Calculating Chain Length on Dual Suspension Bikes
Calculating the correct chain length is a critical and there’s more to it than you may think—especially if you ride a dually. This step-by-step rundown will help you avoid a drivetrain calamity.
At the very least, running a chain that’s too long can lead to dodgy gear shifting and a greater chance of dropping your chain. Shift into the largest chainring and one of the bigger rear cogs with a chain that’s too short and you’ll do some serious damage; the rear derailleur can get torn off, the drop-out can bend or the whole shebang can launch into the wheel and tear half the spokes out!
The major component manufacturers offer some basic guidelines, and their suggestions should work fine on a hardtail with a completely standard drivetrain. But what if you’ve modified the cassette with one of the larger ‘wide-range’ conversion cogs? Perhaps you’re running a mid-cage derailleur on a drivetrain that really should have a long cage mechanism. Maybe your suspension bike has a lot of ‘chain growth’. The following approach accounts these variables and should keep your drivetrain running trouble free.
If you ride a hardtail, you can ignore these initial steps but they are very important if you’ve got a dually. With most suspension bikes the length of the chainstays increases as you go deeper into the travel. Start by noting down the air pressure in the rear shock before deflating it. Use a core remover to pull the valve out completely; this will make the following steps easier.
With the valve core removed, wind the rebound adjuster all the way in. Dialling in more rebound will stabilise the rear end and make it easier to take accurate measurements in the following steps. This usually involves turning the dial clockwise to make the shock rebound slower. Count the number of turns (or clicks) and note it down along with the air pressure.
With the valve core removed, you can move the rear end through its travel. The aim is to find the point where the chainstays are at their longest. While this is usually when the travel is bottomed out, it isn’t always the case. Check the chainstays at a number of points through the travel and find spot that gives the longest measurement. With this DW-link Pivot, full travel gave the longest reading and the chainstays grew in length by 26mm—that’s a substantial variation that needs to be factored into your chain length. Other systems and short travel bikes may only change by 10-25mm.
Slowly and carefully shift into the biggest cog on the rear cassette and the largest chainring (assuming you have a multi-ring drivechain). Then push the suspension into the longest chainstay position. Now check the tension on the rear derailleur cage. If it’s totally maxed out, or if the chain is limiting the suspension movement, you’ll need to make the new chain longer. If the derailleur is close to its limit but still has a little bit of spring remaining, you could always gauge the new chain length off the old one.
Look for a master link on the chain and remove it as per the manufacturer’s instructions. SRAM joiners can be removed by pushing the sides in opposing directions with a pair of pliers or using a special Park tool that makes it easier. Shimano chains don’t use a master link so you’ll need to use a chain breaking tool to remove it.
If you plan on using the old chain as a gauge, lay the two side by side on a level surface. Make sure that the links line up along the entire length of the chain, as wear in the old one may skew the reading. If you are unsure or just want to double check before you cut the new chain, continue with the following steps.
Get an old spoke and bend an angle in either end as shown. Somewhere around 10-15cm between the bends should do the trick and pliers will make it easier to put the two angles in it. It’s best to use a thick plain gauge spoke for this. A stiff piece of wire such as a coat hanger will also work.
Shift the derailleurs into the big-big gear position. Wrap the new chain around the two largest chainrings and both derailleurs—just as you would to ride the bike. If you’ve got a clutch equipped Shimano rear derailleur, position the clutch in the ‘off’ position. If it’s a SRAM Type 2 derailleur, pull the cage forward and lock it there. Pay close attention to routing the chain correctly through the rear derailleur cage and jockey wheels. Most derailleurs have a tab in the middle of the cage and the chain shouldn’t drag across it; it should only run on the jockey wheels.
In the big-big gear, pull the ends of the chain to put tension on the derailleur and pull it tight. Hook your high-tech chain holder (the old spoke) through the chain ends to secure it as shown. Ensure that the suspension is compressed and make the chain long enough to handle this position. It may be unlikely that you’ll bottom your suspension completely on the trail whilst in the big-big gear combo, but it’s best to check that the derailleur can handle it—just in case.
Also check the clearance between the derailleur and the cassette. The upper jockey wheel should be reasonably close but not touching the big cog on the cassette. If it sits too close, wind the B-tension screw in on the derailleur. If the distance seems excessive, loosen the B-tension screw. While this adjustment is not absolutely critical at this point, it’s worth checking that it’s at least somewhere close as it can influence the chain length.
Once you are happy with the chain length, line up the links and mark the point where you want to join the chain. If the links don’t line up perfectly, make the chain fractionally longer—it’s a safer bet than having it too short. Use the chain breaker to cut the chain at the appropriate link; the actual link will differ depending on the joining method (master link or Shimano style joining pin).
Join the chain and then you can remove the chain holder—stash it in the toolbox as it can be handy whenever you need to work on the chain. Make sure that the freshly joined link isn’t tight, as a stiff link can cause the chain to skip across the gears when riding. Flexing the chain gently from side to side will help to loosen a stiff link.
With everything back together, double check that the chain length and derailleur can handle the largest cog combo right throughout the suspension travel. Now you can turn the rebound dial back to where you originally had it, refit the valve core and reinflate the shock.
Now the suspension has returned to its static position, the chainstays will be shorter again. Go through the gears and shift into the smaller cogs. On a single ring drivetrain you’ll want to have tension on the chain right down to the smallest cog on the cassette. If the chain starts to droop, tighten the B-tension screw until the derailleur takes up the slack. On some bikes, say for example a long travel dually with lots of chain growth and really wide range cassette, the derailleur may be unable to cover the complete gear spread. In these cases you may have to avoid using the biggest cog in situations where you could bottom the suspension. With a double or triple ring drivetrain, you mightn’t have chain tension right the way down to the smallest cog combinations; just avoid using those gears on the trail—better to lose a couple of ratios than have the derailleur ripped off by a chain that’s too short.
Alternative Method for SRAM
Rather than fashioning a chain holder from a spoke, you may choose to use a master link to set the chain length. SRAM’s 10 and 11-speed joiners are single-use items, so don’t use your new master link for the initial trial and error process of determining the chain length. Either use an old 9-speed SRAM joiner or buy another type of reusable joiner for the task (you can buy them for less than $10). Start with the chain on the long side and use the reusable joiner to run the chain through the gears and suspension travel. Shorten it incrementally until you have it just right, and then fit the proper 10 or 11-speed SRAM joiner to finish.