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FF: Forks Modified Steering Stem



timetraveller

Well Known and Active Website User
VOC Member
If I've done the sums right then if your spring was correctly 36 lbs/inch at 16.5" long them shortening it to 16.0" will make it 37.125 lbs/inch. I regard that as within the manufacturing tolerances. Shortening or packing by fractions of an inch might be regarded as tuning for the individual. Chris Launders at 280 lbs had to pack his 45 lbs/inch spring by about half an inch. I'm more than happy to get a price for making some of those adjustable length inner spring boxes but I would need to have an order from one or two people to make it worth while bothering people.
 

macvette

Well Known and Active Website User
VOC Member
If I've done the sums right then if your spring was correctly 36 lbs/inch at 16.5" long them shortening it to 16.0" will make it 37.125 lbs/inch. I regard that as within the manufacturing tolerances. Shortening or packing by fractions of an inch might be regarded as tuning for the individual. Chris Launders at 280 lbs had to pack his 45 lbs/inch spring by about half an inch. I'm more than happy to get a price for making some of those adjustable length inner spring boxes but I would need to have an order from one or two people to make it worth while bothering people.
All other things being equal, the spring rate change is calculated as follows.
New spring rate = old spring rate x original number of coils ÷ number of coils after shortening.
The number of coils refers to active coils ignoring the flat end coils so using overall length change is not a precise way of calculating the new spring rate
 

davidd

Well Known and Active Website User
VOC Member
The load the spring will carry is determined by the diameter of the wire and the overall straight length of the wire. Even coil springs can be thought of as a torsion spring, straightened out. If you think of one end of the straight spring wire in the vise and the other end with a vise grip plier attached, you will see that for the same diameter wire a short spring wire of say, two feet, will be quite stiff to rotate with the vise grips. On the other hand a straight spring wire of ten feet will rotate quite readily.

The Vincent springs has many coils, so it is quickly identified as a "soft spring" because it would be quite long if it were straightened out. Every little bit you cut off will make it a little bit stiffer. Generally, if a 36 lb/in spring is 16.5" or 16" is is still 36 lbs/in. The springs just have differing heights (differing spacings of the coils) and both have the same length of straight wire.

Technically, you have to "set" the spring once you get it from the manufacturer. Setting is the act of compressing the spring until coil bind occurs. It will often make the height of the springs more consistent.

Typically, the fewer coils a spring has the stiffer it will be for the same wire diameter. If you count all the coils you may need to subtract 1.5 coils to arrive at the "active" number of coils. Thus, each end of the coil spring that is flattened and ground is not counted as an active coil for 3/4 of its length (this formula may vary a bit). Any coils that are close enough to touch each other after a very small amount of movement will become inactive after they touch and bump up the stiffness of the spring.

David
 

timetraveller

Well Known and Active Website User
VOC Member
OK Chaps, always one to learn from others and so thanks for the help and advice. However, I suspect that with a long spring like the front suspension ones, as opposed, say to a valve spring the difference will not be very great. In the search for accuracy I have just counted the number of coils in one of the 36 lbs/inch springs and there are 52. Removing three from this figure as advised above to take into account the flattened end coils gives 49. Half an inch of shortening means removing about three coils so using the above calculation means that (49/46) x 36 gives 38.3 lbs/inch. My crude calculation gave 37.125. Near enough I would have thought.
 

vibrac

Well Known and Active Website User
VOC Member
Not wishing to further complicate matters :p but I suppose that after 70 years and thousands of owners I am not the first to notice that Vincent inner valve springs fit smoothly into upper spring boxeso_O broom handles are not the only way!
 

Bill Thomas

Well Known and Active Website User
VOC Member
If you cut one down, It can go inside a standard front shocker !, To give a bit more bounce for my comet forks.
Cheers Bill.
 

timetraveller

Well Known and Active Website User
VOC Member
I just had a call from Robin Stafford who fitted one of the JE steering kits some months ago. I originally supplied him with 36 lbs/inch springs for his twin as with his weight I thought that they would be about right. Robin found them too strong and so I sent him some 33 lbs/inch springs. He found these much more comfortable but under heavy braking the front end dipped more than he liked. His solution has been to fit some original Vincent inner front springs. These were cut down in length so that they only come into play after about 1.25" to 1.5" of movement. He finds this has cured the problem and effectively gives him a two rate spring. Note that earlier Gary Gittleson shortened his 36 lbs/inch springs by about half an inch. These two examples are the kind of feedback that allows or encourages users to try to tune for their own use.

On a different front I am having trouble finding some thin wall stainless steel tube to allow the tuneable inner spring boxes as required by Comet Rider, a few posts earlier. If there is anyone out there who can offer advice then it will be gratefully received.
 


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