Front Fork Travel Specs or limits

[MisterCinders]

With some sexy new progressive springs and a fork brace on the bike, I searched for more guidance on fork travel and sag limits. Not finding an answer, I must post.

The springs fit right in without a space (per the instructions) and I used a 50:50 mix of 15w Rotella and ATF. Measured out the 6 oz. and used the bottle pump method to level the oil off at 206mm, per my manual.

So far the ride feels pretty good, though it's been too wet to push any corners or test some hard braking. The ride is definitely smoother over bumpy city streets though.

As I continue to feel out this new suspension set up, I am curious about other fork travel limits, etc. I left the zip ties on the forks and zeroed them out before my ride today. The initial static sag travel looked pretty good. After an hour of riding around town, over Chicago's super-cool "speed bumps" (i.e., crappy pavement ), but without any excessive braking or hard cornering (again, it's been pretty rainy) the ties had moved up the forks a good amount, but looked pretty even. Nothing felt or looked to bottom out, so there's that.

So are there any things to look for with respect to fork travel beyond the static test? Obviously bottoming out would be a big negative, but if there are some other ways to monitor suspension performance short of that, I love to hear about them. For instance, is it worth keeping an eye on average travel with those zip ties, and is there a "danger level" where they might indicate too much sag/travel?

 

[Billy Ricks]

How much sag did you set up with your weight in the seat?

 

[koolaid_kid]

Well, you really only need one (if they are different you have real issues), but I always kept one on my GS. You specifically use it to set initial sag, which should be spec'd somewhere. But I used it to see how close to the bottom tree my forks would get. If it gets too close the springs are too soft. I used it to see how hard I was pushing it in the curves, when I used to push it. I have slowed down since then, so the GPz doesn't really need one.

 

[MisterCinders]

To check sag we put zip ties on boots with the forks extended. Then I sat on the bike, rolled back a bit and then rolled forward and cranked the front brake. Leaving one tie there, we zeroed the other tie with the forks extended again and then I sat on the bike.

Comparing the two, the static tie came in well below the rolling brake tie, and there was a good bit of space left above the higher tie. The static tie was definitely within 1/3 as high as the other tie.

Everything looked pretty good. I am just curious about other things to check out as I explore the new front end.

 

[bwringer]

Here's the real poop for a way to measure static sag while eliminating the effects of stiction: http://racetech.com/articles/SuspensionAndSprings.htm

Most GS models have about 4 inches of suspension travel. Static sag on any road-going vehicle, as a rule of thumb, should be set at 1/4 to 1/3 of total travel depending on your speed/comfort preferences. Therefore, you want between 1" and 1.3" of static sag. More sag is a bit more comfy, less sag gives a bit more control.

As far as a travel limit, it's completely irrelevant on the street, since you don't know what you're going to encounter. Street suspension is always a compromise based on average conditions, so the "rule of thumb" mentioned above works very well.

On a track, where the track is the same each time around, you might need to adjust spring rate to suit the track.

For example, if it's a very smooth track and your rider comes back in with the zip tie showing she's only using 2/3 the suspension travel, you might be able to gain a bit of compliance and speed by using a softer spring. You have to balance this against cornering clearance, of course -- a softer spring might let her put the power down sooner out of corners, but if the bike is sitting lower as a result it might compromise cornering clearance too much.

If, on the other hand, the telltale zip tie shows that she's using all the suspension travel and the rider reports bottoming, you may need a stiffer spring (and/or more compression damping, or maybe less rebound damping...) to maintain control. Most tracks are somewhat wrinkled on corner entries and exits from the abuse of cars, so it's more common to need all the suspension travel you can get.

Dialing in damping for a rough track is a separate topic that will make you tear your hair out, which is, I think, why so many crew chiefs shave their heads.