Wednesday, 5 November 2008

In Court 21, Wednesday morning.

Witness evidence concluded this morning with the cross examination of the defence technial witness by the claimant's counsel.

Although I have so far only commented on the technical evidence of the witnesses, I feel I ought to say here just how competent this witness came over. Not only was he very clear in his technical evidence but his style of delivery and the way he had approached his report and his time in the witness box marked him out as someone who obviously has a lot of experience in givening expert testimony as well as standing up to thorough cross examination. Claimant's counsel had a difficult time finding any significant weaknesses in this witnesses testimony.

Going back to his key assertion, that there are additional forces working on the axle under braking than have previously been described in all other theoretical technical testimony, he advanced that there are at least three or four further forces that he was aware of that ought to be taken into account when trying to distil the phenomnen into something as over-simplified as a force diagram.

Firstly, as mentioned before, there is the frictional force acting against the force component in the direction of the drop out opening.

Secondly the normal force resulting from the effect of gravity on the rider's mass.

Thirdly the backward directed force as a result of the action of deceleration/braking

and finally the effect of force sharing through the fork arch to the other leg.

This is where my very naive grasp of physics really gives up. Ill leave it to others to have a stab at guessing what the final resultant force on the axle may be!

However, notwithstanding a very clear explanation of the possible/probably existence of these additional force components (which obviously all have components AWAY from the drop out opening) the witness was unable to provide any measurement of these forces (or indeed a force diagram) and could only posit that since he could not recreate a forced wheel ejection except under unrealistic conditions in testing (very low downward force on bars and very high braking force) then he could only conlcude that the combination of these forces was significant enough to cancel out the resultant force generated at the axle by the braking forces at the pad/rotor interface.

He made a comment that of the QRs tested at the facility in California (CID, "Collision Incorporated Dyanamics", great company name. Only the Americans could come up with a name like that :), which he had bought from a nearby LBS, all of them had plastic thread locking inserts. Whereas the claimant witness was testing QRs bought from LBSs in Bristol (3 guesses?) which seemingly none had plastic thread locking insert. It wasnt made clear in court just how prevalent or not it is to find locking plastic inserts in QRs. I cant say Ive ever checked myself...

That brings to an end the court testimonies. The closing arguments are pencilled in the diary for wednesday week the 19th. I may be able to be there to hear the summing ups. I have no idea how much time the Judge will need to consider his findings though.


James Annan said...

I'm a bit puzzled by this, because of course forces 1, 2 and 3 are all key forces which are accounted for even in my simplified force diagram. Together with the ~4000N at the pad they are clearly the dominant first-order loads. As for the 4th factor of load sharing, even the most optimistic viewpoint that the load is equally shared between dropouts (which is certainly incorrect) would still leave the resultant force massively exceeding the ISO standard.

Mark said...

apologies James, I missed your R and B on your diagramm. Out of interest how do your calcs come out with a resultant force at the axle from the calliper component only of 61 degrees below horizontal with a 70 degree headangle as in the case.

Mark said...

oh and I cant see a reference to a frictional force back up the dropout.

James Annan said...

Mark, I didn't draw the friction force but calculating what frictional force would be required was the whole point of the diagram and accompanying discussion. The friction vector is not known a priori but only arises in opposition to the total resultant of the other forces acting on the wheel.

As discussed in the text, we can hope for a frictional force of up to about ~1000N under good conditions (ISO standard) but with my figures the ejection force is clearly greater than this. Applying the brake force at a 61 degree angle and using a 70 degree HT would make a significant change to the calcs - altering a cos(18) factor for my assumed 72 degree HT in the force diagram to cos(20+29) (assuming I have interpreted what the 61 degree angle represents). So that reduces the ejection component of the brake pad force by about 30% which makes the total resultant fairly close to the ISO retention standard. Of course changing my guesstimated 2460N to Hayes' measured 4340N would almost double it up again...but really, given the persistent reports of axle slip (including from people such as Brant Richards, hardly my greatest fan) the fine details of the calculation seem a bit angels-on-a-pin like. Nevertheless it moves!