Here's closer pictures of the design.  I'm connecting my parts to the stock steering stem.  The upper clamp doesn't worry me too much, tolerances not too tight and pretty easy welds on substantial pieces.  The lower clamp will clamp on and has higher tolerance to fit steering stem. The thinner sections are more likely to get pulled out of shape by welding but i think i can be minimized by welding the flat piece to the long tube first and then weld the steering stem tube to the assembly.

SS has got another talented builder !!!
Can't wait to see this bike.
What a great bike , stock or just begging to be customized.

I don't even weld that good in my dreams.

Good job stacking the coins? Welding is on my "must learn" list.

I love what you're going for. And I'll confess to a bit of envy at the quality of your work. Ok, a lot of envy!

Still trying to puzzle out the geometry on the girder. If the lower shock mount is attached to the lower forward pivot, the vertical link that shares the lower shock mount and then connected to the upper shock pivot arms (for lack of a better term) would appear to make for nearly a solid parallelogram effectively taking the shock out of the equation. Also, instead of the shock being mounted to the upper tree, it would seem the mechanical advantage of the pivoting arm for that mount would require an extremely robust coil over shock. Or, at least make for a challenging calculation to size the shock.
But then, I'm neither a designer nor a mathematician. Am I not visualizing things correctly?

This is a design sketch of just the shock linkage in the two extremes of travel up and down.  The neck is the big rectangle, just the lower pivot arm to the fork is shown, the upper pivot arm to the fork is left out.  The shock only has 2 inches of travel but I want 4-5 inches full travel on the fork.  The shock is shown in blue, you can the length of the shock changes 2 inches from top to bottom of travel.  The total fork travel shown is 4 inches.

Gary_in_NJ wrote on 08/07/17 at 12:46:02:


The shock is an air shock for a mountain bike.  I don't have the specs for the one I have but spring rate is variable.  They can be pressurized to make them stiffer and I think they are inherently progressive.  With zero pressure it takes about 100 pounds to travel the first inch.  And it's got two adjustments for damping, one for compression and one for rebound rate.  

I estimate the stock front spring rate to be about 130 pounds per inch by measuring travel and the weight on the front wheel.  So I figure to match that I need to be in the 260 pounds/inch on my shock.  I'm not sure the pressure it will take to achieve but that's well within the typical spring rates advertised for spring based MTB shocks.

For this version to go with the stock frame I'm not changing the rake.  I plan to run the stock wheel/axle/brake and fork length.  If I like the way it rides I may change things up when I do the frame build.

The shock is an air shock for a mountain bike.  I don't have the specs for the one I have but spring rate is variable.  They can be pressurized to make them stiffer and I think they are inherently progressive.  With zero pressure it takes about 100 pounds to travel the first inch.  And it's got two adjustments for damping, one for compression and one for rebound rate.  

I estimate the stock front spring rate to be about 130 pounds per inch by measuring travel and the weight on the front wheel.  So I figure to match that I need to be in the 260 pounds/inch on my shock.  I'm not sure the pressure it will take to achieve but that's well within the typical spring rates advertised for spring based MTB shocks.

For this version to go with the stock frame I'm not changing the rake.  I plan to run the stock wheel/axle/brake and fork length.  If I like the way it rides I may change things up when I do the frame build.

The OEM spring are a dual rate progressive spring with an initial rate of 19 lbs-in  ( 0.34kg/mm) from its static position to 4 inches and then 23 lbs-in (0.41 kg/mm) from 4 to 7 inches.

On my cafe racer I'm using linear rate springs that are 40 lbs-in (0.70kg/m) each. I think you need to recalculate your spring rates as 260 lbs-in will be non-compliant.

Also, I would also caution you on using a shock that is "air sprung". If the bladder fails at speed the bike will become unstable. A traditional coil would be a safer choice.

Finally, I don't believe you want a dual rate spring for this application. Given the geometry of your linkage, your spring will inherently load-up during compression.