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Measuring sag - seated or standing


ozgti's picture

By ozgti - Posted on 19 July 2014

Hi all,

I am a bit confused about measuring sag. I've always thought that you measure sag sitting down in the attack position. Using this method, I do have an issue getting the sag right in that it doesn't sag that much. Instead I more look at how much travel I'm using then adjusting the pressure that way. Now a recent trip to the bike shop the guy got me to stand in the attack. Using that method I do actually get a sag of 20%. So the question is have i been going about this all wrong and that I should have been standing up when measuring sag? When I did get the sag using the standing method, the fork feels more stiff and doesn't use the last 2.5 cm of travel. Any comments would be helpful.

Thanks

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Flynny's picture

Depends on the suspension design. Most Single pivot and steady arch designs firm up when you stand and shift your weight forward anyway

For the most part I would have thought you'd want optimum suspension setting when you are sitting down and pedalling. How much time do you spend standing and sprinting over rough terrain?

fairy1's picture

There is a seated attack position? Also is this just on a hardtail? If so id doesn't matter all that much as long as you like the way it feels and it's not bottoming like crazy.

hawkeye's picture

On my Fox-suspended bikes I use the conventional wisdom and set it seated and at 25%. All good. Comfortable ride, uses all the travel once per ride without harsh bottoming and without a lot of pedal bob, whether riding for fun or racing.

Now that my race bike (2013 Scalpel) has RS suspension, all that went out the window. It's taken me awhile to get my head around it.

I kept backing off the pressure so that I was using the full travel appropriately.

I also had to get the fork damper setup updated to 2014 spec (done by CSG at no charge) and fiddle with the volume spacers in the front air chamber (running 25mm instead of the stock 60mm spacer for more linear spring rate), which is bog easy to do but the trial and error was an irritating consequence of being an early adopter of the new bootless Lefty fork. (I wish they'd do their bloody testing in house first instead of leaving it to retail customers!)

That's ended up with me running around 30%+ sag at each end, which initially I found a bit of a scary thought on a mere 4" travel bike, but I'm not bottoming harshly, it seems as smooth on the fast choppy stuff as a 4" bke can be, and it's a fun and comfortable ride around Manly Dam without being at all wallowy.

Bottom line is: change one thing at a time to see what it does, don't get wed to a particular assumption about what is right (eg 25% sag, stock air volume) and be prepared to experiment.

ozgti's picture

It thought there was a seated attack position? The bike is a specialized fsr. Most of the time I'm seated apart from going on more rocky terrain or downhill sections. Maybe it's my riding position being too far back therefore not registering much sag when sitting? The fork is rockshox Reba. Certainly feels better when I set it to use full travel. It's just .that I see in the magazines they are seated when adjusting sag. The shop that I bought the bike from never told me how to set the sag. It was a different shop that showed me the standing way.

hawkeye's picture

Seated, you'll have more weight on the rear. Standing, more on the front.

Ultimately, as I learned, either method is only a rule of thumb that may or may not be accurate for getting the desired performance on the trail. And that is what matters: how it performs on the trail.

It's just a pity I'm such a slow learner. Sticking out tongue

ahein's picture

Does the bike have autosag for the rear suspension? When I bought my spesh the guy in shop just pumped up the rear shock to 300psi got me to sit on the bike and pushed the autosag button and it set it at 25% sag and the bike has felt good since day one

ozgti's picture

Nah no auto sag. Rear shock setup is fine. Just the fork.

Oldernslower's picture

The problem for most of us (not all), especially those new to the dark art of suspension set-up, is that we just don't know how bad settings feel. Most of us (not all) can't tell the difference between a couple of clicks on compression or rebound adjustment, or even 10 pound extra air pressure, or 10% more or less sag. Or what too much compression or rebound feels like. This applies to front and rear and HT. So what happens is we make the adjustments around the middle of the adjustment process, maybe starting with the manufacturers suggestions, maybe setting sag first then fiddling with the various front/rear clicks. We may look at the travel and realise we are bottoming/topping out, or not using full travel, then we add or subtract air pressure (rather than change compression or rebound). We end up with something we think is ok and adjust our riding to those settings.

Next time you ride make both ends (or front if a hard tail) firmer by two clicks on compression and quicker rebound by one or two clicks, go out and ride your favourite trail - notice any difference? For most of us we won't, but we tend to continue to adjust until we have something that uses the travel and has the 'required' sag (which is nominal at best and a guide only - hence my concern re auto setting sag) I'm not saying it does not work, but you are now adjusting the suspension based on a preordained nominal air pressure for your weight and seating position (or maybe standing as noted in another post). People got fed up of adjusting two air pressures on the front, so RockShox went to solo air. The same is happening to sag, let the manufacturer do it for you. Just because I'm using the full travel on a particular route does not mean I have the optimum settings on the bike.

IF you are one of those noted in the first para and can't tell the difference, you can try the following to get a better idea of what difference the various adjustments make. This will take a day of patience (maybe two) but is worth it in the end, Trust me, I was a psychologist Eye-wink.

Write ALL of your current settings down Download and save this XL spreadsheet form if you wish. Go to your favourite loop of around 6 km that has a variety of terrain and you are very familiar with, one of the local XC tracks may be ideal or a short section of the KC as long as it has variety and a good climb. Set everything to middle of its adjustment, compression, rebound and recommended sag (and negative pressure if your forks have this) and tyre pressures at your normal pressure (note your tyres are part of the suspension system) - write all settings down. Warm up and go ride the loop. When you get back note your impressions of the ride and handling.

I would suggest you don't do jumps on the following trial settings the bike may not react how you expect!

Now back off the fork compression to zero - i.e. minimum damping. Go ride the loop but not at racing or Strava PB speed!! CAUTION - the bike will not handle the way it did before so don't overdo it especially on jumps and downhills and braking, (front will (should) dive under breaking!) Write down your impressions of the handling. e.g. bottoming out, pedal bob, vague feeling or no front end feel, felt like I would go AoverT (err over the bars) on downhills, dives 'orribly on braking, very twitchy, fast steering (oversteer), can't hold a line, whatever. If you are uncertain go round twice.

Now put compression on to maximum damping, go ride the loop, CAUTION - the bike will not handle the way it did before so don't overdo it, especially on jumps! Write down your impressions of the handling. e.g. Harsh front end, hits not absorbed, Travel not used, front end bounces and goes off chosen line, front end washes out easier, understeers can't hold a line - it is generally - whatever.

Put compression back to the middle setting and adjust rebound so it is maximum fast ie little damping. Go ride, CAUTION - the bike will not handle the way it did before so don't overdo it anywhere! Write down your impressions of the handling, e.g. pogo stick like? topping out, bounces, bike is unsettled on rough stuff, steering vague, whatever write it down

Put rebound setting (clicks) to maximum slow, go ride. CAUTION - the bike will not handle the way it did before so don't overdo it especially on jumps and over stutter or ripples, the forks may gradually fully compress as they don't have time to extend as the rebound is too slow, or available suspension movement may decrease to less than 40mm)! Write down your impressions. E.g. Oversteers, increasingly harsh over rough ground, steering tightens on fast berms, an AoverT (OTB) experience on rough downhills – whatever - note it down.

Put the front suspension back to the middle settings. You now have some idea as to what too much and too little compression and rebound feels like. Now do the same for the rear, one setting at a time, CAUTION - the bike will not handle the way it did before so don't overdo it especially on jumps and rough sections, or ripples, or anywhere!!

NOTE you only change one setting at a time! Changing the front end can affect how the rear reacts and change the rear can affect how the front feels. Note it all down.

Now set everything to the middle of its adjustment. Change suspension air pressures in the front so it has half the sag than recommended, go ride. CAUTION - the bike will not handle the way it did before so don't overdo it especially on jumps! Write impressions down.

Now adjust so front has an extra 10% more sag than recommended, as per above, write impressions down. Put back to standard.

Now do the same for the rear.

You now have an idea what the various deficiencies feel like. Yeah, I know, you could start in the middle with everything and work something out by gradual adjustment. But what does too much rebound damping feel like TO YOU? The objective here isn't to get the correct settings but to get an idea as to what bad settings feel like.

The suspension is a system comprising of all elements of the bike including the rider. Change one setting (e.g. riding position) and it is likely to change the others. There is no perfect setting as everything is a compromise – but there are a lot of bad settings Smiling

Welcome to the dark art of suspension set up.
Or you can do a WTF, this feels ok Eye-wink

fairy1's picture

Good lord, I can tell you used to charge by the hour, hahaha!

I just put an X-Fusion Vengence fork on my bike and hate the linear feel so I threw an extra 15mL of oil in the air spring to make it ramp up more and will run slightly more sag than recommended.

I have only owned DH bikes and hardtails but set them all while standing because that is when you are having fun and want the bike to handle at it's best.

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