bar on gives you a very adjustable tool. You can play with the stiffness quite easily, so tuning is less of a problem. BUT it works by moving weight from the inside wheel. So in the above example, it will indeed stiffen the front outside wheel, but will do so at the expense of the inner. The result MAY be more grip at the outer, but you can go from underloaded to overloaded very quickly, since the inner tyre used to work a bit, but now doesn't work at all.

I'm not syaing they're a bad thing - Lotus for one swear by them - but they're not he be-all and end-all. Formula cars tend to use them at one end only or not at all. Other formulae may use them all over the place, or in the case of Monoshocks, essentially run nothing but a roll bar.

And that seems a good place to stop before I blow my own mind up, never mind yours.

[GaryFitz]

I regularly run my cars with or without rollbars of different thicknesses depending upon what I am trying to do with it. If we know that the rally will be very rough or have lots of tight slippery turns then we run without roll bars so that we can flick the car, use a small front bar only. As Martin articulates, running without the bars enables a greater amount of weight transfer from one side to teh other which is exactly what you want on slippery gravel roads. On the other hand if the rally is fast, flat, and open then we will run with bars....but.....you need lightening fast reflexes to catch the slide and often we will run a stiff front bar and a soft rear bar so the rear slide is more predictable.

On tarmac or on circuits I would always run with rollbars tuned to whatever handleing you prefer.