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Front Spring Boxes - pressure relief hole

rwhitne2

Active Forum Website User
VOC Member
Rread w/ interest recent articles by L Thornton & N Higgens re front spring boxes. Have the Thornton set up on my own 55 Shabby Shadow w/ lighter front springs than Thornton recc. Works pretty well but always looking to improve - Maine roads are horrible. Air holes to relieve pressure drilled in FF 15 - what size and where? I have old FF15s, new lowers. Will order new FF 15s w/ PTFF bushes - will an air vent be useful with these? Thanks, Mike Whitney
 

timetraveller

Well Known and Active Forum Website User
VOC Member
This should stir up some trouble!! I have real trouble believing that air relief holes are going to make much difference unless they are very large. Imagine if the front fork inner was a piston like fit and wanted to move upwards about 2" in a very small fraction of a second and then move down again, up again and so on going over a series of bumps, pot holes or whatever. Either the air has to compress/decompress and act to some extent as a spring or it has to escape. Take a normal pump for use on pedal cycle tyres and then push it rapidly inwards leaving the outlet totally open. Pull it out and then pump it backwards and forwards a few times. You probably cannot get it to move as quickly as it has to do on a motorcycle going over bumps but the dimensions are similar and that gives you some idea of what is involved when the outlet hole is several millimetres in diameter. I know that the Vincent spring boxes are not as good a fit as a pedal cycle pump and its piston but if PTFE bushes are fitted then they are getting to be near to it. Of course the proof is in the pudding, as they say, but I have real reservations that making any holes much less than, say, quarter of an inch in diameter will make any difference. That size hole or larger is likely to suck in rain and road dirt.
 

Black Flash

Well Known and Active Forum Website User
VOC Member
I personally believe it should be worth a try. even if it helps only a little bit it is better than no help at all. if you drill a hole between the two lower spades at the bottom the hole is quite well protected against grime getting in, i think.
Bernd
 

rwhitne2

Active Forum Website User
VOC Member
thanks - yep maybe some pondering here!! Good thoughts , I will follow up w/ those who have done it. Cheers
 

vibrac

Well Known and Active Forum Website User
VOC Member
Cant think it makes any real difference. getting the spring pressure right is more important, air is a spring anyway and the effect will be constant so why worry?
Certainly the half inch or so of broom handle I finally arrived at and fitted to the racer 20 years ago does it for me and those who have ridden it since
 

ernie

VOC Assistant Secretary
VOC Member
An old thread but rather than start a new one - my problem is not air but friction. Even at partial compression the inner and outer are locked solid. The inners are 10 1/2" long and go in well past the bottom swaged bit which seems irrelevant unless you shorten them by 6 inches. That explains my very hard ride on what now seem like very bumpy roads. Any one else with such a problem?
 

Chris Launders

Well Known and Active Forum Website User
VOC Member
I'm convinced the swaged section is only cosmetic, so the scoring where they rub together doesn't show. If your spring boxes don't slide into each other easily there is a miss match between them.
 

davidd

Well Known and Active Forum Website User
VOC Member
The Factory put them up top. I think Alumibronze drilled his where they would not be as visible.

I don't know how effective or necessary they were, but we know the Factory tried it on the Flash. I would guess it is the sharp bumps that are the issues. Very quick movements of the boxes would trap the air. The holes do look small, but the quick bumps would not be very long in duration.

Spring Box Hole.GIF


David
 

greg brillus

Well Known and Active Forum Website User
VOC Member
It's all about friction........The lighter the spring rates the less friction you want. If the spring boxes are sticky and jam when compressed with the springs removed, then the front end will suffer badly. I cant imagine the holes in the cases have any effect at all, the clearance from the inner to outer cases would be more than enough. Best to wash all the old grease and grit out of the cases with solvent and a good sized bottle brush. Small dents will jam them easily too, but harder to remove, and probably easier to replace. Apart from other tight spots in the front end, the spring cases are a major cause of stiffness in the front end. If you want a softer front end, remove the inner springs if they are still in there, and run the outers only. Just simply cleaning all the old hard grease and grit and reassemble with a light coating of fresh grease will help a lot.
 
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