Muddymoles mountain biking in the Surrey Hills and Mole Valley

How to: Check your sag

Posted by Colin | December 17, 2007 | 1 comment so far

When was the last time you measured your sag?

I’m not talking about the expanding waistline or how far your (man) boobs are from the ground. I’m talking about your suspension. Most of us when speccing our new bikes go for the high-end forks such as Talas or Pikes, which on their own cost more than we all spent on our first car! But do we understand what we’re buying here?

In a lot of cases the answer would be no. They get fitted and we rack up the miles without much thought for their set-up and how they could be tweaked to further improve things.

I was a classic example. Despite knowing a bit about suspension from fettling my track motorbike, I had fitted some Talas forks to my Giant in the summer and never gave a thought to setting them up.

Then I got to thinking a few months later that my front end was quite wooden, not forgiving (or ‘plush’ as they like to say in MTB speak) and gave me little feedback. So I measured the sag. It was way off the mark.

Sag is the amount of the fork’s travel used in supporting the weight of the bike and your good self. It is a really important setup fundamental that is often overlooked. Sag is basically governed by the amount of pre-load there is in the spring (or air chamber as the case may be).

Too little preload and the bike will sit down far too much, use too much of its available travel just to support you and leaving too little travel for when you hit that massive depression, root or rock.

Too much preload and the suspension will be stiff, unresponsive, wooden, vague, etc. Most importantly, if you don’t have enough sag, at the point when you and the bike become weightless after a small crest, stone or root, etc the fork won’t extend to keep the front wheel in contact with planet earth. Not a problem in a straight line, but if cornering, that means the front will tend to skip and slide rather than feeling planted or surefooted.

I’m no suspension guru so give it some thought, measure up and adjust to see if you can improve what you have at the moment. Having just got some Pikes I’ve been playing around with the setup and its amazing how different they can feel from some small adjustments.

How to measure:

  • Get a small cable tie and attach it to one of your fork stanchions
  • Slide it down to touch the top of the fork seal
  • Another person is very useful to hold the bike whilst you carefully get on board, being careful not to bounce. Stand in the attack position, ie arse off the seat, weight through your feet
  • Carefully get off the bike, again being careful not to bounce it
  • Measure from the top of the fork seal to the bottom edge of the cable tie

This’ll give you your sag. As a guide, you should aim for 20-25% of your forks total travel. If it falls outside this range, increase or decrease your air pressure accordingly. Not so easy if you have coil sprung forks as that’ll mean changing the spring, so speak to your LBS.

It’s quite interesting to leave the cable tie there and see how much travel you are using over the course of your rides in future – slide it down when you set off and keep an eye on it every now and then while you’re out. This will then give you some pointers regarding your compression and rebound damping – but that’s for another day.

Ponder on the words of an Ohlins suspension technician who I spoke to a few years back

‘the suspension should be as soft as possible but as hard as is required’.

What that means (I think) is on a given ride you should be using as much of your travel as possible without it bottoming out. Easy to apply on a motor racing circuit when things are consistent from one lap to the next, but not so with MTB when its all so much more variable…

If you are still thoroughly confused by it all, speak to the guys at TF Tuned Shox – very helpful, enthusiastic and knowledgeable MTB suspension gurus.

Filed under Tips in December 2007


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There is 1 comment on ‘How to: Check your sag’

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  1. Muddymoles says:

    So, what tyre then?

    With summer unbelievably (given the arctic chill) on the horizon, Matt turns his thoughts to tyres and bike set up.

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