redrover
Well-Known Member
Ron should set us all the homework and then he can read out the results in front of the whole class ;-)
That's a great find, I've never heard of this approach.
Uprated ARBs no longer available from Torsion Performance
- Thread starter redrover
- Start date
Hi Mr T,
If your 25.5mm bar were to be made from the same steel delivering the same mechanical properties, then the increase in size means that the larger bar is 406% stiffer. In other words, its ability to resist a certain degree of deflection is just over 4 times that of the original. There is also the assumption that the bar is prismatic with no reduction is cross sectional area at the points of retention.
Ron.
Hi Peter,
That is exactly right. Tapering at the ends reduces the larger diameter bars to the same capacity for shear resistance as the originals, assuming the bars have the same mechanical properties, which they probably don't. Maximum shear occurs at the point where the ARB exits the retaining brackets with the top links. The larger cross sectional area through the centre section of the ARB does mean that across that length the bar is more resistant to deflection, but that advantage is lost at the ends. The ARB is also subjected to torsion as one top link moves independently of the other. A bar that presents a larger cross sectional area and is prismatic will have a definite advantage over the orginal bar, assuming the same mechanical properties.
The biggest advantage of all would come from a hollow ARB, provided the same material properties and the same cross sectional area. A bar that is hollow will always be stiffer with greater resistance to deflection than a bar that is solid, provided their mechanical preoperties are the same and their material cross sectional areas are the same.
Ron.
Gargo
Active Member
Don't let this fall on deaf ears. It has to be the easiest and cheapest solution. Any once you source a 306 bar, a local machine shop should be able to sort you out. Remove your existing ARB so they have something to work too, and tell them hex needs to be offset to allow it fit. I.e. the hex machined onto the 306 bar should not be central as the extra diameter will foul on the chassis.
This is a photo of the offset hex, the drilling is the centre of the org 306 bar. It is a machining by Peugeot, you can see the hex has been machined to one side. In the second photo you should note two things.
1. The important radius between the transition from the machined hex to the standard circular section. This is to reduce the stress raiser of a sectional change.
2. One side of the hex has been extended well past the other 5 edges, where the cutter is in the photo. This for clearance on the Rover Chassis. Otherwise the extra ~1/8" of the radius will rub on the Rover chassis.
; talk this through with the machine shop.
The setup:
No, not at all.
This will work with existing springs and dampers. It is a true win win.
The modification reduces chassis roll, which in turn reduces all the negative qualities of the std Rover suspension. The reduced roll reduces front the camber change, which reduces the tendency of the front tyres to fall over and in my opinion reduces passenger discomfort.
Hi Chalky,
The function of the ARB is to reduce the ease with which the top links will move which in turn has an influence on the degree of body roll. The stiffer the ARB the higher its resistance to torsion. This increase in resistance will mean that the bolts that secure the ARB to the top links will face an increase in tensile loading assuming the bar is prismatic. For all else there would be no detrimental effects.
Ron.
redrover
Well-Known Member
That's a brilliant post, thanks Gargo. I've just had a quick look on eBay and can see a few for sale in the £50-70 bracket. I might see if I can pick one up locally from a scrap yard and take to my machine shop.
Do you have measurements or any datum points on what the offset should be? Are the two bottom vertices in your photo parallel with the OD of the round bar? And when blending it up smoothly to the round bar, what sort of length does the wasting take place over?
Thanks
Gargo
Active Member
Adding to what Ron says:
The rate of resistance to roll is increased, not the maximum resistance to roll. For a given lateral force (cornering G) the resistance is the same. The increased rate means it happens at reduced roll.
The resistance to roll is a combination from the springs and the ARB. By increasing the ARB resistance to roll, the resistance to roll from the springs is reduced. Hence as Ron says, the bolts will see a larger force, but the springs will see a lower force...
I've had the 306 bar in for many years now and had no problems at all.
The rate of resistance to roll is increased, not the maximum resistance to roll. For a given lateral force (cornering G) the resistance is the same. The increased rate means it happens at reduced roll.
The resistance to roll is a combination from the springs and the ARB. By increasing the ARB resistance to roll, the resistance to roll from the springs is reduced. Hence as Ron says, the bolts will see a larger force, but the springs will see a lower force...
I've had the 306 bar in for many years now and had no problems at all.
I'll do a drawing this evening and post it here.
mrtask
Well-Known Member
The hex machined at either end of my Classeparts ARB is not offset, it is in the centre of the bar. The long taper is the same length from all six sides of the hex. My ARB does not foul the base unit anywhere. At the same time I also fitted 130% uprated and 1" shorter front springs from the same supplier.
I don't understand what is meant by the ARB steel being described as "prismatic", but not to worry – it all does what it is supposed to do on my car!
I don't understand what is meant by the ARB steel being described as "prismatic", but not to worry – it all does what it is supposed to do on my car!
corazon
Well-Known Member
Credit for Peugeot bar idea goes to Gargo Tim/Gavin by the way.
I did some careful measuring of my car at the time and decided mine didn’t need to be offset either. I had a taper of around 40mm turned after the hex radius to help with clearance.
As Mike said earlier on, I also drilled out the clamps and re tapped my bellcrank castings to accept the next size UNF bolt, 3/8.
Jim
I did some careful measuring of my car at the time and decided mine didn’t need to be offset either. I had a taper of around 40mm turned after the hex radius to help with clearance.
As Mike said earlier on, I also drilled out the clamps and re tapped my bellcrank castings to accept the next size UNF bolt, 3/8.
Jim
sdibbers
Well-Known Member
I made my end centered so the springs were still predictable under torque (front ARBs are really torque springs). I made sure to have them properly hardened (which gives you the correct crystalline structure in the steel.
Also, I replaced the original bolts with grade 8 ones with loc tite to hold them in place. I’d noticed them backing out under load with the originals. Had them on for five years and never regretted it.
I do like the idea of the pug read arb conversion however do we end up over constraining body control for road use doing so?
sdibbers
Well-Known Member
Couple of observations on gargos part. I would add a radius at the end of each cut, the sharp internal edge is a huge stress riser. Also, I’d do what I did which is to stagger cut lengths so you have clearance to easily fit them to the car and again reduce a single plane stress riser.
I read what has been said but in principle, the answer is no. Because the cornering forces put into the bar is exactly the same as before hence the reaction applied against the car to resist these is the same. The difference is only the degree of deflection in the bar to achieve it. So the pressure applied on the same contact patch on the mounts should be the same. I know it does affect the bolts holding the retainer but my limited mechanical engineering knowledge doesn't know why.
It definitely compromises the ride on uneven roads too.
sdibbers
Well-Known Member
I've found the compromise not too big if I'm honest, considering the state of roads here in NY and NJ that's saying something! Typical roll with the 1" bar can be seen in my profile photo. That was coming out of a complex at Lime Rock Park. I'd say I find the car far more predictable in direction changes.
As an aside, this how I machined my ARB ends:
mikecoombs
Active Member
I kept my diameter down to 23mm in order to keep the stiffening effect in corners and over corrugated roads down. That is, same ride except less roll in corners. The bolt issue arises because the hex is trying to turn inside the clamp and with a stiffer bar their is more force available. In addition if the hex is slightly larger, like mine is, the exposed bolt length is slightly longer and there is reduced thread length as a result. Adding to all that is the nature of the fixing which is not central to the pivot point of the link so ther is a small turning moment across the clamp in the other two planes which tries to bend the bolts. Breakages were all fatique fractures. I went one size up as i said but with Grade five bolts as i couldn't get Grade eight in either size without ordering fifty...
Sorry JP, that is not right. The standard for polygon measurement is either to use the apothem (a), which is distance from the midpoint of any side and the centre, which when substituted into the formula 2atan(180/n) where n is the number of sides will then deliver the side length. Alternatively, the long diagonal is used which is 17mm for 17mm hexagon. The distance from the flats from one side to the other is not used to determine side length. There are always standards for obtaining measurments of geometrical shapes, else with no uniformity there would be no consistency. For a regular hexagon, twice the side length equals the diameter. For an irregular hexagon, there is no defined diameter.
Ron.