Target Fork / Shock Ratio
    [Equal Bump Travel]
 Target fk/sh ratio  
  • Forks should be 1.25 - 1.65 times stiffer than the shock, depending on the application.
  • [1.25 - 1.35 - 1.45 - 1.55 - 1.65]
    • Bert XC:  3791 fork with 3873 shock.
      • Forks are 1.25 times stiffer than shock.
    • Stelzig MX: 4029 fork with 4117 shock.
      • Forks are 1.44 times stiffer than shock.
    • Fellows MX:  3885 fork with 3733 shock.
      • Forks are 1.79 times stiffer than shock.
  • By design, XC and Woods forks will be at the lower end of the 1.25-1.65 scale.   [* see soft-7]
  • By design, MX will be at the upper end of the scale.    [** see stiff+6]
  • fk/sh ratio   
    • fk.ratio   [1.25 - 1.65]
  • fork target   
    • fk.targ    [soft-1    soft    aver    stiff    stiff+1    etc]
  • -
  • vdb / excel / sh_fk_ratio_interpolate.xlsm
 
 Setting fork target and fk/sh ratio
  • Pick a fork target and adjust the shock target to get your fk/sh ratio.
  • Fork targets are scaled using soft-aver-stiff with +/- 15% increments.
    • If the fork target goes from 'aver' to 'soft-2' the fork comp force drops to .66.
    • If the fork target goes from 'aver' to 'stiff+2' the fork comp force increases to 1.52.
  • However, the shock target does not change by the same percentage as the fork.
  • The shock target is adjusted using the scale below.
  • For example:  The fork starts at 1.25 times stiffer than the shock.
    • If the fork target dropped to .66, the shock target would decrease by .865 to maintain 1.25 fk.ratio.
    • And if the fork increased by 1.52, the shock target increases by only 1.225 to maintain the ratio.
 
 
 Shock Target Increments and c.force
     
 
  • Shock c.force is the compression force from the main piston.
  • It's a good reference to compare overall stiffness, and the numbers fit nicely in the range of 300 - 600 - 900, with 600 in the middle.
  • 600 lbs c.force is typical for light MX.
  • 900 lbs is pretty stiff for AX and SX.
  • 300 lbs is extremely soft for rocks and roots and would be be the minimum you would ever consider running.
     
 
 TARGET   Fork / Shock Ratios
     2017-2022 KTM    .472 fk spr    4.55 sh spr    2.52 LR
  soft-7
 
  • Baseline fork target:       soft-7           27.6 lbs co wogas at 100ips  [.944 kg spr]                             fk.targ [soft - aver - stiff]      fk.ratio range [1.25 - 1.35 - 1.45 - 1.55 - 1.65]  
  • Baseline shock target:    109 x .730     439 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.24
  • fork target [fk.targ]       fixed at soft-7
  • fk/sh ratio [fk.ratio]       1.24 - 1.49        Fork stiffness cannot be greater than 1.49 because shock c.force is at minimum 304 lbs
  • Minimum c.force . . . The upper range [1.65] of the fk.ratio for soft-7 is limited by minimum c.force     As sh.targ comp goes down, fk.ratio goes up     304 lbs c.force in softest you can go
    • * By design, this fk.ratio cannot reach the upper end of 1.65 as it is limited by minimum c.force.
done

 
   
  soft-6
 
  • Baseline fork target:       soft-6           30.2 lbs co wogas at 100ips  [.944 kg spr]
  • Baseline shock target:    109 x .730     439 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.26
  • fork target [fk.targ]       fixed at soft-6
  • fk/sh ratio [fk.ratio]       1.24 - 1.49        Fork stiffness cannot be greater than 1.52 because shock c.force is at minimum 304 lbs
done
   
  soft-5
 
  • Baseline fork target:       soft-5           33.1 lbs co wogas at 100ips  [.944 kg spr]                              fk.targ [soft - aver - stiff]      fk.ratio range [1.25 - 1.35 - 1.45 - 1.55 - 1.65]   
  • Baseline shock target:    109 x .775     467 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.24
  • fork target [fk.targ]       fixed at soft-5
  • fk/sh ratio [fk.ratio]      1.24 - 1.55        Fork stiffness cannot be greater than 1.55 because shock c.force is at minimum 304 lbs
done
   
  soft-4
 
  • Baseline fork target:       soft-4           36.5 lbs co wogas at 100ips  [.944 kg spr]
  • Baseline shock target:    109 x .820      494 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.23
  • fork target [fk.targ]       fixed at soft-4
  • fk/sh ratio [fk.ratio]      1.23 - 1.58        Fork stiffness cannot be greater than 1.58 because shock c.force is at minimum 304 lbs
done
   
  soft-3
 
  • Baseline fork target:       soft-3           40.5 lbs co wogas at 100ips  [.944 kg spr]                              fk.targ [soft - aver - stiff]      fk.ratio range [1.25 - 1.35 - 1.45 - 1.55 - 1.65]   
  • Baseline shock target:    109 x .820      494 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.26
  • fork target [fk.targ]       fixed at soft-3
  • fk/sh ratio [fk.ratio]      1.26 - 1.62        Fork stiffness cannot be greater than 1.62 because shock c.force is at minimum 304 lbs
done
   
  soft-2
 
  • Baseline fork target:       soft-2           45 lbs co wogas at 100ips  [.944 kg spr]
  • Baseline shock target:    109 x .865     521 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.25
  • fork target [fk.targ]       fixed at soft-2
  • fk/sh ratio [fk.ratio]      1.25 - 1.66        Fork stiffness reached 1.66 at minimum c.force 304 lbs
  • The fk.targ range is [1.25 - 1.65].
done
   
  soft-1
 
  • Baseline fork target:       soft-1           49.6 lbs co wogas at 100ips  [.944 kg spr]                               fk.targ [soft - aver - stiff]      fk.ratio range [1.25 - 1.35 - 1.45 - 1.55 - 1.65]  
  • Baseline shock target:    109 x .910      548 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.25
  • fork target [fk.targ]       fixed at soft-1
  • fk/sh ratio [fk.ratio]      1.25 - 1.65        Fork stiffness reached 1.65 at c.force 331 lbf.
  • The fk.targ range is [1.25 - 1.65].
done
   
  soft
 
  • Baseline fork target:       soft              56.1 lbs co wogas at 100ips  [.944 kg spr]
  • Baseline shock target:    109 x .955      575 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.25
  • fork target [fk.targ]       fixed at soft
  • fk/sh ratio [fk.ratio]      1.25 - 1.65        Fork stiffness reached 1.65 at c.force 358 lbf.
  • The fk.targ range is [1.25 - 1.65].
done
   
  aver
 
  • Baseline fork target:       aver              63 lbs co wogas at 100ips  [.944 kg spr]                               fk.targ [soft - aver - stiff]      fk.ratio range [1.25 - 1.35 - 1.45 - 1.55 - 1.65]  
  • Baseline shock target:    109 x 1.00      602 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.26
  • fork target [fk.targ]       fixed at aver
  • fk/sh ratio [fk.ratio]      1.26 - 1.66        Max fork stiffness 1.66 at c.force 385 lbs     In this case fk.ratio has exceeded upper range 1.65 
done
   
  stiff
 
  • Baseline fork target:       stiff              70.8 lbs co wogas at 100ips  [.944 kg spr]
  • Baseline shock target:    109 x 1.045     629 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.28
  • fork target [fk.targ]       fixed at stiff
  • fk/sh ratio [fk.ratio]      1.28 - 1.67        Max fork stiffness 1.67 at c.force 412 lbs     In this case fk.ratio has exceeded upper range 1.65 
done
   
  stiff+1
 
  • Baseline fork target:       stiff+1           79.8 lbs co wogas at 100ips  [.944 kg spr]                               fk.targ [soft - aver - stiff]      fk.ratio range [1.25 - 1.35 - 1.45 - 1.55 - 1.65]  
  • Baseline shock target:    109 x 1.090     656 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.31
  • fork target [fk.targ]       fixed at stiff+1
  • fk/sh ratio [fk.ratio]      1.31 - 1.70        Max fork stiffness 1.70 at c.force 439 lbs     In this case fk.ratio has exceeded upper range 1.65 
done
   
  stiff+2
 
  • Baseline fork target:       stiff+2           90.3 lbs co wogas at 100ips  [.944 kg spr]
  • Baseline shock target:    109 x 1.180     710 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.31
  • fork target [fk.targ]       fixed at stiff+2
  • fk/sh ratio [fk.ratio]      1.31 - 1.68        Max fork stiffness 1.68 at c.force 494 lbs     In this case fk.ratio has exceeded upper range 1.65  
done
   
  stiff+3
 
  • Baseline fork target:       stiff+3         102.3 lbs co wogas at 100ips  [.944 kg spr]                               fk.targ [soft - aver - stiff]      fk.ratio range [1.25 - 1.35 - 1.45 - 1.55 - 1.65]  
  • Baseline shock target:    109 x 1.270     765 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.32
  • fork target [fk.targ]       fixed at stiff+2
  • fk/sh ratio [fk.ratio]      1.32 - 1.68        Max fork stiffness 1.68 at c.force 548 lbs     In this case fk.ratio has exceeded upper range 1.65 
 
   
  stiff+4
 
  • Baseline fork target:       stiff+4           116 lbs co wogas at 100ips  [.944 kg spr]
  • Baseline shock target:    109 x 1.270    819 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.34
  • fork target [fk.targ]       fixed at stiff+4
  • fk/sh ratio [fk.ratio]      1.34 - 1.70        Max fork stiffness 1.70 at c.force 602 lbs     In this case fk.ratio has exceeded upper range 1.65
done
   
  stiff+5
 
  • Baseline fork target:       stiff+5          131.9 lbs co wogas at 100ips  [.944 kg spr]                              fk.targ [soft - aver - stiff]      fk.ratio range [1.25 - 1.35 - 1.45 - 1.55 - 1.65]  
  • Baseline shock target:    109 x 1.495     900 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.35
  • fork target [fk.targ]       fixed at stiff+5
  • fk/sh ratio [fk.ratio]      1.35 - 1.69        Max fork stiffness 1.69 at c.force 683 lbs     In this case fk.ratio has exceeded upper range 1.65 
  • For comparison . . . Fellows 3885f + 3733s     fk.ratio 1.79     127.3 lbs fork comp     608 lbs shock comp
done
   
  stiff+6
 
  • Baseline fork target:       stiff+6          150.1 lbs co wogas at 100ips  [.944 kg spr]
  • Baseline shock target:    109 x 1.585     954 lbs c.force at 50ips   [4.55 kg spr]
  • Baseline fk/sh ratio:      1.41
  • fork target [fk.targ]       fixed at stiff+6
  • fk/sh ratio [fk.ratio]      1.41 - 1.75        Max fork stiffness 1.75 at c.force 737 lbs     In this case fk.ratio has exceeded upper range 1.65 
  • Minimum c.force . . . The upper range [1.65] of the fk.ratio for soft-7 was limited by minimum c.force     (the upper range for stiff targets are not limited)
  • Maximum c.force . . . The stiff+6 fork target has the stiffest possible matching shock compression resulting in the beginning fk.ratio of 1.41     range [1.25 - 1.35 - 1.45 - 1.55 - 1.65]
    • * By design, the stiff+6 fk.ratio's lower range [1.41] is limited by maximim 654 lbs c.force     If c.force were increased the fk.ratio would go down, but we are assuming 954 lbs to be maximum c.force 
done
   
 
  CONCLUSION  
  • Pick a fork target and adjust the shock target to get your fk/sh ratio.
    • Then decide if the fork comp target numbers look reasonable for the application.
    • Then decide of the shock c.force target numbers look reasonable.
  • Adjust accordingly.
  
    [Equal Bump Travel]