Straight Rate to PDS Conversion

[LogicTech]

Hi Jeremy

Is there an easy way to calculate the conversion of a PDS spring rate to a straight rate spring? The reason I ask this because I calculated a matrix for 2004 KTM’s with straight rate springs. I extended it to cover riders and gear up to 280lb. Now that I have the high end spring rate calculated I want to convert that value to PDS progressive spring rates. So far I found that if I multiply the straight rate by 1.15 and then match it to the PDS high spring rate it matches up fairly well. I also find that the higher PDS rates work much better for most people. When I follow the forums I can’t believe how soft of springs are being recommended for heavy bikes and riders.
Currently I have a 2004 200exc with 44’s and a PDS-4 for a 235lb rider with gear. The bike measures up very good using stock methods, 35mm shock sag and 35mm fork sag. The bike performs very very good. Great steering and handles the deep whoops with ease. How dose this sound to you?

LogicTech

 

[DEANSFASTWAY]

logictech where did you come up with that assumption for the math ,I thought before of doing the backwards math . Pretty cool . You know sometimes I like to use progressive springs on woodsie bikes and heavier thumpers (non PDS) but sometimes cannot find correct rates or applications , sometimes we try to adress this by shaving down the first few coils of a shock spring but I never learned a true rule, some springmaker knows though.

 

[LogicTech]

logictech where did you come up with that assumption for the math ,I thought before of doing the backwards math . Pretty cool . You know sometimes I like to use progressive springs on woodsie bikes and heavier thumpers (non PDS) but sometimes cannot find correct rates or applications , sometimes we try to adress this by shaving down the first few coils of a shock spring but I never learned a true rule, some springmaker knows though.

DEANFASTWAY
I used the KTM 2004 manual and extended the charts out for the heavier weights. The SX bikes are also listed. They use springs two levels higher than the EXC’s. Knowing that I still use the EXC recommendations, but try to stay near the top of the 10Kg usage window or the next higher rate. Using a heavy PDS spring I tripped over this one. The PDS-4 was used one older 300MXC which was 25lbs heavier than the new 200. I guess I put the bike on diet rather than myself. After having great success with the PDS-4 I wanted a way to reference a PDS spring back to the calculations with the straight rate springs. This is where the 1.15 factor comes in. I matched the PDS-4 high spring rate. PDS springs are much softer than the specifications indicate. The PDS-4 measures 115mm race sag with the 235lb load. Calculating spring loads with straight rate springs seems to be good method to determine the high rate. Calculations with the Progressive spring are too complicated.
LogicTech