Just a bit of a cautionary warning. We recently had an aftermarket ride height rod fail on one or our cutomers bikes. Picture attached. Product failures of safety items are the stuff of nightmares for manufacturers and dealers and individual mechanics all of whom can be liable for subsequent vehicle damage or far worse personal injury. Lest we ever forget here is just one sobering news story where both the riders race team and the component manufacturers were subsequently sued 700,000 pounds for the component failure that ended Robert Dunlop competitive career. Dunlop settles for £700,000 in TT crash case | MCN
I would love to know the circumstances of this failure as that rod is only ever in compression, what caused the sideways load?
Yeah, compressive strength, was having a brain fart and couldn't remember the correct term, so just put "tensile" :Angelic: .
Good question. No evidence of any external sideways load. No marks on the rods or rod ends other than wear the outer radius of the bend came into contact with other parts when it collapsed. I have attached a picture showing a photo of the inside radius of the bend. Rod ends were in good working order as was the rest of the suspension. No damage to front forks, etc. Bike was fitted with an up rated spring for touring with passenger. Quotation from our customer "oh, one other thing.... We had done other trips on that bike with the factory ride height rod and the same amount of luggage and suspension set up without any problems." When it collapsed the bike fell on to the rear tyre smashing the hugger and breaking the plastic under the seat. The exhaust was also damaged. Luckily the rider was able to bring the bike to rest from open road speed without crashing. The head Ducati mechanic at the repair shop where the bike was taken after the event felt that the rod was just too weak. Things to note the force that the ride height rod must withstand is several times that of the force the components on the other side of the rocker arm must resist due to the rocker arm ratio being about 3:1 (need to precisely measure this up yet). IF the components on the other side of the rocker arm were the same length as the ride height rod they would need to be only one third of the strength of the components on the other side of the rocker shaft. The Ohlin's shock shaft is made of high tensile steel (I have machined the Ohlins shafts in the past to retrofit them into standard ST2 shocks because of unavailability of the standard ST2 shock shafts and they are bitches to machine - the depth of penetration of the case hardening on Ohlin's shock shafts is very impressive) but of course when the rear suspension is experiencing the bigger loads the shock is compressed and is effectively a shorter and hence stiffer component. A shorter length of any material is harder to bend. The aluminium shaft bent at the point where the rod ends and the shaft had not yet become solid again. There was no swaging at this point as per the original Ducati one and as is common practice to distribute the loads over a longer section, so this is not only the weakest part of the rod it is also the stress concentration point so the loads are concentrated into the very weakest part of the link. American supplier of the part ran for cover but we of course refunded the customer and paid their insurance premium as you do... but the staff at manufacturer Corse Dynamics/Motowheels have now asked us if we think it should be made of steel. I think as well as a review of the material selected a better design is actually required.
They must be a slick outfit if they are asking their customers if it should be made of steel! Perhaps they don't know that there are thousands of different specifications of aluminium alloy and many of them have very different characteristics. They could make it from steel and then excess loads would make it crack and break instead of bend, depending on what steel they make it from. After many years in aviation, I would still be loathe to believe that this damage came only from compressive forces. It will never affect me though as I have the standard alloy arm on mine and will never take a passenger on the back.
I can't understand why someone would modify critical components with stuff from third parties without any form of type approval. This is complex stuff that needs to be designed by people who know what they are doing.
bit of an aside, i read recently a notice in another workshop in town recently stating they wouldn't fit parts supplied by customers due to successful prosecutions being made against workshops when the parts fail. customers eh?. who needs them.
shit ive just fitted one of them to my track bike!! that came from moto wheels!! maybe i should ring um and ask if there safe??
I have aftermarket ones on a number of trackbikes and road bikes and never bent one. That failure needs more investigation to determine root cause.
Thanks for all the good input. I am now coming to the same opinion and many of the safety critical products we supply have TUV, DOT certification, etc and we will be stipulating this more now. Lesson learned. Ducati do make adjustable ride height rods for some of the STs and these use swaged ends ...we will only be using these in the future where the lengths are suitable. I suspect you will be probably be OK as tracks are relatively smooth and you won't be carrying a pillion or be anywhere near the weight limit and if you crash on the track it is usually safer than crashing on the road....less furniture to hit, etc and it is on your bike not mine or one of my customers... I concur. See above, If you are not carrying a pillion I doubt you will have a problem. Agreed, we had sold a couple of them before without problem. This was the first one we sold on a touring bike. Just because this one failed does not mean that other companies are not making perfectly satisfactory ones with very good safety margins. There are some very clever engineers making fantastic quality aftermarket products (particularly in Australia, NZ and Europe). The engineers who looked at the rod before we returned it to Motowheels to do their assessment do not believe it was a manufacturing fault or flaw but a design weakness. We are going to do some load testing on the other end of the rod that has not deformed when we get it back from Corse Dynamics/Motowheels, and another one as we think it should be made. I will publish the comparative load test results. We work with some very good engineers who can assist us with this. However this is not a product line that we will be manufacturing. Well our customer certainly didn't want a replacement of the same type!
Buckling failure force equals Pi squared x E x I over L squared. If a force greater than that is applied, the rod will buckle. Once it buckles, the rod will deflect sideways. If the force is still applied, this will result in plastic deformation which will typically manifest itself in a deflected shape such as the one illustrated. Classic Euler buckling failure. The variables in the above formula are: E - Youngs Modulus - dependent on material used I - second moment of area - dependent on cross sectional properties of rod If either E or I (or both) of the aftermarket rod were less than the original part, the allowable compressive buckling force would be reduced accordingly. The rod would then buckle at a lower force and then fail at the most critical section.
RickyX thanks for that, I note however that you missed 'n' out of your simplified equation. F = n π2 E I / L2 (1) where n = factor accounting for the end conditions which we think are significant contributory factors to this failure. Using the equation in this case is difficult because we don't have a rod with either a constant Pi (different diameters, and both hollow and solid tube) or constant Youngs modulous (different materials used for rose joints and rod). Eulers equation in it's basic form is also not very good at dealing with columns (rods) that also included significant stress concentration points like the ride height rod has. But is very good at pointing out the rods strength needs to increase by a square of it's length. Something that I suspect that the guys at Motowheels / Corse Dynamics probably didn't realise. So if a customer sets this rod longer than the standard rod the strength needs go up exponentially and the hollow section (the weakest part) between the rod end and and the bottom of the threaded section it screws into gets longer and hence the rod becomes much weaker than the standard part even if it was made of the same material and section. Just because they can cut and shape metal doesn't make them satisfactory design engineers for safety critical parts. Regardless the Motoswheels/Corse Dynamics has not been able to provide us with any engineering calcs they did to determine their design or any load testing. So an update: They have not reimbursed us for the direct costs our customer incurred (which we gladly paid and would do so again in a similar incident without hesitation) nor have they provided any engineering assessment of why they think the rod failed (despite asking us to return it so they could make an assessment) and they now won't return the rod that we still own for us to do load testing on the end that has not failed. A lot of after market motorcycle accessories are designed and manufactured by passionate, skilled and highly experienced engineers (particularly in the UK and Italy ) and the final products often work better, are more durable and more beautiful than the original parts. A well designed product manufactured by a true craftsman is always a beautiful thing to behold and I am amazed and envious of the skills of these craftsmen and I like the rest of us lust for their products.....and then there are the others.
I did...and I didn't... In this case, the end case conditions with spherical bearings at each end results in a rod that is pinned at both ends, so n =1. Therefore, although I didn't include n in the equation, it doesn't make any difference for the case in question. Basic summary: copying a design but without appreciating the importance of materials, cross section and finesse of detailed design requirements and an understanding of structural mechanics equals crap product.
