FF: Forks M

[Pushrod Twin]

I find it interestinhg that the steering feels 'sluggish'. I thnk that most of the 500 cc Vins racing are using either the JE stem or the DaveD one, both of which do essentially the same job. Clearly those chaps find the handling acceptable so what is it that creates two different impressions and yet seems acceptable to several users. Certainly the nearly 300 road users seem to be fairly happy or they would have let me know by now. Anyone got any ideas?

Norman, I'm not criticising the handling at all, It is totally acceptable, and as as I said, very stable. I found it interesting yesterday, when I asked the owner of the standard bike his opinion of the modified steering, his response indicated that he had almost not noticed, his conversation concentrated on the very noticeable improvement in suspension and road holding. I noticed the difference, perhaps I am a bit sensitive.

 

[timetraveller]

No Problem. It is just that it is only through feedback that there can be any improvement so I am always looking for information which might help the cause.

 

[Pushrod Twin]

A question here, did both bikes have the same tyre wear/pressure, if not then any of those could have made a difference, as an example my own twin used to have 90/90 19 Roadriders, last change I fitted a 3.25 19 Roadrider that looks identical and Avon list similar dimensions but it feels considerably more "sluggish", I also find the tyres susceptible to a small pressure drop.

Good point Chris, by a fluke of nature, both bikes still run the "standard" 20" wheels with ribbed Avon tyres on the front and 3;50 x 19 Avon SMs on the rear. I run the modified-steering-bike at 28PSI front. The standard bike has electronic pressure sensors reading out on a handlebar display. I spotted 30something on the readout and expressed my surprise. The owner informed me that the readout reads several pounds above his favourite old analogue gauge and believes he has 32PSI in his tyre.
I take your point regarding the possible difference in feel that this can make, but I am feeling something more in the steering difference than a few PSI in a tyre.

 

[Pushrod Twin]

I don't have the numbers in front of me, but in general, using a concentric shortens the trail from stock because it is moving the lower link halfway to the sidecar position. The sidecar position is closer to no trail because lots of trail makes steering very heavy for a sidecar machine. In the old days, many owners used a short top link for sidecar work because the eccentric was a compromise and did not shorten the trail as much as someone with a double adult sidecar would desire.

In the US, almost all of the Vincent racers are using the DD steering stem, but the slow steering is caused by the large rake and not the steering stem. All of the racers using the new stem have also steepened the rake. So, it may seem like the two are connected, but they are not.

The easiest way to alter the rake is to use the Reg Bolton mod described years ago in MPH. This mod also gives more ground clearance as an added benefit.

David

Thank you for pointing that out David. I did a very simplistic drawing the other day and came up with the outrageous increase in trail without taking the eccentric effect into account. It was so weird that even I suspected a calculation error but the penny didn't drop. Now that I have accounted for what appears to be about 3/16" change from eccentric full rearward to centre, my rough drawing calculation works out much more in line with John Reynolds accurate calculations.
Yes, steepening the rake certainly sharpens the steering. My Egli is a degree or two steeper than the original Fritz figure and with telescopic forks turns in nicely, but there is evidence that the mudguard had made contact with the header pipe when the suspension is well compressed in a turn.

 

[Michael Vane-Hunt]

Norman. Are shims put in here? Not the little shims you included. The bearings are slightly wider than the eye. Is this where the FF9 thrust washer goes?

 

[timetraveller]

Hi Michael, I am surprised that the bearings are wider than the eyes. I have assembled fifty or sixty of these and on those it has always been that the two bearings, 7 mm wide each one, total 14 mm, are narrower than the eyes. You will notice that there is a 0.5 mm raised section on the inside face of the long part of the concentric and on the face of the washer which faces into the bearings. These were designed in to take up some of the side clearance and to centrlise the bearings in the eyes. The rear reason for the inclusion of the shims is that after all this time the lower links are not all the same shape whereas all the new steering heads are, or should be, the same thickness. The shims are intended to fit wherever they need to in order to cut down any side clearance. I supply three shims for each side, one off of each of 0.1 mm, 0.25 mm and 0.5 mm. I fit them where ever they are needed to take up any side clearance and to try to ensure that the lower link is as near central on the base of the new steering head. No one has ever asked for more shims and there should be no need for any extra shims. In fact in general no one needs to use all six shims. So where does this leave you?
Could I ask you to fit the two bearings to each side and when pushing them in try to get them as central to the lower link eyes as possible. Then put the long part of the concentric in from the outside of the link and see if there is still clearance between the side of the eye and the outer side of the link eye. Note that the raised centre of the long part of the concentric should ensure that there is clearance. If not then try one of the shims, start with the 0.1 mm and then inspect it. If you are happy with that then push on the washer from the other side of the lower link eye ensuring that the 0,5 mm raised centre is facing the bearings. There should be clearance between this washer and the side of the lower link eye. Try to rotate the concentric in the bearings and check that the is clearance each side and that the concentric turns freely. If there is no clearance then use some of the shims to space the washer out fromwhere it is contacting the side of the link eye. When that is done do the same for the other side. When you have both sides assembled and turning freely check the clearance between the inside of the washer, that is the flat side, and the side of the new steering head. Additional shims are normally used to ensure that there is just about no clearance. Once you have done that please come back to me and let me know what you have found. There should be no need for any extra shims and normally there is no need to use all three supplied. Norman

 

[Michael Vane-Hunt]

Thanks Norman. I have yet to visit one of the section Gurus to check the link and make sure it is OK. It is from eBay so it would be prudent to have it looked at. This may take awhile. It has also been painted inside the eyes so I need to clean that out. Mike.

 

[greg brillus]

Mike, you will need to remove the paint, and hone the bores parallel to not more than 7 tenths of a thou interferance........any more and the bearings will become tight as the outer race will collapse too much......... I generally push the pair of bearings inward so they are flush with the inner face of the link eye, then shim as per Norman's instructions to keep the link central to the new column........This honing procedure is best done by a local engine reco shop using a Sunnen hone, like they use to re size connecting rods.

 

[timetraveller]

Hi Michael, I have just measured the width of eight eyes in four lower links. These are all in assembled steering heads so accuracy is not as good as it could be but non of the eyes is narrower than 14.5 mm and most are 14.5 tp 14.8 mm. Note that non are as narrow as 14 mm so either the one you have has been trimmed down or there is some other problem. When you have a chance please measure the width of both eyes on the link you have and let us have the measurements. Even if the link has had the eyes thinned down I think that we can still find a way of using it so all is not lost. Norman

 

[Michael Vane-Hunt]

I will sand blast all the paint off later today.