Thick Front Suspention Setup Q

[Plax]

My MTB has Suntour XCM 100mm travel suspension forks with pre-load adjustment (the Stanchion's are 28mm).

My book says to tie a cable tie to the stanchions just above the wiper seal. Then I'm to sit on the bike and get off it again and measure the distance between of the wiper seal to the cable tie. This will give me the amount of "sag".
To get my "suggested sag" I'm then to multiply the "range of motion measurement" by .25. I presume then that this is 100mm x .25 which will be 25mm sag? So I need to tweak my pre-load adjustment until my sag is 25mm?

 

[Bigtwin]

Probably.

I wasted a complete afternoon p1ssing about with sag settings on my full sus MTB.

It made absolutely no noticeable difference to the ride at all. "A reasonable about of free" would have saved me several hours.

 

[maurice]

Generalising here, for XC you want around 20-25% sag of the total travel.

Note that the 100mm is probably 'advertised' travel, with real travel being a bit less. I would start with 20% sag (in this case 20mm), and see how it feels on the trail.

 

[mickle]

The spring in a suspension system carries the weight whilst the damping controls the speed at which the suspended weight moves in response to bump force input. Pre-load is a very blunt tool, all it does is compress the spring which simply adjusts the amount of bump force required to get the fork moving. More complex systems allow you to change the spring rate. I don't know the fork but I assume it features an undamped coil spring sitting on a stack of elastomers. The elastomers do provide some rudimentary damping but in such a fork you'd need to actually replace the internals in order to adjust spring rate or damping. Pre-load doesn't make the fork any 'stiffer' nor does it actually have much effect on sag. What it does is remove the springs ability to respond to small bumps, making it feel stiffer. Once you get beyond that initial pre-load the suspension behaves in exactly the same way.

Shame some of the fork manufacturers don't know this. My recommendation is that you fart about with it until you like the way it feels.

 

[RedBike]

Yes, thats it in a nut shell.

Put the zip tie on the forks stantion (top bit of the leg), Push the zip tie down to the bottom of the stantion. Then sit on the bike

The forks will sag pushing the zip tie up the stantion. Hopefully when you get back off the bike the zip tie will remain in the raised position so you can see how far your forks actually compressed/sagged.

As with all things 25mm is just a guide. It doesn't really matter if it moves 1/2 an inch or two inches, so long as it moves. Have a play around and see what suits you best.