Long-travel street bike set up

[Gary856]

I’m trying to think thru the suspension set up for my 950SM (7.9” front travel, 8.2” rear) and I’m getting a little confused by the “purpose and goal” of longer-travel suspension for street use, and how to best set it up.

In concept, when suspension travel is increased from 4” to 8” for a street bike, does one:

1. Keep the spring rate and shock damping the same – this way the 8” travel bike can ride as crisp as a 4” travel bike “normally”, and the longer travel suspension only gets activated on bigger hits. Or,

2. Soften spring rate and shock damping – this way the 8” travel bike dives/rises/wallows a little more but it's more active to deal with rough roads and more comfortable.

Right now the fork pre-load is backed out all the way (CCW), but there’s very little sag, and the zip-tie shows I’m only using ~ 5.25” of fork travel, less than my 690SM and Multistrada 1200, but is that necessarily bad? Not sure how much travel is ideal for the street before it crosses over from firm but harsh to too soft and wobbly? The spring rate feels ok. The ride feels firm, crisp and handles well, but the front tire seems to push a little more than it should at higher lean angles.

Let’s say I want to get front sag from ½” to 2”, how do I know if I should change a) spring rate, b) spring length, or c) spacer length?

Another thing that’s interesting – I’ve always read/heard that straight rate springs are preferred, but now I’m reading (on a Racetech page) that it’s only because straight rates are easier to set up, and that progressive springs aren’t necessarily bad - they just need more careful setup to match rider weight. It almost feels like progressive springs would work better for a long-travel suspension street bike.

 

[stangmx13]

Spring and spacer length are determined by fork and cartridge length. U need 10-25mm of actual preload on the spring for it to work well. So u can’t change lengths outside of whatever lengths produce that range.

If 10mm doesn’t produce enough sag for your liking, the spring rate is too stiff. If 25mm doesn’t produce too little sage, the spring is too soft. But I’ve always felt this importance placed on sag is misguided. I tune springrate more for midcorner feel and bottom out than sag position.

My forks are extended because it offers control of brake dive w a lighter spring. Lighter springs produce more edge grip. So go as light as u can get away with might not be a bad rule of thumb.

There’s no point of progressive fork springs. The forks total springrate is already progressive because of the air spring. U don’t need to make it more progressive, esp not more than OEM oil levels.

Riding style and brake application are just as important for selecting springrates and preload as rider weight. In roadracing, rider weight is least important of those 3.

 

[Gary856]

I'll take it to a suspension shop for setup eventually, but I want to at least think it thru so I can discuss the direction with the shop. Interesting back-to-back comparison below of firm (race) vs. soft (trail) dirt bike suspension setup for different applications. This essentially addressed the compromises in setting up a long-travel suspension between option 1 and 2 in the OP.

https://dirtbikemagazine.com/ktm-suspension-shootout-pds-vs-linkage/

From Dirt Bike Magazine:
"When you factor in the intended use of these two bikes, it gets more complicated. The XC is a race bike and the XC-W is a trail bike. Accordingly, the XC is much stiffer. When you ride the two bikes back to back, that fact overwhelms everything else. Both ends of the XC ride higher in the stroke, while the XC-W sinks deep into its cushy little world. On technical, low-speed trails, the XC-W is a much more comfortable motorcycle. It sucks up little bumps and rocks while the XC shakes you up. When you go a little faster, the XC is more stable, while the XC-W dives and moves around excessively in its travel. All of this has nothing to do with the linkage, or lack thereof. It’s just that the two bikes have different applications."


Below is the Racetech link I was reading. Refer to item 5 and 8 on progressive spring. They do recommend straight rate springs for forks since, like you said, Robert - air already makes it progressive.

http://www.racetech.com/page/id/30

 

[usedtobefast]

Not sure of the etiquette in this subforum ... is this just "Ask GP" and "GP answers"?

I have a 990 SMT, same long travel challenge. I spent some time making sure my springs were right for my weight and getting the front & rear preload set right.

But ... if I set the clickers for a ride that absorbs bumps nicely (so a great nasty road, pothole, kind of set up), then when going a faster pace (on a less bumpy road) the bike moves around too much ... too much forward when hard on brakes, basically lots of chassis movement.

If I then tighten up the comp/rebound to control the chassis properly it becomes a great fast carving bike, but then when on bumpy nasty roads it is too harsh.

Guess this is why the new KTMs get the electronic on the fly configurable suspension.

Seems a basic challenge is the clickers mostly affect slow speed comp ... so clicking those open enough to handle bad roads then means too much movement on dips, braking/accelerating.

I'm thinking if I got a revalve to lighten the high speed compression, then maybe that will help with the nasty roads (sharp hits, fast fork/shock movement) and my compression clicker low speed can be tight enough to control chassis movement.

 

[stangmx13]

Seems a basic challenge is the clickers mostly affect slow speed comp ... so clicking those open enough to handle bad roads then means too much movement on dips, braking/accelerating.

I'm thinking if I got a revalve to lighten the high speed compression, then maybe that will help with the nasty roads (sharp hits, fast fork/shock movement) and my compression clicker low speed can be tight enough to control chassis movement.

all OEM compression valves that ive seen are not preloaded and don't use preloaded shim stacks. so there is no separation between low and high speed damping. the clicker changes the whole curve, just adjusting the slope of the line. thats probably y u feel some harshness with a stiffer clicker setting.

a revalve can fix this. but u gotta add preload to the stack to accomplish what u want. its not guaranteed that ull get a preloaded compression stack from every tuner. so make sure to have that convo with the tuner.

 

[rrrdbw]

How does all this suspension adjustment relates to the newer model BMW R1200 GS/GSA that's self adjusting?

Recommended upgrades? How good are the stock suspension? I ask because a very fast rider I know upgraded R1200 GSA front and rear with Ohlin's he rides mostly street on it. He discouraged me from upgrading because he did not get the results he was expecting. His opinion the stocks are comparable. Thoughts?

 

[Gary856]

I haven’t experienced active suspension yet; my reservation with them is the repair/replacement costs. A well set up standard (non-active) factory suspension (like the ones on newer GS, Multistrada and KTM Adventure) works well enough for me to be happy on any pavement. I rather not get into the headache and cost to have to upgrade.

I read that stock suspension on older GS like mine (’08) is crap and needs replacement after ~ 12k miles. Mine’s still under 10k miles, and the stock suspension (non-ESA) still feels good. I’m ok with spending a couple of thousands to upgrade the suspension if the improvements are significant, but I’m hesitant when results are uncertain. It feels like a leap of faith. It would be great if whoever’s doing the work has a demo bike with the upgraded suspension to showcase the results and to help convince potential customers.

 

[stangmx13]

Upgrading suspension without first identifying issues or problems can often produce that disappointment of not noticing improvements. Some OEM damping curves and springrates are way off, maybe 100% off for damping and 20% for springrate. “Upgrading” from that will be noticeable to almost everyone. But if the differences are only 10% for either, no one is going to feel that outside of the racetrack.