Ever won­dered how sus­pen­sion works? We get the pros from K-Tech to talk us through it and tell you how to get the best from yours.

The mysteries of sus­pen­sion are un­rav­elled right here, right now, with the help of some rather clever peo­ple at K-Tech.

Fast Bikes - - CONTENTS -

It can be very dif­fi­cult to ob­tain that per­fect sus­pen­sion set-up. In rac­ing, with so much ad­justa­bil­ity, it can be a chal­lenge to find the com­pro­mise be­tween how the bike re­acts into a cor­ner, dur­ing a cor­ner, and on the exit – and nearly ev­ery cor­ner on a track would, ideally, need its own set­ting so find­ing the right com­pro­mise for all is never easy. In off-road rac­ing and es­pe­cially mo­tocross, with ever-chang­ing track con­di­tions it’s even trick­ier.

On a stock road mo­tor­cy­cle, built to a spe­cific bud­get yet de­signed for a huge range of rider weights (with the op­tion to carry a pillion and lug­gage) get­ting it spot-on for every­one is, un­der­stand­ably, a near im­pos­si­ble task.

Open range

A bike man­u­fac­turer has a huge win­dow when de­sign­ing a stan­dard road ma­chine – from the phys­i­cally smaller, lighter Asian mar­ket, to the of­ten larger US own­ers. Will the rider be car­ry­ing lug­gage? What kind of road sur­face will they be on? A bike with too wide ad­just­ment could, in the wrong hands, be set up to han­dle dan­ger­ously – give enough to suit a sin­gle Asian rider, and a cou­ple who load their bike up ex­ces­sively in the States could, the­o­ret­i­cally, set the mo­tor­cy­cle to han­dle dan­ger­ously.

Due to the liti­gious times we live in, this could prove costly for man­u­fac­tur­ers; hence an av­er­age is ac­com­mo­dated for. It’s this lack of real ad­justa­bil­ity that con­trib­utes to many rid­ers not be­liev­ing they un­der­stand sus­pen­sion – un­til you can ac­tu­ally feel what’s hap­pen­ing as you tweak it, it will al­ways seem some­thing of a dark art.

Good = well bal­anced

A lot of OE sus­pen­sion is set up on the softer side, to be more com­fort­able. But good sus­pen­sion, es­pe­cially when de­signed around a spe­cific rider, doesn’t have to be harsh. Aprilia’s RSV4, for ex­am­ple, sports an in­cred­i­ble setup from the box that re­mains sporty but pli­ant on the road, and ra­zor sharp enough (for most) on track.

A bike that’s too soft on the front al­lows the forks to dive too far, chang­ing the ge­om­e­try and putting more weight over the front tyre, po­ten­tially over­load­ing it. It’s all to do with how the bike sits on its sus­pen­sion – of­ten, firm­ing a bike up will ac­tu­ally make it more com­fort­able. But it’s not as sim­ple

as putting in a stiffer spring – the damp­ing needs to match it. An af­ter­mar­ket man­u­fac­turer like K-Tech can look at an in­di­vid­ual rider’s cir­cum­stances and rid­ing style to give them an ideal setup and ad­vise them on the cor­rect ad­just­ments. So, let’s dive right in to what does what when it comes to sus­pen­sion…

What does what?

Take a ma­chine like 2016’s Yamaha Tracer 700 – it has no pis­ton in the fork – it’s a ‘vis­cos­ity damp­ing fork’ that is sim­ply an in­ner tube with holes in the bot­tom go­ing through an outer tube with oil in it.

Fit­ting a more ad­vanced car­tridge fork sys­tem doesn’t just in­tro­duce ad­justa­bil­ity, it has bet­ter con­trol of the oil, us­ing a reg­u­la­tor valve that lim­its oil flow dis­placed as the in­ner pis­ton moves. There’s also a shim stack fit­ted to the pis­ton, which is ba­si­cally sev­eral thin steel discs that can de­flect un­der higher pres­sures, re­duc­ing the re­sis­tance of the pis­ton in the oil by open­ing up holes ma­chined into it (this is what al­lows the fork to be more com­pli­ant when hit­ting harsh bumps that cause the fork to move faster) – it’s ba­si­cally the same in a rear shock. Good sus­pen­sion will give more com­fort, feed­back and con­trol.

A mo­tor­cy­cle works best as a bal­ance, so it’s typ­i­cally ideal to im­prove the front and rear of a bike, but peo­ple of­ten tend to change the front first as it’s what they feel most. Sus­pen­sion is sim­ply the con­nec­tion be­tween the tyre and the ve­hi­cle, con­trol­ling sprung mass. The spring, be it in your fork or on your shock, con­trols the mass of the bike. The damp­ing con­trols the speed at which the spring is al­lowed to move. Here’s what you need to know…

PRELOAD: The most mis­un­der­stood part of sus­pen­sion. In­creas­ing preload does not make a spring ‘stiffer’, it sim­ply in­creases the ride height of the bike. How­ever, it does af­fect the break-out force – the point at which it starts to move. If you have a 100lb/ inch spring, and you put one inch of preload on it, to make it move you have to add an­other 100lb.

Once it’s mov­ing it feels the same, as the spring rate is un­changed, but this in­creased break-out force can make a bike feel more harsh as greater loads are trans­ferred to the rider be­fore the spring starts to re­act. Equally, if you add 50lb of lug­gage to a bike with a 100lb spring that is bal­anced to you as a solo rider, an ad­di­tional half an inch of preload will even out the ad­di­tional weight and main­tain the cor­rect static sag. In­creas­ing the ride height can speed up the steer­ing, but in or­der to do this with­out in­creas­ing the preload, many af­ter­mar­ket shocks come with an ad­juster that length­ens the shock with­out com­press­ing the spring.

STATIC SAG: This is the amount the bike drops as the spring takes the weight. It’s the dif­fer­ence in height be­tween the ma­chine sit­ting nat­u­rally, and the bike be­ing lifted to the point where the wheel is only just touch­ing the floor. Sag al­lows the wheel to drop into dips in the road sur­face with­out tak­ing the bike with it – it’s sim­ply slack in the sus­pen­sion. Pic­ture a Dakar racer blast­ing across sand dunes – the bike stays level over dips while the wheels drop into them be­cause of sag.

AIR GAP: Air acts as a spring and is gen­er­ally also part of a fork’s char­ac­ter­is­tic. It’s sim­ply the space be­tween the top of the oil and the fork cap. Air can also be used to re­place a spring al­to­gether, as in K-Tech’s Bul­lit units de­signed for bikes like the Du­cati Scrambler, Har­leys, In­di­ans and more. Like the oleo struts used in tanks and air­craft land­ing gear, these ni­tro­gen-charged shocks give con­stant damp­ing con­trol over a wide range of loads and sur­faces.

PRO­GRES­SIVE SPRING: These are sim­ply springs that get stiffer as they’re com­pressed fur­ther, mak­ing them re­turn more force as the sus­pen­sion in­creases its stroke.

PRO­GRES­SIVE LINK: Also known as ‘ris­ing rate’, the link joins the swingarm to the shock, and causes it to re­quire more force to be moved as the sus­pen­sion is com­pressed. As with a pro­gres­sive spring, the prin­ci­pal is to al­low a softer feel un­der nor­mal con­di­tions, with­out let­ting the sus­pen­sion bot­tom out.

COM­PRES­SION DAMP­ING: Oil in­side the fork or shock re­stricts move­ment to slow the speed at which the spring is com­pressed. Too lit­tle and the spring’s move­ment is un­con­trolled, too much and it’ll cre­ate a harsh feel as the com­po­nent can’t move smoothly.

RE­BOUND DAMP­ING: This con­trols how quickly the sus­pen­sion can un­load. Too lit­tle and the bike will bounce back from a com­pressed state. Too much and the sus­pen­sion can ‘ratchet down’; imag­ine rid­ing over a series of close bumps with too much re­bound damp­ing on the rear shock – the tail of the bike will never get time to re­turn to its nat­u­ral po­si­tion, and get quickly lower un­til you’re kicked up the bum. This could be re­duced by de­creas­ing the amount it moves in the first place with more com­pres­sion damp­ing, but then the ride would be a lot harsher. Sus­pen­sion is all about the bal­ance of all the set­tings, so for the very best ride, the spring has to be

Ian Hutchin­son’s won nu­mer­ous TTs on K-Tech sus­pen­sion.

If you ever feel the urge to strip your sus­pen­sion, hope­fully this pic makes you see sense.

Ask any racer and they’ll tell you setup is every­thing. Once this guy’s mas­tered scis­sors, the pos­si­bil­i­ties are end­less.

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