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Suspension: Setup and Technical Talk

Oldbear

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Whether it's Kayaba, Showa, WP, Sachs, Öhlins or something else, the basic principles – and many of the technical bits – are the same. So in order not to bury any useful info in the bike-specific threads, let's discuss dirtbike suspension setup and technical issues here.
 

Oldbear

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So, the basics. This is from an old KTM/WP shock manual:

Springs and damping

To meet the high demands put on our suspensions, our suspension systems have a spring and a damping system. The spring absorbs the wheel movement, the damping system controls the spring movement.

The spring and the damping work together to fulfil all of the different suspension tasks: Comfort, Vehicle Balance and Safety.

When looking at the total stroke of the suspension, the suspension tasks mentioned above can be linked to different parts of the stroke. If the stroke is divided as following:
  • The first 25 % of the stroke mainly takes care of comfort.
  • The middle 50 % of the stroke takes care of balance.
  • The last 25 % of the stroke ensures safety.
These three factors always work together, because they influence each other.

If all of the suspension components work together properly, the result is a comfortable, balanced and safe motorcycle.

Comfort​

Comfort can best be seen as the amount of vehicle movement that is transmitted to the rider. Maximum comfort would mean that no movements are transmitted to the rider.

Balance​

Vehicle balance is determined by the weight distribution between the front and rear wheel. The motorcycle is primarily balanced by its springs. Therefore it is very important to make sure, the springs are suited to the rider's weight and/or use of the bike.

Safety​

In order to maintain a level of safety, the tire must be in contact with the ground at all times.
This means that at least a small amount of suspension travel should always be left.
If there is no suspension travel left (when the suspension 'bottoms-out'), grip will be severely compromised.

Springs​

The tasks of the spring are:
  • Supporting the bike's weight (at standstill and during driving).
  • Absorbing the wheel movements.
  • Preventing the fork / shock from being fully compressed.
  • Returning the motorcycle to its 'neutral' position after being compressed.

Spring rate and preload​

The springs mounted on the bike have a rate which suits the weight and area of use of the bike.
The first step in maintaining the bike's handling characteristics is making sure the springs are preloaded correctly and have the correct spring rate.

Spring rate​

The spring rate is the force (N) needed to compress the spring a specific amount (mm.). The spring rate is measured in N/mm.

Spring preload​

The spring preload is the amount (mm.) the spring is compressed by the preload adjuster/preload bushes.

NOTE:
The spring 'rate' and the spring 'preload' are two very different things! The spring rate is a fixed value which stays the same. The spring preload can be altered in order to set the correct ride height. Changing the spring preload does not alter the spring rate!

Suspension sag​

Each bike needs a predetermined amount of 'sag'.
A certain amount of sag is needed to make sure the wheel is able to move down when it passes over a hole in the road.
If the bike wouldn't have any sag at all, the wheel would only be able to move up, which would compromise safety and comfort.

Sag is adjusted by altering the spring preload.

Correct sag settings are needed to ensure the bike has the desired ride height and steering geometry. The sag can be seen as the 'fine-tuning' of the spring preload.

The sag can be separated in:

Static sag​

Difference between when the vehicle is entirely unloaded (jacked up) and when it is resting on the ground with only the weight of the vehicle (stationary on ground without rider)
If this value is too low, it will have a negative impact on vehicle response. It should be approx. 10% of the total suspension travel.

Riding sag​

The difference between when the vehicle is entirely unloaded and when it is loaded with the rider in full gear.
Dynamic sag has the greatest impact on the adjustment of the spring preload. It should be approx. 1/3 of the total suspension travel.

NOTE: When measuring the sag, the measurement should always be taken between a fixed point on the frame and the wheel-axle. If the fork is equipped with a preload adjuster, the preload can easily be adjusted at any time. Without a preload adjuster, the fork must be opened and the preload bush must be changed.

Damping​

The tasks of the damping are:
  • Control of wheel movement.
  • Control of spring movement.

Compression and rebound damping​

The wheel can move in two directions (up and down), both directions require a different damping rate.

The damping can be separated into two different kinds of damping: Compression and rebound

Compression damping​

When the wheel moves up, the suspension component is compressed (against the spring force). The damping that controls this movement is the compression damping.

The compression damping can be separated into two different kinds of damping: LOW-SPEED and HIGH-SPEED.

a. Low-speed damping
If the suspension is compressed slowly (low piston velocity), the movement will be damped by the Low-speed damping.​
Low-speed damping regulates the damping up to a piston velocity of 0,5 m/s (shock absorber) / 1 m/s (Front Fork). Above this speed, the high-speed damping will take over, the influence of the low-speed damping will then be negligible.​
The "low-speed" setting has the greatest impact on the change of the total compression damping - comfort or sport setting. When adjusting the low-speed damping, small changes in "comfort" can be felt as well.​
b. High-speed damping
If the suspension is compressed rapidly (high piston velocity), the movement will be damped by the High-speed damping.​
High-speed damping regulates the damping when piston velocity is above 0,5 m/s - 1,0 m/s. Maximum piston velocity lies around 3,5 m/s (shock absorber) / 7,0 m/s (Front Fork)​
High-speed damping has the biggest effect on the "balance" of the motorcycle.​

Rebound damping​

When the wheel moves down again the suspension component extends (extension is forced by the spring). The damping that controls this movement is the rebound damping.
 

Oldbear

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And here is a useful troubleshooting guide, from WP/KTM:

OLCl6sk.jpg
 

Yinzer Moto

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Great write up. It should also be said, rider sag should be the first thing that is set, before clicking damping. Front sag is very difficult/impossible to determine and it Is usually best to go off of a chart to select a spring.

Measuring sag with all the riding gear on is the only way to do it. Add luggage if that is how the bike will be ridden most of the time.
 
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woods

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I've got Cogent in my DR650. Kind of wish I went one click softer on their recommendations. For jumpy stuff its fine, but the light stuff like frost heaves on the road there wasn't much difference over stock.
 

SnakeOiler

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Ya, But it’s really light and super cheap to make. The lightness is for the rider. The cheapness is so KTM can make max profits on the 10k plus dirt bikes. Then with all the extra profits they can buy up all the euro boutique brands and kill them. :photog

Who's next in line for ktm?
 

SnakeOiler

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Let's talk about shock spring rating.

I find myself board line between spring rate. Today I swapped out the stock 42 spring with the next rate down of 39. I needed to turn the preload 4mm greater than the manual called for. Before with the stock spring I needed to go around the same in the opposite direction, 4mm less.

Which is better? Or should I load the bike up, lol?
 

Oldbear

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Which one feels better? Sometimes a "wrong" spring can be the right one if it feels better.

What kind of race sag numbers are you getting with those preload adjustments?
 

SnakeOiler

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Which one feels better? Sometimes a "wrong" spring can be the right one if it feels better.

What kind of race sag numbers are you getting with those preload adjustments?

That's a good question. Well I'm surprised to find the static to be more than the 1.4" the book called for, it's about 1.7". As for feel, I haven't rode the lighter spring (3.9) yet. The 42 wasn't really that bad at higher speed. But in the slow single track it definitely suffered. And to be honest I swapped just to see if it really was worth it. But I definitely needed softer in the forks, and that helped.
 

SnakeOiler

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Okay I'm putting the stock spring back on the gal. It's too soft and bottoming out too often. I'll play with the preload more on the 42 rate and deal with it.
 

Yinzer Moto

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Okay I'm putting the stock spring back on the gal. It's too soft and bottoming out too often. I'll play with the preload more on the 42 rate and deal with it.

Did the lighter spring get the sag number correct? Bottoming resistance is more about having the compression damping settings correct.
 

SnakeOiler

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Did the lighter spring get the sag number correct? Bottoming resistance is more about having the compression damping settings correct.
I understand that but I was at like 20mm preload to get close to the sag measurements. I feel more comfortable when I was at 5mm preload and light with the sag
 

Yinzer Moto

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I understand that but I was at like 20mm preload to get close to the sag measurements. I feel more comfortable when I was at 5mm preload and light with the sag

I am not a suspension expert. I thought if you can get the sag numbers correct, the preload number does not really matter. The only time the preload number matters is if you are running into coil bind. If the other spring felt better, then that is what you should be running.
 

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