Workshop - Setting up your rear suspension
By BikeRadar | Thursday, September 25, 2008 8.00am
Tools - The essentials you’ll need:
- Shock pump
- Tape measure
- Zip ties
- Shock manual
1. Locate Air Valve
Many of our readers will be owners of air shocks. These are the best choice for general-purpose cross-country and all-mountain style riding. Air shocks require air to be added to the spring, and this is done via a Schrader valve ﬁtting that’s found on the shock body. Before you think about adding any air, you should ﬁrst remove the air from the shock and fully compress it. This will show you the total available travel of your particular rear shock.
2. Add Zip Tie
Fit a zip tie around the shaft of the rear shock. Make sure it’s tight — it won’t harm the performance of the rear shock, and will stop it moving from vibrations. The zip tie will show you the maximum travel you get during every ride (remember to slide it back to the lip seals to reset it before the ride), and you’ll know at a glance whether everything is working as it should. Air loss is rare, but can happen over time, and this will help spot it.
3. Extend The Shock
Fully extend the rear shock and measure the distance between the lip seal and the oil ring to get the ﬁgure for the shock’s total travel. Check this ﬁgure against that given by the manufacturer. Remember that 40mm of shaft travel doesn’t mean your bike has 40mm of rear travel: the frame it’s ﬁtted to will convert that 40mm into anything from 80-140mm of rear-wheel travel, depending on the linkage used between shock and frame.
4. Add Air
Re-spring the shock. To get the shock back to its original state, use an air pump that you know to be accurate, and add air to the main chamber using the manufacturer’s guidelines as a starting point. These are often based on rider weight, and are speciﬁc. If you don’t know your exact weight, then weigh yourself on some accurate scales.
5. Know Your Numbers
Knowing the total travel available allows you to understand what differences the changes you’re making to the shock are actually having. Make a written note of the total travel available, your weight and the exact pressure applied to the air chambers. This will allow for a faster set-up next time around and make detail changes accurate.
6. Add Air
A good rule of thumb for inﬂating air shocks is to add 1psi of air for every pound you weigh. Therefore if you weigh 140lb, then begin the set-up process at 140psi. Remember to use an accurate shock pump, as old pumps often have leaks and faulty pressure dials. If you’re only guessing at the changes you’re making, it’ll take a lot longer to nail the settings you need. Check your manufacturer’s shock manual for precise guidance for your set-up.
7. Understand Sag
In order to let the rear wheel ‘track’ the surface of the ground, we allow some of the rear-wheel travel of the fork to be used when supporting the rider’s static weight — this is called ‘sag’. The basic idea here is to have around 25% of your bike’s total travel as sag. This allows the rear wheel to drop into dips and holes while maintaining contact and traction. Remember the rear wheel isn’t working in isolation, so the front and rear wheel sag should match.
8. Check Sag Level
When you’ve added the equivalent of your body weight (measured in pounds) in psi, carefully sit on the bike and then ease off the saddle, being careful not to ‘bounce’ the rear suspension as you get on and off. It’s often easier to do this with the help of a second person to steady the bike. The zip tie you ﬁtted earlier will ride up the shaft and show you the level of sag you have.
9. Adjust Sag Level
Add or remove air to dial in the sag until you reach the ‘optimum’ 25% sag setting. Correctly adjusted, you should bottom the shock once or twice per ride (on suitably big hits). The difference of even 5psi in some shocks can make a big difference in ride feel. Spend some time sorting sag (some riders like more or less than 25%, depending on individual riding style and terrain).
On some rear shocks there is also a negative air chamber, and this is used to add progression to the end of the spring rate and counter the tendency for harsh bottoming. Some models automatically ﬁll this chamber as you add it in the main chamber, while others have a separate valve. On separate valve models, don’t add too much — you want to bottom the shock, but only on hits of a certain size/severity.
11. Spring Rate
Air has a linear spring rate, and changing the air pressure changes the ﬁrmness of the shock but doesn’t alter the spring rate curve, which will remain (relatively) linear. To make the shock ﬁrmer, add air. If the shock feels about the right ﬁrmness, but is being activated by the transmission of the design of the frame, switch on the Pro Pedal setting. This will allow you to ride with a supple rear suspension, but dial out the shock movement.
12. Coil Shocks
Coil shocks use coiled metal as a spring, instead of air. If you’re over or under the desired sag level, you should deﬁnitely swap out the spring for a heavier/lighter version (most shops carry a few spares for swapping on new bikes, so don’t be afraid to ask them for help). Don’t be tempted to ﬁx coil spring rate issues with preload. This is a ride height adjustment, so again refer to your shock manufacturer’s manual for precise set-up advice.
13. Rebound Adjustment
Some shocks have external rebound damping adjustment (normally a small red anodised knob). This adjustment controls the rate at which the spring’s energy is delivered back to the rider. Too little and the rider will get a jolt though the saddle after each hit; too much and the shock won’t extend to its full length in time for the next impact, potentially causing it to ‘pack down’, losing available travel with each hit.
Begin with the adjustment all the way off (turn the knob towards the minus arrow). This will allow the shock to re-extend to its full length at maximum speed. Usually this is too fast for most riders, though very light riders may ﬁnd it acceptable. Add rebound one click at a time and ride the bike through some typical trail sections. Keep adding rebound damping until you feel that the suspension extends quickly but without ‘jolting’ or ‘packing down’.
15. Compression Damping
Occasionally air shocks also have a blue dial or switch (other than the Pro Pedal lever we mentioned earlier in the walkthrough). This is a Compression Damping Adjuster — controlling the rate at which the spring is allowed to compress under a given load. Begin adjustment in the fully off position, adding a click before testing the effect and deciding if more is needed.
Some air rear shocks also have a lockout facility. This allows the rider, at the ﬂick of a remote bar-mounted switch or turn of a shock top dial, to cause the shock to remain at its full ﬁxed length. This is handy to use on smooth road or for long smooth climbs. Most rear shocks with a lock out option have a blow-off feature, which ensures that the shock will begin working as normal should you hit an obstacle while riding locked out.
When you’ve got the front and rear suspension set up, ride along and stand up with your pedals level and your weight squarely between the wheels. Now squash the bike into the ﬂ oor. The front and rear suspension should compress and return at the same rate. Use the reﬂection from a shop window to check – the front triangle of the frame should remain level through compression and rebound strokes.
Check the mounting hardware for wear and adjustment at regular intervals. Worn bushes, bent mounting bolts or loose nuts will only serve to reduce shock performance and (much worse) potentially cause permanent damage to your frame’s shock ﬁxing points. It really is worth taking the time out to check these things regularly, as they will potentially save you loads of money in the long run.
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