What types of shock ab­sorbers do bikes use?

Motorcycle News (UK) - - Garage -

An­swered by Gareth Evans, Re­ac­tive Sus­pen­sion There are three main types. The sim­plest is an emul­sion shock which has a pis­ton and shims to gen­er­ate damp­ing on the end of the shaft and a vol­ume of pres­surised ni­tro­gen and damp­ing oil in­side the shock body. Rid­ing the bike moves the shock up and down, mix­ing the oil and ni­tro­gen to form an emul­sion. This emul­sion has less vis­cos­ity than oil alone, so there can be an in­con­sis­tent damp­ing ac­tion.

If the shock has an ad­juster it will gen­er­ally be a re­bound ad­juster, how­ever with the ad­juster nearly closed it will act as a com­bined com­pres­sion and re­bound ad­juster.

A more com­plex ap­proach is to keep the ni­tro­gen and oil sep­a­rate us­ing a ‘float­ing pis­ton’. Keep­ing the oil and gas sep­a­rate of­fers more consistent per­for­mance. A ba­sic float­ing pis­ton shock will still only have sin­gle damp­ing ad­just­ment.

Next up is a float­ing pis­ton shock with a sep­a­rate reser­voir for the ni­tro­gen, ei­ther piggy-back­ing on the shock body or re­motely at the end of a hose. The ex­tra vol­ume of the shock helps to keep the shock cooler.

Hav­ing a sep­a­rate reser­voir al­lows the oil flow­ing to it to be me­tered, so you can have sep­a­rate re­bound and com­pres­sion ad­just­ment. This can also sep­a­rate that com­pres­sion ad­just­ment fur­ther into just lowspeed, or low and high-speed.

Fi­nally, there’s the twin-tube shock de­sign. In­stead of a pis­ton run­ning up and down in­side the shock body with a shim stack to con­trol the damp­ing, there is a solid plunger. The shock body has two tubes, one in­side the other, with by­pass holes at each end. On com­pres­sion the plunger pushes oil out of the in­ner tube into a valve hous­ing on the reser­voir. The valve hous­ing has fixed com­pres­sion and re­bound pis­tons in­side it. The oil flows through the com­pres­sion pis­ton, by­passes the re­bound pis­ton and flows around the out­side of the in­ner tube to the other side of the plunger. On the re­bound stroke as the shaft is pulled out of the body, the plunger pulls the oil out of the cen­tre tube, into the valve hous­ing, and through the re­bound pis­ton. The oil by­passes the com­pres­sion pis­ton and flows to the other side.

On a sin­gle-tube shock a pis­ton mov­ing through oil gen­er­ates damp­ing. On the twin-tube oil is moved through the pis­ton to gen­er­ate damp­ing.

Not all bike shocks are cre­ated equal

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