Ask The Geek


Sport Rider - - Front Page - BY AN­DREW TREVITT

Re: The Per­fect Setup

In your sus­pen­sion ar­ti­cle “The Per­fect Setup” (Dec./jan. ’16), you said that front sag “de­pends on what’s in­side your forks as far as springs and top- out springs,” and that we shouldn’t worry about it “un­less a rep­utable sus­pen­sion per­son gave you a num­ber to aim for.” Can you elab­o­rate on that? How am I sup­posed to set preload if it’s not to get a cer­tain amount of sag? Why does this ap­ply to the front sus­pen­sion but not the rear? And how do the top- out springs af­fect sag?

Jesse Daniel Palo Alto, CA

Top- out springs work against the main spring at the very top of the travel, and their orig­i­nal in­tent was to pro­tect the fork or shock from top­ping out metal to metal with a hard stop. But over time these short, stiff springs have evolved into longer, softer springs, as they can change how the sus­pen­sion works near full ex­ten­sion. For ex­am­ple, if your bike has no top- out springs in the fork, it can take 10 or 20 pounds of force (or more) to over­come the preload just to move the front sus­pen­sion from fully ex­tended. As you can imag­ine, small bumps do not work the fork at all when this is the case, and the tire will skip over these bumps un­der ac­cel­er­a­tion, caus­ing a loss of com­pli­ance and trac­tion. But with top- out springs, the fork ac­tion can be changed so that light loads and those small bumps do move the fork, mak­ing the sus­pen­sion much more com­pli­ant dur­ing ac­cel­er­a­tion.

Be­cause the top- out springs act to com­press the sus­pen­sion, they need to be taken into ac­count when it comes to sag. For ex­am­ple, a fork with no top- out springs may need 30mm of sag to work prop­erly; if we were to add a set of top- out springs with a length suf­fi­cient enough that they are partly com­pressed at the sag mea­sure­ment, sag may need to be, say, 40mm if the fork is to work the same fur­ther down in the travel. The sag mea­sure­ment is still very im­por­tant and goes a long way to de­ter­min­ing how the sus­pen­sion works, as it in part de­fines the work­ing range of the fork. But be­cause there is such a va­ri­ety of top- out springs be­ing used, it’s dif­fi­cult to give a gen­eral num­ber for sag that will ap­ply to the ma­jor­ity of bikes. This is how a sus­pen­sion ex­pert, who knows what’s in­side your fork, can de­ter­mine a sag num­ber that will work with your setup.

Few stock shocks have top- out springs, but most af­ter­mar­ket shocks do; just as with the fork, top- out springs in the shock al­low the sus­pen­sion to track over small bumps at the top of the travel, such as un­der heavy brak­ing. Typ­i­cally, shock top- out springs are short enough that they are at their full length and out of the equa­tion at the sag mea­sure­ment. As a re­sult, we can still use 25 to 30mm sag as a good start­ing point for most bikes.

With­out a sag num­ber to use as a base­line for the front sus­pen­sion, you can ad­just preload based on how it af­fects front ride height and ge­om­e­try be­cause that is mostly what you are chang­ing when you ad­just preload. The main con­cern with ven­tur­ing too far in one di­rec­tion or the other is that the sus­pen­sion will top out or bot­tom out in nor­mal use. When that oc­curs, you can look into rais­ing or low­er­ing the fork tubes in the triple clamps to ad­just ride height di­rectly rather than us­ing preload, or a change in spring rate may be in order. SR

The top- out spring in a fork or shock works against the main spring at the top of the travel and will be com­pressed against a stop when the sus­pen­sion is com­pletely ex­tended.

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