15 replies to this topic

[chazzybaby]

Hi Guys,

 

I am looking into designing and making a set of billet radius arms for fitment on Mini's. I have access to CNC machines and all the software needed to make these and would like to make some quality looking parts for my Mini. My plans are to use rose joints in the radius arms to adjust the tracking and CNC'd brackets for mounting and camber adjustment.

 

I am posting this as a feeler to see if anyone would be interested in some. I will machine a batch rather than just the pair for my car. Obviously I would subject them to as much testing as I could before supplying as I do not want a failure on on of these items.

 

If there is enough interest I will be aiming to get them on my mini in time for the heritage show at Gaydon on the 7th July.

 

Thanks

[jonny f]

price dependent. Yes.

[L400RAS]

Hi,

Now i am no materials expert, but my (heafty) "gut" feeling is that aluminium is too soft for this appliciation / brittle?

 

R

Edited by L400RAS, 11 June 2013 - 02:13 PM.

[jonny f]

Hi,

Now i am no materials expert, but my (heafty) "gut" feeling is that aluminium is too soft for this appliciation / brittle?

 

R

 

http://www.twinkam.c...14&aid=SC052.B

[Vipernoir]

Specialist Components already do something along these lines, though it is only suitable for coil-over conversions.
KAD do very light standard arms in aluminium.

 

I'm not knocking what you want to do, just pointing out that you will need to be radically different and cheaper than than the competition.

[brum,brum,mike]

Would you make them to accept standard cones? or coil overs only? Rear beam only or would the standard subframe be used?

[Dan]

SC found that it isn't all that straightforward to get enough metal around the knuckle in a milled billet radius arm, the knuckle cups were tearing off of their arms.  That's why they only make coil over ones now.

[Cooperman]

It seems unlikely that aluminium alloy would be a suitable material in view of the fact that it has a limited fatigue life. Would be OK for race applications on smooth tracks and where items are changed on a regular basis so long as it was properly and professionally designed and stress analysed.
Machining from solid steel would be expensive and it would be better to design and make a jig to enable an arm to be fabricated from steel plate and arc welded, then finally machined to position the mountings accurately.
One might wonder where the figures to enable the stress analysis to be done on the proposed design would come from as to equip a car with the necessary strain gauges and other instrumentation and put it on the test track at M.I.R.A. or Millbrook would be very expensive. Without such data any design could be fatally flawed.

[chazzybaby]

7075 aerospace grade aluminium would be more than strong enough for this component. I will be retaining the standard cone suspension, and standard subframe as this is what is fitted on my car. 

 

The same could be said for every aftermarket suspension part available today... how do you know that the adjustable suspension component fitted on your car is strong enough for the job? ie: every adjustable front lower arms, tie rod and rear camber brackets. I want to see the MIRA data for those. (I can say this... I have some on my car and will be looking at Billet items again)

 

I would be stress analysing these arms to the best of my ability and will break (or attempt to) a set just to prove they are strong enough... I bet you the 4 bolts that locate the locating bracket onto the subframe fail first. As proof of the tensile strength of 7075 alloy, I would like to point out that the company I work for make billet uprights for use on Lotus Exige's and a customers tested them with over 600bhp on many track days and not had any trouble. 

 

You want to tell me that the amazing accuracy of fabricated parts and weld fractures compare to a part designed tested and machined to an accuracy of 5 microns (metric)? One might wonder about your pessimism. I would read up on Tensile strengths of 7075 alloy and compare it to steel. 

 

Designing of these parts starts Thursday when I get the mini onto the ramp and start measuring up. After initial designs I will put up an accurate price I can make these for if anyone is interested. As I said none will be sold until I have personally tested a set to destruction or given up trying. 

[Cooperman]

The problem with aluminium alloys is not the basic measurable ultimate strength, it's the fatigue cycle. That is why commercial aircraft structure made from aluminium has a finite airframe life as determined by the structural test rig which must always be run in excess of the highest hours aircraft of that type.
So for racing, which is where such lightweight components will be useful, it will not be a problem due to the fact that replacement of such components will be a regular thing.
Fatigue failure of alloy wheels, which are always grossly over-engineered, is not unknown.

[Captain Mainwaring]

7075 aerospace grade aluminium would be more than strong enough for this component. I will be retaining the standard cone suspension, and standard subframe as this is what is fitted on my car. 

 

The same could be said for every aftermarket suspension part available today... how do you know that the adjustable suspension component fitted on your car is strong enough for the job? ie: every adjustable front lower arms, tie rod and rear camber brackets. I want to see the MIRA data for those. (I can say this... I have some on my car and will be looking at Billet items again)

 

I would be stress analysing these arms to the best of my ability and will break (or attempt to) a set just to prove they are strong enough... I bet you the 4 bolts that locate the locating bracket onto the subframe fail first. As proof of the tensile strength of 7075 alloy, I would like to point out that the company I work for make billet uprights for use on Lotus Exige's and a customers tested them with over 600bhp on many track days and not had any trouble. 

 

You want to tell me that the amazing accuracy of fabricated parts and weld fractures compare to a part designed tested and machined to an accuracy of 5 microns (metric)? One might wonder about your pessimism. I would read up on Tensile strengths of 7075 alloy and compare it to steel. 

 

Designing of these parts starts Thursday when I get the mini onto the ramp and start measuring up. After initial designs I will put up an accurate price I can make these for if anyone is interested. As I said none will be sold until I have personally tested a set to destruction or given up trying. 

 

Accuracy of machining isn't that important. Especially bolted to a car that flexes 1/8"...

 

 

You mention about 7075 UTS, which is to be honest not far away from steel dependent on the heat treatment - untreated, it doesn't have much in it's favour, it's weaker than MS.

Hardness can be on par.

It's lighter than 1144 but nowhere near as strong.

I'm not sure how you'll test them. I have a work colleague who designs hydraulic test rigs for NASCAR suspensions, I'm sure he could test it to destruction for you if you wanted.

 

Now I know the answer to this is "because I can", but why would you want to spend all that money on lightweight arms to save 5Kg when you run a standard rear subframe. Wouldn't you provide it as a kit with an alloy subframe too? (yes I know about unsprung weight).

Edited by Captain Mainwaring, 11 June 2013 - 11:23 PM.

[chazzybaby]

I am fitting these on my car which I intend to take passengers in... I am definitely over engineering these arms. Fatigue is the main failure for almost every part on a car. I have yet to own a car which feels like it did out of the factory after it has aged 10 years and traveled 100,000 miles. Or for that matter for it to not have had a number of parts replaced because of failure.

 

These arms will not break. If you would like to help me in the design I look forwards to learning what your career in aerospace has taught you. 

 

I have a Friend who teaches Stress analysis at university and is willing to help. If he does half a job I will definitely contact you Captain Mainwaring.

[Captain Mainwaring]

I have contracted for Westlands (rotor blade balancing) by I'm not an aircraft engineer although I do deal with material specs and stress analysis.

 

I'm interested to look at what you come up with and if I can suggest anything, I will.

 

As stated here, I think you major issue will be supporting the knuckles...seems others have tried and failed.

[Ethel]

I'm wondering how you're going to get it on spherical joints and still have it fit a standard subby.

[tiger99]

As Cooperman suggested, the very major problem is that ALL alloys have a limited fatigue life. You can pick an alloy that is almsot as strong as steel, but its fatigue properties are nowhere near. Steel stressed to below about 45% of its ultimate load capability has an essentially infinite fatigue life, alloy does not. How are you going to do testing equivalent to several hundred thousand miles of driving, on a variety of roads, some not very good? Basically, doing the testing to ensure that they are safe is unfeasible, and the manufacturers of similar items have probably not been able to do it either.

 

I have seen one where the main part of the arm, instead of the standard cast tube, was basically a channel section, with far less torsional stiffness, possibly as mauch as a factor of 20 compared to a tube. The main load on the lever part of the arm is in torsion, as the wheel is outboard of the end of the arm. The vertical load should, with a normal sized wheel, pass through the hub bearing centre, which is clearly outboard of the arm, hence the torsion. Torsional flexing, which must happen, is not good for the handling as it causes camber change, but far worse than that, it will certainly lead to fatigue failure.

 

Those parts currently on the market are only suitable for competition use, where the mileage is limited so the accumulated fatigue damage is far less than in a road car.

 

I suggest that you abandon this idea before you kill someone, as you surely will if you proceed. Or, as has been suggested, make fabricated STEEL arms, but do make it tubular as the tube is the lightest shape which gives adequate torsional stiffness. The early arms were fabricated in that way. Issigonis knew exactly what he was doing.