ARB sizes

[Nefarious]

How big are yours?

What are the best sizes? Is bigger necessarily better?

I measured up at the weekend for my nice new poly-bushes and found 25mm at the front and 28mm at the back (which is apparently larger than "standard", whatever standard is). I had expected to find a thicker one on the front than the rear??

Seeing as I have no intention of taking everything apart again after the new bushes go in, is there any mileage in fitting the "best" ones now?

(Car has a "later" type rear ARB, is running Nitrons, and is soon to have the Steve Heath ARB droplinks)

[shpub]

There are two important factors in the ARB: the bar diameter and the length of the arms. The thicker the bar the stronger the ARB response is given that all other dimensions are the same. This is why the arm length is important as the shorter the arm length, the stronger the ARB correction force. So you can have a 25 mm ARB that is stronger i.e. more corrective than an 28mm simply because of the dimensions.

So... it doesn't naturally follow that the 28mm bar is heavier than the 25mm front one. IIRC the 28mm bar has longer arms than its 25mm early predecessor so is actually a weaker setup.

[Nefarious]

Is the 28mm bar standard fit for the later design then?

FYI - The drop links on the later design are much longer than on the early one, but still not as long as the fronts.

[shpub]

It is not the length of the drop links that matters but it is the horizontal distance of the arm on the ARB itself. All the drop links do is to transfer the force between the suspension and the arm. The length doesn't change the ARB characteristics providing the links are the same length. If the link length is different this preloads the ARB by introducing twist into it which is not usually done on a road car.

The ARB is a U shaped and consists of a horizontal part that the ARB pivots on and that goes across the car. The arms are the bits of the ARB that are at 90 degrees to this part. Some adjustable bars allow the connection point for the drop links to be moved along the arm to effectively allow the arm length to be changed and thus the ARB's strength

On some bars the arm is at 40 degrees which makes life a little bit more complicated as the vectors etc have to be resolved to calculate the arm effect. It is these arm dimensions that are critical.

Edited by shpub on Tuesday 9th December 11:00

[Nefarious]

I see what you mean now.

So how does one go about calculating the optimum arm effect? Can you calculate a best guess relative to the spring rates, or is this another aspect of the 'black art' of suspension set-up that just requires lots or experience, tiral and error?

[shpub]

It is a bit of a black art. The maths to calculate the ARB's contribution is straightforward (I did applied physics which helps) and you need to understand how much suspension movement there is and from that you can then calculate the proportions and make some approximations to what you want. You then take the car out on a track and start measuring performance and handling and then make appropriate changes and so on. This latter part is where the black art comes in, especially as changing the settings is more of changing compromises rather than finding the optimum. An improvement in one aspect is often at the cost of something else.

The main problem with the ARB design is that the rubber bushes on the OEM drop links cause the ARB correctional force to be applied in a very unpredictable way and this leads to increased body roll and twitchyness. This can be corrected with a stiffer suspension but now the ride quality can be compromised. With the rose jointed droplinks, the suspension can often be softened as it no longer needs to compensate for the ARB which means that you get a softer ride without the body roll and twitchy ness.

[Nefarious]

shpub said:

With the rose jointed droplinks, the suspension can often be softened as it no longer needs to compensate for the ARB which means that you get a softer ride without the body roll and twitchy ness.

This is the aim of the current endevour. The whole suspension system should be completely rubber-free by the end of january.

I'm just trying to get it right first time, rather than strip and rebuild it all, pay for a full geo and then find that I would have been better moving to a bigger/smaller ARB.

[markh1]

Nefarious said:

shpub said:

With the rose jointed droplinks, the suspension can often be softened as it no longer needs to compensate for the ARB which means that you get a softer ride without the body roll and twitchy ness.

This is the aim of the current endevour. The whole suspension system should be completely rubber-free by the end of january.

I'm just trying to get it right first time, rather than strip and rebuild it all, pay for a full geo and then find that I would have been better moving to a bigger/smaller ARB.

I think there are a few of us trying to achieve this over the winter! I am going to be getting rose jointed drop links but I did not realise they would make such a difference to the handling.