Stiffer Anti Rollbar

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Re: Stiffer Anti Rollbar

Postby bobbrown » Fri 29.08.2008, 13:56

Nige wrote:As I understand it stiffening the rear anti roll bar on a FWD vehicle reduces body roll at that end and the effect on the handling is less understeer, conversely stiffening the front will reduce body roll at the front but will increase understeer. So it comes down to a question of handling balance and preference.

Body roll is not necessarily bad, it only becomes bad when it exceeds the ability of the suspension to keep the contact patch of the tyre firmly planted on the tarmac. When wheel lift or less than ideal wheel contact is reached reducing body roll simply pushes the point that this occurs further out.


Not sure that is strickley true as it depends on how stiff the chassis is and on the elan it is very stiff.
If the car has large amouts of body roll you are reducing the grip on the inside front tyre, it is not so bad on the rear but on the front you then raise issues of putting the power down, as it will tend to spin the wheel that is lightly loaded.
A bigger rear sway bar will move the handling more towards oversteer, and will also help in putting the power down on the way out of corners because it will help to keep the inside front tire planted.
Note: Going from say a 22mm diameter roll bar to say 26mm diameter will just about double its stiffness.
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Re: Stiffer Anti Rollbar

Postby RayD » Fri 29.08.2008, 16:25

Nige wrote:
...stiffening the rear anti roll bar on a FWD vehicle reduces body roll at that end and the effect on the handling is less understeer



Is this the reason for making the change, and are there any others?

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Re: Stiffer Anti Rollbar

Postby Nige » Fri 29.08.2008, 17:07

RayD wrote:
Nige wrote:
...stiffening the rear anti roll bar on a FWD vehicle reduces body roll at that end and the effect on the handling is less understeer



Is this the reason for making the change, and are there any others?

Ray


It's the main reason I'm considering it but I would prefer to go for a complete track biased package rather than a piecemeal approach.

Unfortunately I missed the rear ARB on e bay (not outbid just was doing other stuff when the auction ended).
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Re: Stiffer Anti Rollbar

Postby RayD » Sat 30.08.2008, 20:19

With regard to the understeer...While waiting for a solution to arrive would it be worth trying different aspect ratio tyres on the front and rear, low profile on the front, needless to say.

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Re: Stiffer Anti Rollbar

Postby F1 LOTUS » Sat 30.08.2008, 20:57

I'm just wondering if it's possible to 'stiffen' the rear bar by designing a sliding drop link that would effectively shorten lever-arm portion of the bar - it may have to be done in combination with and adjustable bar-mounting bracket such that as the lever-arm is shortened the bar is moved rearwards to keep the drop-link angle as-stock?
Maybe a similar concept for the front anti-roll bar too? :smt102
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Re: Stiffer Anti Rollbar

Postby Monty » Sat 30.08.2008, 21:22

F1 LOTUS wrote:I'm just wondering if it's possible to 'stiffen' the rear bar by designing a sliding drop link that would effectively shorten lever-arm portion of the bar - it may have to be done in combination with and adjustable bar-mounting bracket such that as the lever-arm is shortened the bar is moved rearwards to keep the drop-link angle as-stock?
Maybe a similar concept for the front anti-roll bar too? :smt102



Brian, that went straight ove my head lol could we have one off your drawings plz :wink:
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Re: Stiffer Anti Rollbar

Postby Nige » Sat 30.08.2008, 22:07

F1 LOTUS wrote:I'm just wondering if it's possible to 'stiffen' the rear bar by designing a sliding drop link that would effectively shorten lever-arm portion of the bar - it may have to be done in combination with and adjustable bar-mounting bracket such that as the lever-arm is shortened the bar is moved rearwards to keep the drop-link angle as-stock?
Maybe a similar concept for the front anti-roll bar too? :smt102


I think that would have the desired effect of stiffening the bar but whilst I like the idea very much (because it's clever and elegant :) ) I would have thought a stiffer bar would be easier (maybe a move from solid to tubular as well)?
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Re: Stiffer Anti Rollbar

Postby F1 LOTUS » Sat 30.08.2008, 22:11

The concept is straight forward enough: -
The short arm of the bar is the lever arm - if you apply load at the end (where it mounts to the wishbone) you have maximum leverage, if you shorten that arm (or grab it closer to the bend) then you'll have less leverage - the bar twists less = stiffer bar.
One way I've seen this done is to slide the drop link along the lever arm and clamp it closer to the bend.
It's not an ideal illustration but quicker than me modeling something...
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Nige - Agreed.
Maybe the solution I've hinted at could serve to establish the required increase in stiffness before fab'ing new bars, although the option to easily revert back to stock must be desirable?
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Re: Stiffer Anti Rollbar

Postby Nige » Sun 31.08.2008, 09:15

Brian,

I like the adjustability piece of your idea as well as the ability to return to standard. It would certainly permit the optimum (preferred) stiffness to be found.
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Re: Stiffer Anti Rollbar

Postby bobbrown » Sun 31.08.2008, 12:02

I doubt this would work, as the ARB acts as a torsion bar, increasing its thickness is the way forward.
Getting new ARB's made up is not a problem, what is a problem is knowing what size it should be.

My solution was to clamp a standard bar with the ends cut off to the existing bar in effect doubling its stiffness which would be about the same as going from a 22mm thick bar to a 26m one.
This is a cheap solution as a test and would be easy to remove if unsucessful. If this proved to be a move in the right direction a thicker one could be made up (just a bar of the correct material with a couple of bends in it) and that tested or even a thinner one if double was too much.

Once the total area of the bar has been established then a new one can be made up.

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Re: Stiffer Anti Rollbar

Postby Monty » Sun 31.08.2008, 14:17

The stock bar is about 14.5mm, so maybe a 20mm might do the job :?

Wouldn`t mind attaching my spare bar but once i`ve chopped it then i`ve go no spare to use aas template if we decided to go with a single thicker diameter bar. :? I`ve got a trackday at outlon park in october so could give this a test, but what would be the effect if the rollbar is too stiff? as i don`t want to risk anything at oulton as it`s known for eating cars :roll:

So has anyone else got a spare rear anti rollbar before i cut mine up?
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Re: Stiffer Anti Rollbar

Postby bobbrown » Sun 31.08.2008, 14:53

Front wheel drive cars tend to understeer. This is the case because the front tyres are forced to perform two tasks – transferring torque to the road and also turning the car.

It’s best to fit a thicker anti-roll bar to the rear of these cars, this helps hold the car flat, counteracting understeer without causing the same problems as a bigger front ARB which tends to lift the front inside wheel, worsening the understeer.

But can the rear roll stiffness be too great? Yes, it can. A FWD car with an over-stiff rear will have throttle-off oversteer. This can be a little tricky, especially in wet conditions or with an inexperienced driver. Someone who lifts off sharply when the car starts to understeer might be a little surprised when the back suddenly comes out! Very sporty FWD cars (and those set up for circuit use) frequently lift the inside rear wheel right off the ground because of the very stiff rear roll stiffness.
But in a front-wheel drive that understeers, upping the thickness of the rear sway bar is a great place to start, it’s cheap too.
If it does not work and you are on a track day 8 bolts and its off! but you should have an idea of what it is like on the road first .
http://autospeed.com/cms/A_2746/article.html

If you mark the bar when it is cut (scribe a line along the length of the bar and mark left and right) you still have a template to get a thicker one made just weld it back together.
The increase in thickness to stiffness is aprox to the power 4 or 14.5 x 14.5 x 14.5 x 14.5 so I would say an increase to 17.5mm should double the stiffness of the rear anti roll bar.

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Re: Stiffer Anti Rollbar

Postby RayD » Sun 31.08.2008, 22:21

bobbrown wrote:
... can the rear roll stiffness be too great? Yes, it can. A FWD car with an over-stiff rear will have throttle-off oversteer.

Bob


Does an Élan, on a race circuit, have pronounced throttle-off oversteer? There are a number of references to it elsewhere on the site, albeit on the road.

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Re: Stiffer Anti Rollbar

Postby Nige » Sun 31.08.2008, 22:38

RayD wrote:
bobbrown wrote:
... can the rear roll stiffness be too great? Yes, it can. A FWD car with an over-stiff rear will have throttle-off oversteer.

Bob


Does an Élan, on a race circuit, have pronounced throttle-off oversteer? There are a number of references to it elsewhere on the site, albeit on the road.

Ray


In my experience you can generate lift off oversteer in an Elan, easier and safer to do on the track where you can do it deliberately than on the road where you usually do it unintentionally. However, I wouldn't call it pronounced, it's more a case of balancing it on the throttle.

The Elan will lift an inside rear wheel on track, as illustrated :-D .
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Re: Stiffer Anti Rollbar

Postby F1 LOTUS » Mon 01.09.2008, 00:40

With the Elan being a relatively highly refined (it's what Lotus do :D )car in terms of balance I'd expect it to be sensitive to roll bar changes, making a big jump in rear [alone] roll stiffness could easily unbalance the car? That's my reasoning for thinking to adjust the stock item first (if practicable). If it's feasible to reduce 'R' by welding different ends (that enable adjustment) on the stock bar it should be possible to learn something incrementally? :smt102
My guess would be that both rollbars will need changing for track use if a significant improvement is sought whilst maintaining good balance - should make an interesting talking point with the experts at Lotus on the 15th :D
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Re: Stiffer Anti Rollbar

Postby bobbrown » Mon 01.09.2008, 09:58

From the picture I would say it is more roll in the corner than anything else.
If you look at both inside wheels they are both tending to lift and most of the load is being carried by the outside wheels due to roll.
Uprating the rear anti roll bar will increase the load on the outside rear in a corner and the force is transmitted to the front inside wheel helping to keep that wheel planted, works on the diagonally opposite wheel.
Lifting the rear inside wheel is not that uncommon in FWD cars just look at FWD BTCC cars they all tend to cock a rear in tight corners.
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Re: Stiffer Anti Rollbar

Postby Nige » Wed 03.09.2008, 10:52

Bob, agreed, the car is rolling in the corner and the inside front is also lifting although not fully clear of the ground (I'm not that brave) and the inside rear is only just clear rather than the full daylight you see on FWD BTCC cars.

Just thinking this through a stiffer rear ARB will keep the front wheels better planted, ie reduce the front wheel lift which as I understand will reduce understeer. However I can also clearly understand that stiffer springing (with appropriate damping) would also reduce the roll as the ouside springs would not compress to the same degree for the same force applied and therefore the acr would corner flatter.

The big question for me is which is the priority, springs and dampers or ARB? I suspect both will ultimately need to be done for a track day setup, but which to start with?
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Re: Stiffer Anti Rollbar

Postby RayD » Thu 04.09.2008, 21:47

Nige wrote:The big question for me is which is the priority, springs and dampers or ARB? I suspect both will ultimately need to be done for a track day setup, but which to start with?


Most people would agree you could make quicker progress through corners with stiffer springs than you could with the 16 year old standard ones.

If a stronger rear ARB was a good idea and it were possible to identify exactly what strength it needed to be, it would be different in the case of standard and uprated springs.

The right ARB for the standard springs would be wrong for uprated ones. It would be wrong by about the same amount as the springs had been uprated.

With this in mind it might be better to change the springs first and in the mean time try higher profile tyres on the rear.

Considering there are two people in it, I didn’t think Nige’s looked too bad for a road car.

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Re: Stiffer Anti Rollbar

Postby Doug » Thu 04.09.2008, 22:51

so who's gunna design the "active suspension" with the electrohydraulic units then?
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Re: Stiffer Anti Rollbar

Postby Kuching » Tue 23.09.2008, 19:21

I’ve just caught up with this thread and it sounds like its getting quite interesting – especially for me as I have a bit of previous with such matters, and as Brian was ‘kind’ enough mention me, I probably ought to reply.

I often hear misconceptions about anti-roll bars and how they work but I’ll spare you my theories on vehicle dynamics because ultimately the spec is defined by what works on the track, regardless of what the maths tells you.

Having stuck my head under the back end of the car and it looks to me like it would be fairly simple to produce a stiffer version. Adjustable blades would be nice but would probably cost in excess of £1000 a set, so may not be practical. I’m thinking of a range of interchangeable tubular centre sections mounted onto fabricated arms. The arms could have 2 or 3 alternative hole positions at the upright end to change the effective lever-arm length (similar to what Brian suggested) and act as a fine adjustment. Once this has been tested and we have a better idea of the optimum stiffness, the design could be simplified with perhaps no need for interchangeable tubes.

I'll knock up a scheme when I get chance and post it on the thread.
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