[Closed] Why do we set rear sag by SHAFT travel, not wheel travel?

25% shaft travel may be, say, 40% of wheel travel for a progressive curve.

I doubt that very much.

Go on then, measure the travel on the back of your bike. There may be a couple of % difference, but is easy to measure the shaft.

You are essentially. I've always taken the usual 25% sag as a starting point, a general rule of thumb, and you then tune according to your own bike, suspension geometry and set up and personal preferences. I usually end up with alot less than the usual 25% to 30% sag levels.

Stood up vs sat down is just a weight distribution thing. I guess you decide where you'd like the sag to be optimised...when you're 'attacking' the trail stood up, or at some other point where you're sat down - I guess if you ride alot of XC you'd spend more time in the saddle so might set things up in the seating position. If do more gravity riding then you'd set up in the attack position.

It's much easier to measure the shaft travel. They may not be perfectly linear, but it's still a reasonable guide.

I thought it was because the shocks are set to a certain amount of damping at around the sag percent, so you need to be there regardless of what that ends up being at the wheel.

Sat down. Umm cos....yeah.

I'm not sure that we don't OP.

We might set initial sag with the o-ring on the shaft, but then we ride our bikes and adjust the pressure according to whether we're bottoming out or not getting full travel.

Granted, we refer to the o-ring again to see how much we've used - but it's just as much about feel when riding.

It’s much easier to measure the shaft travel. They may not be perfectly linear, but it’s still a reasonable guide.

It's easier to measure the shaft travel, agreed.
But.. specialized could EASILY say to me that my "50mm travel shock on my stumpjumper should be sagged at 10mm to equate to 25% wheel travel" ....

Also agreed it's unlikely to be vastly out! I might even try and work it out from my bike's leverage ratio!!!

DrP

Because measuring rear wheel travel is a complete PITA in real life.

Distance through the shock stroke is (relatively) easy to measure and once you have it sorted is easily repeatable.

Because Shaft is "damn right!"

Regarding the stood up or sat down technique, I think the jury is still out on that but I think the key bit is that it doesn’t matter as long as you stay consistent and use the same method each time and tune it to your preference.

Just checked. Sat down I am at 28%. Standing ready position - 24%.

Bike companies probably also want to avoid lots of people finding out that advertised rear travel and actual rear wheel travel can be a bit different.

If you actually measured rear wheel travel there would no doubt be a few surprises.

Why do we set rear sag by SHAFT travel

We don't, or at least I don't. I put it at 25-30% ish to begin with, and then I adjust it based on feel as I get to know the bike. This is what everyone should do, if they care.

Seems an odd question. Are you on the right forum, or is the answer actually "a logburner"?

🙂

I might even try and work it out from my bike’s leverage ratio!!!

Unless it's a single pivot you'll struggle...

Repeating some other people's points, possibly all these points are repeating other people's points, but here you go:

1) It's easier.
2) If it was massively non linear, you could easily set the sag at a different percentage of shaft travel.
3) It's not massively nonlinear.
4) It's often just an approximate starting point, you can tweak if you want to.

Unless it’s a single pivot you’ll struggle…

Nah. you'd just need the leverage ratio graph and plot it out..
Find out the 25% wheel travel point,and work out the shaft compression from there.
But.. i actually can't be bothered !

DrP

does anyone actually run recommended settings? i suppose if you are bang on the average weight you might be ok. its just a starting referance position and you tweak to suit. i run alot less than 25% sag front and back. I cant stand squidgy suspension.

On my Nicolai Geometron at roughly 20% through the stroke the rear wheel is 25% into it’s travel.  I was sad enough to take points from a graph and work it out when I was wondering why a Shockwiz wasn’t giving me the results I wanted

i run alot less than 25% sag front and back. I cant stand squidgy suspension.

You should probably be looking at damper settings rather than spring settings in that case

does anyone actually run recommended settings?

I don't even read them.

:S

I run closer to 35%. Anyone who gets on both mine and my boys bikes think they're very squidgy... But it's what we both like.

This is why you ask the bike MFR what sag to run.

They will look it up on the progression curve and tell you a measurement for the shock shaft that correlates to whatever travel sag you want.

we're kinda digressing now... but... I've always thought you set sag first (at 25% or whatever is recommended for your bike)..
This will be 25% for everyone, BUT..your pressure etc is vastly different.

Then... you change your DAMPING settings to control if you're getting full travel or not... or if you want it squidgy or not...

Anyway, that's what I do..

DrP

Yeah. I go sag, rebound, compression, volume spacers-add/remove in that order.

Regarding the stood up or sat down technique, I think the jury is still out on that but I think the key bit is that it doesn’t matter as long as you stay consistent and use the same method each time and tune it to your preference.

Exactly, sitting down is seen as more consistent. you sit in the saddle, weight in the same place. measure sag from there.

Basically because sag doesn't have to be, and doesn't benefit from, being exact. It's a start point and it doesn't matter at all if that start point's got a 10% or whatever error. On pretty much all bike designs, the shock shaft will give you a usable result

I’ve always thought you set sag first (at 25% or whatever is recommended for your bike)..

No, the 25% is purely a suggestion. Run what you like. You can knock a degree or more off your angles by running more sag if you don't find it too squishy, for example. All a matter of preference.

Plus a large number of bikes (mine included) don't have independent compression and rebound settings. I would like to run about 25% sag because I like the geo that way but it'll be too hard and I'll only get about 75% travel. There are no tokens to remove or compression knobs to twiddle, but 30% is fine. I can also let more out of the fork if I want to even it up, cos the fork is pretty easy going with regards pressure.

Why do we measure our tyres in psi why not sit on the bike and measure the sag ?

what would that achieve?

I used to have a stumpjumper fsr that had lines on the linkage and seat tube, which when lined up equated to a specific % rear wheel travel. I suppose this is what you're suggesting but as has been mentioned, since it's only a starting point for further micro adjustment, it doesnt really help versus using the shock travel.

Are you on the right forum, or is the answer actually “a logburner”?

If you report these we can move them (which I've just done).

i'm sure the early lapierre's had that too - an arrow that lined up with a mark on the seat tube...

DrP

Yeah, my remedy came with s clippy-on plastic thing that everyone instantly lost, except for Singlespeedstu who treasured it and passed it on to me with the frame so that I could lose it

Why do we measure our tyres in psi why not sit on the bike and measure the sag ?

I do, digital shock pump makes it very easy. Otherwise makes no difference to use PSi or mm

you’d just need the leverage ratio graph and plot it out..
Find out the 25% wheel travel point,and work out the shaft compression from there.
But.. i actually can’t be bothered !

Bingo - answered your own question 🙂

it doesn’t matter as long as you stay consistent and use the same method each time and tune it to your preference.

This is the correct answer. The method matters not, repeatability, changing one thing at a time and proper time taken to setup and test is the way it should be done.

Measure sag in Radians of head tube angle relative to the chainstay if you want, doesnt really matter.

Banshee have supplied this info for a long time:

https://www.bansheebikes.com/titan

(look at the bottom of the geometry chart)

"28-32% sag (15.5-17.8mm of shock compression, ignore sag markings on shock)"

Why do we measure our tyres in psi why not sit on the bike and measure the sag ?

I’ve always preferred to measure tyre pressure in fruit units.

Feels like:

A) an unripe avocado - perfect
B) a ripe avocado - remember to pump tyres up before next ride.
C) a banana - I’m going to die on the next tech section.

Isn't the leverage ratio to do with force not travel? The wheel travel is linear with shaft movement? (I am not a suspension designer, so expecting to be wildly wrong.)

"Isn’t the leverage ratio to do with force not travel? The wheel travel is linear with shaft movement? (I am not a suspension designer, so expecting to be wildly wrong.)"

The change in force applied from the shock to the wheel occurs because the leverage ratio changes, and that can only happen if the wheel:shock movement is not linear.

With a very progressive bike, that goes from 3:1 at top out to 2:1 at full travel, you'll get 3mm of wheel travel for 1mm of shock travel at the start of the stroke and 2mm of wheel travel for 1mm of shock at the end. So with a coil spring, the spring rate at the wheel will be 50% firmer at bottom out.

The change in force applied from the shock to the wheel occurs because the leverage ratio changes, and that can only happen if the wheel:shock movement is not linear.

If you have a coil shock the spring rate is linear so the force applied to the rear wheel still goes up with compression even if the leverage ratio has a linear relationship with displacement. It's just that if you have a changing leverage ratio the amount by which it goes up for each mm of displacement changes. And/or if you have an air shock.