Suspension

By Ron Milam

 

  This month I would like to start a discussion on suspension theory and setup. This is an area that can make or break the way your bike performs, regardless of its age. If you start adjusting on your new bike you can easily ruin the turning ability if you do not keep a few simple principles in mind. On the other hand, if you get lucky and find some good settings as I recently did, your old bike can suddenly become “new” again.

     I think a little history may be interesting as an introduction. Those of use that started riding many years ago may know some of this already, but the younger guys were not born when single shock suspensions came about. And any of you that think Yamaha invented the single shock concept, think again. When I was a teenager, I had a 1960’s NSU 150 that had a single shock mounted inside its stamped steel frame. There were probably earlier bikes than this with single shocks, but the current type of suspension that we are familiar with grew out of an idea from a Belgian gentleman named Mr. Tilkins. This was during the early 1970’s. Suzuki ruled the motocross world with the “Dream Team” of Joel Robert, Roger DeCoster, and Sylvain Geboers. Joel and Sylvain were number 1 and 2 in the world 250 class. Roger was number 1 in the 500 class, closely followed by the Swede, Ake Johnnson who rode for Maico. I may be misspelling a few names here. I am going off of memory from nearly 30 years ago. Anyway, the entire Suzuki team was Belgian, and they liked to hang around at Mr. Tilkins place. He was a skilled craftsman who loved motorcycles. He had an idea to mount the rear shock horizontally and connect it to the steering head. His theory was (so the story goes as later told by DeCoster) that the bumps absorbed by the shock would be redirected to the steering head and would push the bike forward. He told the riders about it and tried to sell the idea to the Suzuki team. The Suzuki engineers realized that his theory was wrong and the horizontal shock would not make the motorcycle go any faster. So, he approached Yamaha with the idea. Their engineers also were smart enough to know it wouldn’t work. But, they saw another value in the concept and bought the idea. That idea soon stunned the motocross world, when in 1972 I think it was, Yamaha fielded a 250cc class machine utilizing the concept. The Monoshock was born. In the first G.P. of that year, a Swedish rider who I believe was Hakan Anderson upset both the Suzuki and Maico teams aboard the prototype Yamaha to win first place. After a few races it was clear that the concept was far superior to the existing twin shock rear suspensions that boasted travels of about 4 inches. Fork travel at that time was approximately 6 inches. The Maico team was the first to figure out that the advantage was in the longer travel afforded by the monoshock arrangement. For you younger guys, the original monoshock had no linkage. That was not to come for another 7 years. With no time to design a new bike, the immediate problem was solved by a Maico mechanic who simply moved the shock mounts forward on the swingarm and frame and bolted up the existing Koni shocks (the hot setup back then ). The result was about 6 inches of rear travel. The bike was immediately competitive with the Yamaha. Rapid development in shock design began. Standard shocks could not stand the higher temperatures and pressures of the longer travels. Fade and blown seals were common. The next few years saw progress and fads come and go. Air shocks and air forks without any springs were tried with some success. Then combination air/spring designs came and went. Many vintage bikes still have vents in the fork caps from the era when riders wanted only the influence of the spring, without any effect from the air trapped in the fork. The next big improvement came about 1979 when Kawasaki introduced the Uni-Trak. This was the first modern rear suspension to utilize a linkage to vary the mechanical advantage between the shock and swingarm. Like the Yamaha Monoshock, this was a major advance which resulted in success on the race track. Within 2 years, both Honda and Suzuki also had linkage type systems. Of the 4 Japanese designs, the Honda Prolink system was the one that endured, becoming the forerunner of the suspensions used on most off road bikes today. It took until the mid 1980’s for the concept to appear in trials bikes. However, while front and rear travels of most motocrossers have grown to about 13 inches, modern trials bikes have travels of about 6 to 7 inches. I know a lot of this story was not about trials, but both trials bikes and go fast bikes continue to evolve from the same pool of technology. The have disk brakes and watercooling. We get that and return the favor by giving them aluminum frames and hydraulic clutches. Their past is our past. Their future is also our future. Next month, I will get into some suspension theory.

 

 

   Last month we started a discussion on suspension systems in which you were either dazzled by my amazing recollection of ancient history or bored silly. This month, we will get down to the nitty gritty. Some of you will think this part is boring too. You would rather me just tell you which knob to turn to make hopping easier. Others will understand that if you understand the principles and hardware, your eventual adjustments will be much better. If you are in the first category skip to the last sentence. For the rest of you, this is my opinion of suspension principles.

    So how does it work? Well Grasshopper, an old and well known system is utilized to turn the energy from hitting bumps and such into heat. You may not have thought about it in that way, but you cannot create or destroy matter and energy. You can change the form of either, but you cannot destroy it. If you were to hit a big bump on a bike with no suspension and solid tires and a rigid frame ( as in not springy ) and you were sitting down so you could not absorb the impact with your legs, you and the bike would be tossed into the air. How high would depend on how fast you were going, how much you and the bike weigh ( thus determining the amount of kinetic energy available ), and the size and shape of the bump. The point is that the energy you have riding down the trail is redirected by the bump. Some energy is used to lift you into the air. Since you do not really want to be launched from every bump you hit, you have elected to do your riding on a modern trials motorcycle, complete with pneumatic tires and an oil damped suspension system.

     If you were to ride your trials bike over the same trail and hit that same bump ( hey, you were dumb enough to ride that rigid framed solid tired thing over it sitting down so you might be dumb enough to hit it again! ), some of the energy that you and the bike have available is absorbed by temporarily squashing the tires flat. Some of that energy is converted to heat in the sidewalls, but it doesn’t make a lot of heat so you never notice it. Some more energy is used to compress your fork and shock springs. But remember what we said in paragraph 2. You can’t destroy the energy and you are not doing anything that could convert it to matter, so what happened to it? You are just storing it for a short time. After you get past that nasty bump thing you hit, you can let go of that energy slowly and it will not have much effect on your progress down that hypothetical trail. The tires are going to rebound and there is nothing you can do to control that. Just like those big hoppity balls kids used to sit on and hop, your tires are going to rebound and push you upward. How much and how hard depends on many factors such as tire pressure and how much of the energy gets absorbed by the forks, shocks, and legs. There is a complex interaction between each component that pretty much adds up to an infinite number of possibilities. That is why suspension development continues almost 30 years after the “suspension revolution” and is also why it will never end. Now that I have made the point that tires and legs are part of the system, let’s forget about them for now and concentrate on the forks and shocks.

    As we hit the aforementioned bump, the forks were compressed. The energy that was expended to compress the spring is stored in the spring. But not all of the energy that the bike/rider unit gave up was used to compress the spring. Some was used to overcome mechanical friction in the fork. This was converted to heat. If the bump was sharp, the compression damping of the fork also came into play. This means that the compression speed of the fork was slowed by pumping the oil in the fork through restrictions of some sort. Most folks don’t think of it in this way, but forks and shocks are simply hydraulic pumps that only pump their oil internally. Everybody knows you can’t pump something without using energy of some type. When the fork compressed and pumped oil through the restrictions, energy was used. Since it did this pumping fairly quickly, there was a lot of fluid friction involved as the oil was crammed through the holes much faster than it wanted to go there. In overcoming this friction, some of the energy was converted to heat.

     So now we have rolled passed the bump and the forks are compressed. Some of the energy has been given up as heat, but most is still stored in the spring just waiting to pogo you into the air. But now the rebound damping comes into play, and  the extension of the fork is slowed by pumping oil through some more restrictions. Energy is again given up as heat, and still another portion of it is used to return the bike to the attitude that it was in before all of this unpleasantness started.  

     The point of all of this is that the characteristics of your suspension are simply the way that it stores and releases energy, and how much of it is given up as heat, and when. Next month we will get into the hardware and adjustments. For those bored folks who skipped to the end, turn the blue knob to the left.

 

 

     This is Part 3 of suspension basics. It may take a few more issues to complete this topic, but then we’ve got plenty of time, right? Anyway, I’ve got a lot to say about springs and oils and damping, etc., but I know that the season is moving by quickly and that every one of you is waiting in anticipation for my advice before you touch anything. Sure, I believe that. Well in case at least one of you is waiting, I want to say a few things before you start adjusting.

   First of all, and very important, is to make a record of what settings you currently have. Measure your spring preload settings with the most accurate method you have. If you have Paoli or other forks with an externally accessible preload screw, screw it all the way in and count the turns required to bottom out the adjuster. Write this number down and if you need to return to these settings later, you just screw it all the way to the bottom again and back out the number of turns you wrote down. You can do the same with the damping adjustment. This sounds pretty basic, but many people think they are just going to make a little change here or there, and pretty soon things are all messed up and they don’t know how to get it back to its former, less messed up state. On the rear, things are a little more difficult for a couple of reasons. First, access is very restricted unless you do some disassembly. Second, the spring is very stiff, so even a small movement of the preload adjustment causes a considerable change in the spring force. Be very accurate in your measurement. A good trick if you can gain visual access is to count the number of threads showing under the adjustment nuts. Then put a witness mark such as a punch or chisel mark in a place that lines up with some stationary part of the bike or maybe just in visual alignment with the mounting bolts. If you use a magic marker, the marks may be gone when you come back to them later. Magic marker marks are good however, for temporary marks such as when you are making the adjustment. Later, if after making several adjustments and deciding that the original setting wasn’t so bad after all, you can use the thread count for the rough adjustment and then bring the witness marks into alignment. This will get you so close to the original setting, Dougie couldn’t tell the difference.  

     I know that everybody reading this is a dedicated and responsible mechanic who always uses the right tool for the job and hasn’t used a pair of channel locks in years. Of course as the original trials bike dummy, this is how I try to conduct myself. However- there is a limit to this and on most bikes adjusting the rear shock spring is just on the far side of that limit. I own a complete set of hook spanners and I use them if the shock is off of the bike. But they won’t fit if the shock is on the bike. And to make an adjustment I am notta-gonna spend an hour or more taking the shock off to make a trial adjustment that is probably going to be too much or too little anyway. I just use a long punch and a hammer. If the punch is soft, it will not hurt the nut much anyway. Now that I have publicly made this confession, the 99.999% of the rest of you that do the same can stop feeling guilty.

     The next thing to consider is that any adjustment you make will affect more than just bottoming out or making it easier to hop. This will be the subject of next month’s column.

 

 

    This is Part 4 of suspension basics. Last time we discussed how to make spring preload adjustments. There is plenty to cover in upcoming columns, but we now have enough information to decide if adjustments are necessary and what adjustment to make.

  First, you want your suspension firm enough to not bottom severely, but it should be soft enough so that it does bottom on the very hardest impacts. If you try to avoid bottoming altogether, you will not have the compliance to keep the tires on the ground in some conditions. When you go to make your changes you may find different possibilities depending on your bike.  Next month we will cover various bikes.

   This time I want to talk about steering geometry and how it is affected by suspension settings. Once you have adjusted preload and damping, you will need to make further changes to get your bike to steer properly. There is a great deal of advice and debate about things like bar position and such and how forward positioned bars can help you turn sharper and etc. Furthermore, some folks think they can affect steering by moving their weight forward or back to compensate for bike deficiencies. This can help on up / down hills, but in an average turn, it is likely to make matters worse. I finally learned a lesson that has been taught by many people, but took me a long time to adapt to. When you are turning, you should have practically no weight on your arms. Next time you continually have trouble in a turn, think about this and make a point to keep your arms light. You will probably find the turn wasn’t so hard after all. You will also find that since you do not have you arms propping you up any more, you are forced to balance yourself on the pegs. Now your mass is pushing downward on components located a foot above the ground and much closer to the contact patch of the tires instead of on the bars which are located much higher and which will probably be located on the inside of the tire contact patch if the bike is leaning into the turn. Since you are positioning your body so that you are nearly balanced with little arm pressure, you cannot do things like the typical go-fast guy does and slide your weight forward in turns. And also, since your bike probably doesn’t have an adjustment for steering head angle, you must have your suspension set up properly to be able to carve that tight turn.

    Basic rules are as follows. If your front wheel is “plowing” or not turning tight enough, you need a steeper steering head angle. If it is “knifing” or digging in and causing itself to turn sharper than it should, the angle is too steep. You can affect the angle in several ways. One is to slide the fork tubes up or down slightly in the clamps. Another is to change the spring preload. A third is to change the ride height in the rear by changing the rear preload. All of these methods change the angle of your steering stem with the ground. I finally learned that even small changes can have a dramatic effect. A year ago if some told me that a 1/8 in adjustment in fork preload could even be felt, I would think they were crazy. After all, I could make bigger changes than that simply by moving my weight forward or back. But now since I have learned to keep my weight off the handlebars, I can’t affect the suspension in that way. So I made adjustments until the steering was satisfactory. If your bike is more than a couple of years old and has some wear and/or bad adjustments caused by you or a previous owner, this can be a very satisfying discovery. It can be like getting a whole new bike. Don’t forget the fact that if you change your tire pressures you can affect the steering angle. If you drop the rear pressure a couple of pounds for example; you will make the rear end ride a little lower. This effectively increases the rake angle of the front end and can allow the front tire to plow. If you did this for muddy conditions, you may attribute the plowing to the mud, but it may be partly caused by the rake change. It would probably be a good idea to experiment with suspension settings for an alternate tire pressure at home. That way, when you get to a muddy event and drop your tire pressure, you can slide your fork tubes up a little to compensate.

 

 

    This is Part 5 of suspension basics. Last time we discussed how spring settings affect your steering geometry. This time we will talk a little about making adjustments. Before we get into that though, let’s talk about springs. Let’s take two hypothetical trials bikes

(HTB) and a hypothetical rider named Vern. These two bikes have identical damping characteristics in the forks (The nice thing about hypothetical bikes is that you can make them work any way you want). They are the same brand so the geometry is identical. Vern is offered a test ride on HTB-1 and notices that when he is standing in a balanced position, the forks are compressed 1.5 inches. He rides off of a drop off approximately 6 Crawford units high (about 3 feet for those of you who have not heard Scott describe the height of obstacles). He notices that the forks just barely bottom out. Next Vern takes a ride on HTB-2. As he is standing balanced, he notices that the forks are compressed 2.25 inches. He thinks to himself that those forks feel cushier, but will probably bottom out too easily. When he rides over that 6 C.U. high drop off; again, the forks just barely bottom. He also noticed that on the small bumps after the drop off, the forks seemed to absorb the bumps better and kept the front wheel on the ground better. Wow, he thought. Those softer springs really work better. He goes back to HTB-1 and reduces the spring preload until the forks also compress 2.25 inches while he is standing balanced. He takes it out for a test and rides off that same drop off. The forks bottom harshly and he nearly eats dirt for lunch. What could cause this?

    The answer is that HTB-2 has stiffer (higher spring rate) springs than HTB-1. Wait a minute! Wasn’t HTB-1 the one that compressed less when he stood on it. Yep. And that is the whole reason for this quaint tale about Vern and his hypothetical bikes. In order for both to just barely bottom on the same drop off, both sets of forks had to develop the same spring force as the forks reached the end of their travel. But for the two sets of forks to compress different amounts while at rest, the spring force must be different. Hum. Here’s the explanation. HBT-1 with the softer (lower spring rate) spring has more preload than HTB-2 with its stiffer spring. Spring rate is a measure of the force required to compress the spring a certain amount. I haven’t measured any springs lately, but I seem to recall from the good ol' days, numbers around 40 pounds per inch. This means a 40-pound force will compress it 1 inch. 80 pounds would compress it 2 inches and so forth. If the spring rate is constant (all coils are the same distance apart) and not progressive (the coil spacing varies along the length), then this linear relationship holds true throughout the travel. Most trials bikes have constant rate fork springs. I am going to do a little engineer stuff here that any kindergarten engineer could do and show a little chart that demonstrates the effect of preload versus higher spring rate as a way to adjust forks and shocks. It will compare a 40 lb/in spring with 2 inches of preload to a 45 lb/in spring with 1.1 inches of preload at various compression amounts. Total travel is 6 inches.

 

 

 

 

 

 

 

 

 

 

As you can see, the heavier spring with less preload is 1/3 softer at rest, but the same at the end of the travel.

      If this is too technical and boring for you, too bad. Trials is a technical sport that is won or lost with tiny details. I for one think all this spring stuff is really exciting! ( Not ).

Just file this away for later and for now just consider that if you are running high preloads to compensate for weak springs, you are hurting yourself. By the way, long preload spacers do not necessarily mean you have a high preload. Some bikes have short springs and a lightweight spacer to save weight. If you need heavier springs and your dealer doesn’t have any, you can talk to a performance suspension service and they can supply whatever you need. Scott – if you are reading this you know we love you and I was just kidding about the C.U. Until next month, keep your feet up. And your weight centered. And your outside elbow up. And your shoulders…..

 

After all these months of discussing springs, geometry, conservation of energy, and the like, you guys probably wish I would just get down to the nitty gritty of adjusting your suspension. Well folks this is your lucky month, cause I’m-a-runnin-outta-stuff-ta-say. So here comes the nitty.

   The most important thing is for you to keep the suspension at both ends balanced. This means that with your weight centered on the bike, you compress both front and rear suspensions the same amount. This should also happen while riding through a section. Sure you will hit bumps and such that temporarily affect one end or the other, but in general both ends should compress and rebound together. You might be asking yourself why this is so important. I’m glad you asked that question. ( If you did not ask that question, you might not have an inquiring mind. And you know what the National Inquirer says about that ). Any way, the reason is that you want the steering geometry to always be the same. We already discussed that a few months ago. Many knowledgeable or allegedly knowledgeable people suggest that when stand on a stationary bike, the suspension at both ends should compress about 1/3 of the total travel. This is about 50mm ( or 2 inches for the metrically challenged ).

    Different bikes have a variety of adjustment methods on the forks. Most shocks adjust the same way. Lets do shocks first. The spring adjustment is easy to figure out. The damping adjustment will be a little harder to figure out. The reason for this is that most of them don’t make much of a difference. The suspensions of late ‘90’s bikes are faster rebounding than earlier models. If you want to duplicate this characteristic, you will probably have to replace the shock or have it modified. I have been told that Bob Ginder at B&J Racing is good at this. Non rebuildable type shocks can be rebuilt and the damping can be updated. If you are extremely handy, you can even do it yourself. Good aftermarket units are also available. Try the adjustment screw. It may be enough. If you speed up the rear be sure to do the same to the front. If your motive is rear wheel hopping, consider this. If you need faster rebound to hop, you probably aren’t doing it right. In that case, your scores may actually suffer from the change. Fast rebound is a hindrance in marginal traction conditions. And if you are attempting a splatter, you need the wheel to stick on the obstacle long enough to drive up it. I don’t mean to discourage anyone from experimenting. Just be sure to record your initial settings so you can get back to the original setting if you don’t like the changes.

    Forks are a different matter. They have all kinds of adjustment. I will discuss the ones I know about. GasGas forks have both compression and rebound damping adjustment knobs on the top of the fork caps. Rebound is controlled by the right leg and compression on the left. If you need to change the preload, you do it by changing the spring spacers.

Conventional Paoli forks as used by Beta and probably Montesa, Scorpa, and others, work a little different. They only have one spring. It is in the left leg and it is adjustable externally via a hex key in the top of the cap. The adjustment is limited to about plus or minus 5 mm. If you need more, you can install a spring spacer. The rebound damping is in the right leg and is adjustable by a knob on the top of  the cap. Compression damping is not adjustable except by changing oil viscosity or modifying parts in the left leg. Marzzochi forks as used on Fantics and others, work basically the same as the GasGas forks.  All trials forks listed above use SAE 5W oil. Some month, I’ll get into oil viscosities, etc. But for now you should consider this point. An SAE specification for oil viscosity is not an absolute number. It is a number that covers a range of viscosities. This means that the 5W oil you get from one company may not have the same viscosity as the 5W you get from a different company. Once you find one you like, stick with it. Spring characteristics can be modified by playing with oil levels, but most trials riders stick with the recommended amount of oil. On the Paolis, this amount is 360 cc per leg. On a GasGas, the level should be 100mm from the top of the tube with the leg fully compressed and the spring removed. I don’t remember how much goes in the Marzzochis, but I do remember that they have  air bleed screws in the caps and it is very beneficial to utilize them regularly. This really makes a difference to lighter riders.

    One last suggestion for you guys that are fortunate enough to get a new bike and plan to keep it for a while. As soon as you have enough hours on it to get the suspension broken in, take the time to make some detailed observations and make notes. How much does each end compress at rest? With you on it? How fast does it seem to rebound? Is it very smooth or kind of jerky?  Put numbers on as much as you can and write everything down.  Even how much you weigh. That way you can review your notes a year or more later and compare the current condition to the new condition. Suspension deterioration is a gradual process. It is hard to notice the day to day changes.