Front Spring Box Compressor Tool
- Thread starter Jim Bush
- Start date
Know thy Beast has most of this information, but not spring rates. Page 34, second edition.
I think both springs are the same rate (I think I measured it at 57lb/in,) but the D spring was longer whereas the C used an inner spring as well, timetraveller will have the information as I'm sure as we needed it when starting out specifying springs for the JE modified steering stem suspension mods.
Sorry Chris, this is where I let you and cyborg down. From memory the inner 'C' springs are 10 lbs/inch. I remember working out what the larger 'C' springs are and I will have to go back and do the tests again. What I do is put a spring into a front spring box, put the top under the ball hitch on the front of my trailer and the bottom on some bathroom scales and then see how the spring box changes length with different loads. That would be how I got the spring rates and then worked backwards to design the springs we needed. I have done the same with the rear springs, both standard and Pettiford, and still have the graphs in my PC. But do not seem to have them for the front springs. If it is dry tomorrow and I can find any original springs I will do it again... It is also possible that Dave Dunfey has the measurements as those with long memories will recall that before we went down the route of altering the geometry Dave tried to get the same effect by using shorter but stronger springs. He had three rates if I remember correctly and he must have know what he was working towards.
Peter, thanks for pointing me towards KTB. Once again, I didn’t think to look in there. I should hang a sign in the shop.
The reason why I’m asking is because I unintentionally ordered D springs. I should take the D spares list and lock it away.
I’m using a copy of David’s modified steering stem which moves the spindle down 0.5” if I understand correctly.
When installing the D springs (16.5” according to KTB) , there seemed to be a lot of preload. With everything assembled on the bike the front suspension is extremely stiff. A combination of .5” lower spindle and 1.5” longer springs (C springs were 15”according to KTB)? The spring boxes are also new, so it’s possible something is amiss inside there. When the forks were assembled, they would fall under their own weight, so as far as I know, all is well there.
The reason why I’m asking is because I unintentionally ordered D springs. I should take the D spares list and lock it away.
I’m using a copy of David’s modified steering stem which moves the spindle down 0.5” if I understand correctly.
When installing the D springs (16.5” according to KTB) , there seemed to be a lot of preload. With everything assembled on the bike the front suspension is extremely stiff. A combination of .5” lower spindle and 1.5” longer springs (C springs were 15”according to KTB)? The spring boxes are also new, so it’s possible something is amiss inside there. When the forks were assembled, they would fall under their own weight, so as far as I know, all is well there.
The lower link will add even more preload, so to start with I would cut 2" off each spring, this would take you back to the standard C length minus the 1/2".
The standard springs are stiff anyway, they were that way to keep the suspension from going into unstable areas, even with the JE steering stem fitted and a Koni damper I was only getting 1.25" of travel in normal/ rough road riding.
I weigh 130kg and have 45lb/in springs/AVO with very little damping on now and have 3.5" on the same sort of roads.
Anybody lighter than me would be using the 36/lb/in springs.
The standard springs are stiff anyway, they were that way to keep the suspension from going into unstable areas, even with the JE steering stem fitted and a Koni damper I was only getting 1.25" of travel in normal/ rough road riding.
I weigh 130kg and have 45lb/in springs/AVO with very little damping on now and have 3.5" on the same sort of roads.
Anybody lighter than me would be using the 36/lb/in springs.
I weigh about 77kg. David had mentioned elsewhere that cutting the spring increases the spring rate, so I did a bit of reading on the subject….. which gave me a splitting headache. I’m assuming I should measure the spring rate on these D springs and see what I come up with. Timetraveller has 33’s and 36’s, which are also16.5”
According to KTB, C springs were originally intended to run with inners. It also states that the D springs were increased to 16.5” ( inners only for sidecar use) to soften the ride. Once I know what the D springs are rated for, I can start figuring out which way is up.
According to KTB, C springs were originally intended to run with inners. It also states that the D springs were increased to 16.5” ( inners only for sidecar use) to soften the ride. Once I know what the D springs are rated for, I can start figuring out which way is up.
As I said earlier I think both C and D springs are the same rate, I think all they did was increase the length (and therefore the preload) and not use the inners, this would have given a similar ride height (because of the extra preload) but a softer ride because the inners were no longer there. It's easy to check new ones, just measure the wire gauge and coil spacing.
As the inners are 10/lb/in but I'm not sure of their length and I think the main spring is around 57 lb/in, so in effect C springs were 15" 67lb/in and Ds just 16.5" 57s so of course they would provide a softer ride, and I say the extra length was just there to increase the preload so the bike sat (I think) slightly higher.
The mathematics around springing are not as straight forward as they seem at first, remember the entire weight of the fork and front wheel is unsprung and on a Vincent most of the riders weight is carried quite far forward ( the rider sits 4" closer to the steering head than on a Norton) so a change in rider weight has a larger effect than would be normal.
Then there is preload and ride height to be taken into account, both of which can be adjusted by packers or longer springs, both of which will give different effects even while using the same rate springs.
A way of determine the spring rate if you have them and a milling machine is install the a spring in spring boxes and mount some bathroom scales below the bed and then lower the bed until it's just touching the spring boxes stood vertically. Then lower the bed 1" and take a reading, then another inch and then a third, then take a average.
Also don't forget you can use different rate springs one side to the other in use.
I did a lot of the on-road testing for TimeTraveller covering hundreds of miles over the same roads at the same speeds and got really proficient at spring and damper changing.
For the springs I ended up using a wooden pole and nylon rope with a loop in each end, Bike on centre stand and a trolley jack under the front stand cross piece just to take some of the weight.
Hooked one loop around the top of the spring box and back through itself, put the pole through the bottom loop and hooked it under the drum through the spokes so it couldn't slip sideways, levered down to slacken the bottom bolt, take the top bolt out, tension off, spring out, spring in, tension on, fit top bolt, tighten bottom bolt unhook rope and repeat other side then ready to go, in not much more time than it takes to read this.
As the inners are 10/lb/in but I'm not sure of their length and I think the main spring is around 57 lb/in, so in effect C springs were 15" 67lb/in and Ds just 16.5" 57s so of course they would provide a softer ride, and I say the extra length was just there to increase the preload so the bike sat (I think) slightly higher.
The mathematics around springing are not as straight forward as they seem at first, remember the entire weight of the fork and front wheel is unsprung and on a Vincent most of the riders weight is carried quite far forward ( the rider sits 4" closer to the steering head than on a Norton) so a change in rider weight has a larger effect than would be normal.
Then there is preload and ride height to be taken into account, both of which can be adjusted by packers or longer springs, both of which will give different effects even while using the same rate springs.
A way of determine the spring rate if you have them and a milling machine is install the a spring in spring boxes and mount some bathroom scales below the bed and then lower the bed until it's just touching the spring boxes stood vertically. Then lower the bed 1" and take a reading, then another inch and then a third, then take a average.
Also don't forget you can use different rate springs one side to the other in use.
I did a lot of the on-road testing for TimeTraveller covering hundreds of miles over the same roads at the same speeds and got really proficient at spring and damper changing.
For the springs I ended up using a wooden pole and nylon rope with a loop in each end, Bike on centre stand and a trolley jack under the front stand cross piece just to take some of the weight.
Hooked one loop around the top of the spring box and back through itself, put the pole through the bottom loop and hooked it under the drum through the spokes so it couldn't slip sideways, levered down to slacken the bottom bolt, take the top bolt out, tension off, spring out, spring in, tension on, fit top bolt, tighten bottom bolt unhook rope and repeat other side then ready to go, in not much more time than it takes to read this.
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