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Old 14th November 2015, 15:41   #1
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Default Motorcycle suspension theory and setup guide

Note to Mods: searched but could not find a thread specific to motorcycle suspension. Please merge into suitable thread if there is one.

Hi ,

Calling suspension gurus to chime in, clarify and correct this post.

This is a thread dedicated to explaining motorcycle suspension terminology and as a quick guide to setting up suspension in motorcycles, that have adjustable suspensions.

Pre-load :
Pre-load refers to the amount of force acting on the spring as static load. Suspension components are installed such that even at full extension, with the vehicle off the ground, the spring is still slightly compressed. That is to say, a spring at max extension of suspension, is still shorter than a spring of the same strength (force vs deflection, measured in kg/mm , such as applying 12kg force will compress the spring by 1 mm)same coil length and under no load.

Nowadays, even entry level motorcycles offer pre-load adjust on rear suspensions, but rarely anything more.

Changing the preload, does have some effect on right height which reduces when preload is increased. Changing the preload also affects effective spring length. While it doesn't change the spring rate ( the force required to compress the spring ), it does increase the threshold until the spring compresses. For example, a spring with rate of 5kg/mm and 100mm range, when set to 10mm preload , will not show any compression until the compression force exceeds 50kg ( 5kg per mm ). If the rider's weight is 50kg, there will be no further sag/compression when the rider sits until the rider's weight increases past 50kg.

The purpose of preload is to set the suspension to stay in the active/effective part of its travel , that is, if the spring travel is 100mm , then the suspension should be set to be withing 20mm to 80mm. The motorcycle should be at 20mm into the compression travel - this is called sag. Static sag is when the suspension compresses under the bike's own weight, and normal sag is with rider on board. Nominally, this should be ~25mm , though dependent on the travel range and setting normal sag to about 25% into the travel range is a good starting point.

Compression damping :
This determines the opposing force to compression stroke. Too little compression damping will have the suspension compress rapidly and fail to damp subsequent bumps (prominently so if the rebound damping is also too high). Too much compression damping will result in choppy ride, witnessed as inability to adjust to protrusions/bumps and affect road holding.

Some suspension systems also allow you to adjust high-speed and low-speed damping. This is not related to vehicle speed at all, it's the rate of stroke movement (on compression or rebound). Suspension movement on gentle undulating roads is where low speed damping comes into play. High speed damping refers to sudden, sharp suspension strokes, such as hitting a pothole or speedbump.

Setting or determining what is the right compression damping, is a bit tricky since it is rider dependent, bike type dependent and road surface dependent. What works great on a smooth track may be awful on a road that is free of potholes but otherwise uneven.

Rebound damping :
This determines the opposing force to rebound or extension stroke. Too little rebound damping will have the suspension bounce back like a pogo stick. Too much rebound damping will also result in choppy ride, with the wheel unable to track through depressions in road surface, and not extend back quickly enough.

Generally speaking, both compression and rebound should be within a certain range of each other , too much variation will affect handling and ride. Since there are few situations where the wheel needs to rebound slowly, rebound damping can be pretty much considered to be high-speed only.

The common way to determine if your bike has proper rebound, it to press down on the fork or rear , and release it. The suspension should come back to original position from compressed, in approximately 1 second with no oscillation or overshoot.

Ride height :
Changing ride height is achieved by adding spacers/shims to the rear shock, this raises the rear ride height, and front:rear balance and the steering geometry. Rear ride height changes may require changing the length of the rear shock , and with a linkage involved, not a straightforward or easily reversible mod, so best to leave it to professional suspension tuners.

The front ride height is usually changed by either lowering the forks through the triple clamp to increase front ride height, or raising the forks to reduce the front ride height. This also changes the front:rear balance and fork geometry.

Why would you do it ?
1. To correct handling problems. Riders looking for quicker steering may want to drop the forks a little to get sharper rake without changing seat height much. Taller riders may opt to raise the rear ride height instead to get the same sharper turn in.

2. Drag racers want to lower the centre of gravity as low as possible, so many drag bikes are lowered via both forks being raised and either using a different shock or linkage for the rear.

Coming to my Triumph Daytona 675.
I find the steering to be unsteady if braking and turning at the same time, as well as when there are bumps/depressions while leaning. I have got the steering head checked, and the workshop says steering head/bearings are ok. My only data point is based on front end feeling unstable under braking and turning, and the only other hint I have got is that the rear rebound damping is too stiff, letting the rear wheel slip/slide when the front is loaded , which is felt as the steering feeling unsteady. Current setup is as factory default.

In a straight line, there's no problem. It tracks straight under braking, no pulling left or right, no wobble or shake. So prima facie, the steering headstock bearings look ok, fork and wheel alignment also look ok. No visible bends or damage to rims or tyres. Whatever problem does manifest, does under specific conditions during turns.

I don't think it has anything to do with my technique, though most bikes I have ridden don't take kindly to braking and cornering, long ago I found the R-15 v1 would remain quite stable under braking and cornering at the same time, and I expect at least as good manners of an import supersport, plus there are others (owning standard 675) who also feel the front end instability.

Factory suspension setup for reference :
Front :
preload : 7.5 turns, anti-clockwise
low speed compression :
clicks, anti-clockwise [ 6: track, 12: standard/sport , 14: comfort]
high speed compression :
clicks, anti-clockwise [ 3: track , 3:standard/sport , 3.5: comfort ]
rebound : clicks, anti-clockwise [ 8: track, 12: standard/sport, 14: comfort]

Rear :
preload : not given
low speed compression :
clicks, anti-clockwise [ 7: track, 12: standard/sport , 14: comfort]
high speed compression :
clicks, anti-clockwise [ 3: track , 3:standard/sport , 3.5: comfort ]
rebound : clicks, anti-clockwise [ 7: track, 12: standard/sport, 14: comfort]

I'm about 85kg, with gear you can estimate 95-100kg, which is a 5-10kg more than what the typical European rider this bike would be setup as default. The stock/current setup is as per factory setting for standard/sport as given above. As it is, the ride is stiff, so using the track settings is bound to result in an even stiffer ride. Maybe tuning the damping down (softer) is the way to go - seeking learned opinion from other seasoned riders, especially those with track experience.

I don't have the spring rates handy - and swapping springs if that's what is needed, is not going to be a quick fix since getting specific springs could be a challenge in itself.
Rear preload adjustment is not shared since rising rate linkage used with rear suspensions, make the travel vs impact/force very far from linear. Whatever I have to change, has to be within rear compression/rebound and front preload/compression/rebound.

Any pointers as to what I should set baseline settings to try?

Last edited by Ricci : 18th November 2015 at 21:46.
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Old 19th November 2015, 11:56   #2
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Default Re: Motorcycle suspension theory and setup guide

Thread moved from the Assembly Line (The "Assembly Line" Forum section) to the Motorcycle Section. Thanks for sharing!
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Old 19th November 2015, 14:06   #3
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Default Re: Motorcycle suspension theory and setup guide

I hope the following video from Keith Code helps
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Old 19th November 2015, 15:16   #4
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Default Re: Motorcycle suspension theory and setup guide

Originally Posted by Ricci View Post
Any pointers as to what I should set baseline settings to try?
Why not get back to basics (most of your technical post was a bouncer over my head honestly) and just make the front forks and rear shocks harder or softer in increments and see where you have the max confidence attacking corners fast?

Addendum: I missed the symptom part of your post.

In the absence of a fall or hit and a fork or chassis down tube bend

In the absence of your triple crown nut being over tight

In the absence of your steering races or bearing being damaged

In the absence of low air pressure in your front tyre

I would say the "funny" feeling on turn in is attributable to one fork leg top bolt not being threaded in to the same level as the other.

I had the same issue on my 200 once. A half or 3/4 turn out by the mechanic who immediately diagnosed the problem, and I was a happy camper once again.

Last edited by ebonho : 19th November 2015 at 15:24.
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