Author Topic: How suspension works and how to tune a bike to handle well  (Read 500 times)

woodsracer

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How suspension works and how to tune a bike to handle well
« on: November 16, 2018, 09:46:43 PM »
I was asked to give some insight on suspension tuning, so here is an attempt to explain the basics of suspension function and how to get a track bike set up. This should help you guys understand what the bike is doing so you can make logical changes to deal with issues you are having. It's one of my main interests in playing GP Bikes, that once I get consistent enough at the controls I might be able to use it to solidify tuning techniques so I am more prepared and efficient at the track in real life! I figure there are more of you out there who feel the same.

Disclaimer:

I am not an expert, merely an amateur racer who builds his own bikes. I am an engineer (motorcycle unrelated) and am always eager to test things to see their effects, this is how I learn. Undoubtedly, the best match at the track is an experienced suspension tuner teamed up with a very consistent, intuitive, and aware rider who can articulate subtle things that the bike is doing. You should know before reading further that unfortunately, in the past I have made suspension/geometry changes in the pit, taken it back on the track, and felt little if any difference. That being said, I have also cured major problems and dropped many seconds off my laptimes by making only small adjustments in the pits. So at best I guess I am suspension-competent. This is just one man's experience explained from one man's perspective.

Damping:

Springs are there to support the weight of the rider and bike, fight brake dive in the front, and fight rear compression under heavy throttle application. So springs are necessary. But to make the bike behave well, and move precisely the way you want it to to make the bike turn well, brake well, and accelerate well, you need to control each spring's motion. The purpose of damping is to control your springs. Hold a spring down onto a table from the bottom, compress it from the top and let go. It will bounce around like crazy for a long time before eventually settling. This is because it is undamped, and this is why you need to dampen both its compression strokes and rebound strokes. There is high speed damping and low speed damping. On a typical bike you can change low speed compression and rebound on the forks and shock, and high speed compression on the shock. "Speed" refers to fork/shock shaft speed, speed of motorcycle is irrelevant. Big hits translate to high shaft speeds (hitting pot hole or object for example), and most of the rest you experience is at low speed.

Damping is achieved by requiring that oil must pass through some small passages any time the spring moves. How to do this is very involved, but basically you flow oil through pistons with (usually 4) holes in them, and use a stack of very thin steel shims that must bend to let oil by as a means of controlling flow rate. These shims are stacked in a very particular way using different shim thicknesses and diameters in different positions, which can change the way oil flows at low force/speed and at high force/speed differently. Getting these stacks right consistently would land you on a top race team; it is difficult but when it’s right it is pure magic.

Low levels of compression damping lets the spring compress quickly as if it's by itself – this is because the oil is allowed to flow very freely. Lots of compression damping restricts flow much more and thereby slows the compression of the spring, so you get the controlled feel you’re looking for and you don't lose travel too fast. On heavy braking you don't want the front to dive immediately, this would create a very unstable chassis. You want it to gradually compress. Likewise, rebound damping keeps the spring from shooting back to its original position too fast after being compressed. Without damping, the spring would rebound very quickly, overshoot its original length, and "pogo" up and down until it was finally still again. I'm sure you see that would be extremely dangerous on a motorcycle. Imagine if you let the brake out quickly on corner entry and the forks popped back and started bouncing around. Not good.

You tune damping so that it controls the spring the way you want it to compress and rebound. This is a feel thing but there will always be some setting that will work for your style of riding on your particular bike at your particular track. Generally, rebound should be set so that the fork or shock returns as quickly as possible without overshooting its original length and cycling many more times, it should come right back in a controlled way and come to rest so that the chassis stays composed. Back to the spring on the table example, with damping instead it would shoot back when you let it go, but slightly slower, so that when it reached its original position it would stop. Of course, rebound affects handing drastically and you may end up at a damping setting that is above or below this theoretical ideal, but generally getting the spring to settle back quickly without overshooting will get you close to what you want.

Geometry:

Geometry is mainly determined by frame size/shape, swingarm length and angle, rear axle position, fork height, fork offset, and rear ride height. Frame design defines your rake angle and the positioning of everything to start with, and is basically unchangeable (some bikes like Ducatis come factory with a rake adjuster and there are also aftermarket eccentrics you can use, but I don’t see those much). The rest is a tuner's bread and butter.

The overall geometry is the stance the bike is in with the rider on it. It affects the way the bike behaves because although when you’re on the throttle the rear is low and front is high, and when you brake you are in the opposite position, the general geometry gives you your standard position from which everything else is based. With a low front/high rear stance, the bike tips into turns quickly with little effort and has the feel of a sharp knife. This setup is great for a track with lots of technical turns at lower speed but can be unstable under hard braking, high speed turns, fast straights, and under hard throttle. It can feel twitchy and make you feel very uneasy at high speeds. A high front/low rear setup is very stable overall but takes a lot more effort to turn. It lacks the agility and precision of the former setup but excels at high speeds, heavy braking at the end of a straight, and stability once you are at full lean. You can set geometry so that the bike helps you turn (the former setup) or it doesn’t do anything unless you explicitly tell it to (the latter).

Front ride height is changed by raising or lowering forks up or down in the triple clamps (or "yokes" in other parts of the world). Rear ride height is adjusted (generally) by shock length or adjustable linkage. Some shocks are adjustable (most aftermarket Ohlins and Penskes) some are not but you can modify the shock internals to change length – not a pit modification for most of us. Swingarm length changes wheelbase and rear ride height because as axle goes back it also goes down which pushes the rear up. Fork offset is the distance between steering head pivot (determined by frame), and the horizontal line directly between the centerline of each fork. This changes mainly the trail, and unless you have adjustable clamps like the aftermarket ones (very expensive hardware), they are set and can't be changed. You can also get stock clamps modified with offset eccentrics but it’s still in the $600 range for a machine shop. Most are set somewhere in the 26-38mm range which gives you somewhere around 85-105mm of trail (ideal is around 92 to 100mm).

Geometry is determined by all the above mentioned things, and also (equally as important) spring rates and spring preload. Spring rate is how fast the spring increases in stiffness as you compress it, measured in force per length of compression (as in N/mm or lbs/in). More force on spring, more length it compresses, but also it gets stiffer to resist additional compression. This makes perfect sense to me, but unfortunately a large part of the suspension community goes instead by kg/mm, which is mass per compression length. This conceptually makes less sense, but we are in luck because the acceleration of gravity is 9.8 m/s^2 and is close enough to 10 that a 1.0 kg spring is close to a 10 N spring (number almost stays the same just changes the order of magnitude), but I digress!

Preload is a measure of how much the spring is compressed from its natural length when your fork or shock is resting with no weight on it, at max length. It is NOT related to spring rate. It just makes the force to initially begin to compress the fork or shock higher or lower and is used to change geometry – it changes stance but also changes how the forks work on the brakes and how the shock responds to throttle application (as does spring rate and other things).

My tuning methods:

Start with talking to some suspension guys who have tuned that particular bike at the track in the past. Talk to several of them and see what’s typically done to them. A Daytona 675 for example needs trail, therefore the front is usually set high with the fork caps flush with the top clamp, swingarm pivot is moved up in frame a few mm, adjustable triple clamps are installed to decrease offset (from 38mm down to 30 or even 28mm), some people raise rear and some lower it. A Ducati 996 needs the rear raised significantly as the stock swingarm angle is way off. These things are great to know before you put any work in. See what spring rates people are running in your weight range, and see what sag numbers they’ve been using, and pick something in that range. This gives you some place to start (and ONLY a place to start). Take it out for some laps, get used to the feel, and then determine what it’s doing and what it’s not doing for you. If it’s doing one thing poorly, assess your options and choose your intended solution based on the effectiveness that may have in curing your problem and what side-effects you might get. Choose one that would likely prevent a significant detrimental effect on something the bike is doing well. The only way to know for sure is to test.

Try to use all available travel. If you are bottoming forks while braking hard you can increase spring rate, preload, or fork oil height. The oil height inversely affects the air volume at the top of your forks, which acts as a highly progressive spring to resist compression at the bottom 1/3 of the travel (actually even small bumps are affected too but the effect is small in comparison to the effect on bottoming resistance). Think about air gap not oil height, increasing oil height decreases the size of air "spring" so it reacts faster and has more total effect. Too little air gap could keep you from bottoming but may also make the entire rest of the stroke stiff and this can beat you up over time and be uncomfortable. The frustrating part is that the problem may actually be too much front end weight bias, so if you dropped the rear a bit or increased front height you could solve it that way. You need to try different things to see what feels best and is getting the job done. With some experience you’ll usually know the best options you have, but this is difficult. Why?

BECAUSE EVERYTHING AFFECTS EVERYTHING ELSE.

I kind of choose the solution to a problem based on the effectiveness it will probably have and also the impact it would have on other things (you could have geometry and handling perfect so I would never consider changing this to keep forks from bottoming, would only consider springs or air gap at that point). In the opposite case that you have an extra bit of travel not being used (you know this by putting a zip tie on the lower fork tube, setting it high, and then checking its position after a session to see how much travel you used), you can reduce the spring rate, preload, or oil height. This will give the chassis more travel to work with and soften things a little, however this may not give you the feel you want, so maybe you decide to change it back and deal with less travel, not as important as how it feels.

It's really hard to do all this and is largely feel-based. Generally, if you have the right spring rate, the preload will set your stance and oil height will be tuned for travel use. Compression damping would then more or less give you the feel you want, this is basically what I try to do.

Note that the ONLY way you know if you have your settings close to ideal is if you have tested different settings. For example, for preload you need to change the setting and try with different rates while keeping oil height constant. OR try different oil heights with different preloads with same rate. See how time consuming this is? Is there a “right” setting? Maybe there is an ideal for YOU but will you ever find it? Only if you test every combination. The bright side is that with a reasonable amount of work you can get a “pretty good” setting that is probably not far off from ideal for you without investing a lot in parts to test and a lot of your time. That’s the balance.

Speaking of balance, you want the front and rear to be pretty balanced together, ie rates and damping should have the front and rear of the bike responding evenly if you just stand the bike up and compress front and rear at the same time. Both ends should compress about the same and come back up about the same, this is a good way to test to see if you have something very wrong and is a good thing to do once you get one end dialed so you can try to match that with the other end as a starting point. In practice this can be pretty difficult, I start with the fork rebound: hold brake and compress at the bars as hard as you can, then let it rebound by itself with only very light hands on the bars. This should be done after your first session so the oil is warmed up. Cold oil has a higher viscosity. If it overshoots when it rebounds then increase rebound damping until it comes up to its relaxed position as fast as possible without overshooting. This will get you in the ballpark but you need to tune it further by listening to what your bike is telling you on corner entry and exit when you are out there.

So what works for YOU on YOUR bike at YOUR local track? I have no idea!! You need to test and figure it out for yourself, my friend. It is complicated because everything affects everything else, so there are many directions you could go to solve a particular problem. There may be one right way or there may be several that cure a problem. Each option will affect different things so you need to choose wisely or test a lot of settings.

Symptoms, possible cures:

-Bike is stable but going wide on corner exit: This means geometry is front high/rear low on corner exit. Change geometry to counteract by raising rear/lowering front, or increase rear compression damping (or spring rate or preload). I suggest looking at compression damping first. If you are losing grip after increasing compression then reduce back down and try increasing spring rate. Fork rebound may also be too fast pushing the front end up. This could also be too much shock rebound, keeping the rear from being able to recover from being compressed, so rear stays low too long and front is therefore high. Sometimes the spring rate on shock is too light, causing too much compression on corner exit, but it might feel ok elsewhere.

-Bike is unstable, tank slapping or wobbling on throttle application, diving into turn/unstable braking or at high speed: Geometry is too low in front/high in rear. Could be due to purely geometry settings in which case you should lower rear or raise front, or both. If not then your fork rebound damping is too high and front is staying low for longer than it should which can cause the front end to lift during hard throttle. Could have to do with rear rebound damping being too fast as well, coming up and pushing front end down, think about it and figure out what makes the most sense given everything else the bike is doing, think about what it’s doing well and try to keep any change you make from interfering with that. Hope you can see the connections between all these things.

-Everything is good but rear end is skipping while braking hard: Shock rebound could be too slow so under hard braking rear tire loses contact with pavement and skips as you modulate front brake. If you bounce the bike on flat ground in the pits and it comes up visually slow in rear, try decreasing damping. More than likely, if rebound is quick in the pits, chances are the skipping is more side to side on track and the rear is popping up quickly, coming back down and gripping for a half a second, popping back up quickly, over and over. In that case it needs to be slowed down so increase rebound damping.

Other things to note:

-Swingarm angle affects anti-squat, the mechanical force giving tension to the top of the chain under acceleration which pulls swingarm down. It fights the rearward weight transfer or squat when you get on the throttle, use more angle to counteract running wide. This can be done by lengthening shock/adjuster but that also raises the rear and has other handling effects. The way to do this without also raising rear is to move the swingarm pivot point up (the axis it rotates around through frame). This lowers rear of frame and gives you more trail too. Perfect mod on a Daytona 675 because you need both, but there are some other bikes you would not want to do this to.

-Shock linkage: most stock sport bikes are designed so that if you take a passenger you won't bottom out given the extra weight. For racing this is irrelevant and the linkage ramps up mechanical advantage on the shock way too much so you either can't use the full travel of the shock or worse, you can't use all the travel and also you hit a wall with compression. Meaning it feels like you’re bottoming but you’re not, this can reduce grip a lot.

-Gearing effects: for your gearing choice for a particular track, the chain length may change to the point you need to go up or down a few links. This changes wheelbase and rear ride height, but mostly the former. Sometimes an almost equivalent gearing (like 15/45 and 16/48) are swapped so that rear axle position is changed but gearing stays the same.

-Reading tires: the wear pattern on a tire can tell you a lot about the suspension performance. This takes a lot of experience, of which I do not have a great deal, but you look at leading/trailing tread wear (or wear indicators on slicks) to determine which side is worn more or if there is a raised edge, this tells you if the rebound is working well or not. If it’s way off it can destroy a set of slicks in a few laps. Someone can clarify but I believe if the front edge (looking at top of front tire) is worn more than the back edge, this means your rebound is too fast. Not sure how consistent this is. And I feel like it should be the opposite edge on the rear tire that would indicate this, as the tires get stressed in opposite directions - front is used for braking and rear for accelerating. Can anyone talk about this? Anyway, you basically want to see a nice ripple pattern on your rear tire, like the pattern waves make on a beach over time. There are also geometry tears that indicate too much weight bias on front. There’s a lot to know here.

Anyway, hope all this makes sense. I would be happy to answer any questions.

matty0l215

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Re: How suspension works and how to tune a bike to handle well
« Reply #1 on: November 16, 2018, 09:59:15 PM »
Blimey! :o

That's going to take some reading. Great Work Woodsracer :D



@Mods- Can this be stickied please ;)

Hawk

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Re: How suspension works and how to tune a bike to handle well
« Reply #2 on: November 16, 2018, 10:02:40 PM »
NICE! Well done woodsracer.... Nice work mate! ;) 8)

Gibbon

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Re: How suspension works and how to tune a bike to handle well
« Reply #3 on: November 16, 2018, 10:17:46 PM »
Fantastic. Thank you.

woodsracer

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Re: How suspension works and how to tune a bike to handle well
« Reply #4 on: January 11, 2019, 01:16:27 AM »
Are there any bike setup questions you guys have that you've always wondered about or never understood we could try to figure out?

I think we should start a discussion, this could help quite a few enthusiasts and could be a good time.

Myst1cPrun3

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Re: How suspension works and how to tune a bike to handle well
« Reply #5 on: January 24, 2019, 10:32:56 PM »
My head just exploded looking at that  :o its too late at night to try and read that my brain will end up like them old TVs that lose signal. thats a weekend job, but thanks for doing this as GPB seems to have some unique little aspects that are interesting to compare to IRL setups. My dad gave me an old Superbike Comprehensive Circuit guide which was co-developed by the ACU and features a load of these little tips and stuff, but I've never really delved deep into it, as its 17 pages long in total, and I don't have the time to read it properly ATM.
« Last Edit: January 24, 2019, 10:41:35 PM by Myst1cPrun3 »
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