Suspension

Suspension and its adjustments have an big impact to motorcycles control and runnability. Suspension consists of springs and shock absorbers. Springs are there to damb the power result from bumps, accelerates and brakes, as well  from curve coming centrifugal forces. Shock absorber is there to slow down movement of the spring, when spring doesn’t get to compressed and come back in full force. That way the tyre stays stick to the track as good as possible. Actually springs and shock absorbers are there to do two things, get motorcycle moving forward as smoothly as possible and prevent tyres break away from track surface. Suspension adjustments can improve or weaken its functionality. 

Sprung and Unsprung Mass

Motorcycle mass could divide sprung and unsprung mass. By sprung mass I mean frame, fuel tank, engine etc. By unsprung mass I mean brakes and rims etc. When driving for example to bump, tyre rise upwards compressing springs, which again pursue to rise frame. Motorcycles own mass drag rising and rising movement strenght depends on springs stiffness and tyre, rim and other unsprunged parts mass. So the smaller unsprunged mass is commensurate to sprunged, the better motorcycle can resist tyre movements and stays better in track surface. Tyre loosening to track depends greatly which is ratio of sprunged and unsprunged mass. Example in MotoGp there are aspired to get unsprunged mass as low as possible, by making expensive carbon fiber  back forks and rims. In series-produced those are so far too expensive.

Progressivity

Progressive suspension means suspension, which chances all along stiffer when tyre boundingmovement increases. In that kind of suspension tyre remains in track surface remarkably strictly. Progressivity prevent spring bottoming in bounding, when driving for example to big bumbs. Telescopefork progressivity is commonly achieved by progressive spring. Spring spiral is thicker in the other end of spring. Rarer way is to put two linear springs, which spring stiffness is unequal between each other.

Rearspring progressivity can be achieved by many ways. One simple way is to install linear spring-shock absorber element in sloped angle to rearfork so that upper fastening point is more front than lower fastening point. If spring-shock absorber element is located on an right angle towards to rearfork, then is spring movement range directly comparable to reartyre movement range. In sloped spring-shock absorber element, spring compress in the beginning of the tyre movement only a little, but when tyre movement grow, spring and swingarm angle chances to greater and in the same time spring movement and stiffness increases.

Progressivity could be achieved also by progressive spring or by putting two spring stiffness unequal springs. These options aren’t very common in this day track made motorcycles. Half the time progressivity is achieved by variable ratio levermechanism also known as linkage. This kind of structure gets spring progressivity work just like designer wants. This is also cheaper to achieve, than for example by using progressive spring.

Suspension Parts For The Meantime

Before I told, that we bought a crashed year 2004 Suzuki GSX-R 750 cc motorcycle from damaged vehicle center. Shock absorbers were quite good guality in Suzuki, so we ended up to use those in our project. In front there was telescopic fork, which inner tubes were lower so it was so-called upside-down front fork. Upside-down fork benefit to combared to the right way fork is notably stiffer structure. In front fork there was also adjustments for shock absorption bump- and rebound movement absorption power and for springs pre-load. These adjustments are actually necessary if the goal is to get best possible suspension for track. Rear shock absorber spring stiffness and shock absorption could also be adjust.

Parts which we get from Suzuki aren’t under no circumstance best suited to our project. Example in WP or Öhlins shock absorbers is some more better adjustments and those are also lighter,which again degrease unsprung mass and improve drivability. Later idea is to chance shock absorbers, or actually as soon as budget allows it.

Next time my intention is to tell, which kind suspension solutions we ended up in our project.

Designing the Frame

Designing of the steel tubular space frame, where to start? What is the mission of frame in track motorcycle? Engine, wheel axles, swing arm to frame axle, seat, shock absorbers and fuel tank are in certain places. All these affect to the lap time. Frame combines these all together. It has to be stiff, so that chassis will work and light so that overall weight doesn't be any more than it has to be. Balancing once again in the world of different kind of terms.

Designing is understanding, you have to understand matter widely as possible while you keep our eye on the goal. By understanding and often over time with designing software you can get insight, which will take you closer to goal or in worse situation further away of it.

I will tell about designing of our track motorcycles frame, one part at the time. I will start from the attachment of the front shock absorbers till I come to the rear wheels axle attachment.

Front shock absorbers attachment:

1. Front wheel axle

2. Lower attachment bearing of front shock absorbers

3. Upper attachment bearing of front shock absorbers

Designing front shock absorbers attachment main boundary conditions are force generated by maximal braking situation and front rake angle defined by chassis design (a). Force generated by maximum braking situation comes from friction of tire and asphalt. Rubber type and condition of asphalt affect to this considerably. Force moves via wheel axle to shock absorbers, from those to attachment bearings and from there to frame. Force is descriped in picture by yellow arrow at front wheel axle point (F). Measument A descripes the distance from lower attachment bearing to front wheel axle. This distance creates moment to force F. Longer the distance is, more force is going to frame. This distance is affected by wheel size and shock absorbers maximum stroke. Forces generated by maximum braking is descriped at the points of bearings by green arrows. Distance B act as so-called counter moment, longer distance is, less have to frame support at point 1, but this lengthens shock absorbers and some tubes in frame. As good as possible middle course in this once again.

We decided distance between bearings to be 170 mm, this is possible by the parts that we got from Suzuki and this is justifiable by FEM calculations. So purpose of frame is to endure forces that are descriped by green arrows at bearing points. This is possible best if the tubes and shock absorbers are in 90 degrees angle (perpendicular) in other words direction of maximum forces at bearing points (see picture below). Tubes 4 and 3 are not excatly in this angle because it was better to take them directly to the engine attachment points. Because engine and air filter box has to have room frame circles them, this provide also needed torsion stiffnes to frame. Tubes 5, 6, 7, 8 and 14 are there to provide the proper triangulation. This is what I explained briefly at my last post (Frame Type and Material). Tubes 9 and 10 carry the loads forward. Ideal situation is, example in braking situation is that all the tubes in frame take forces, then frame can be very light. With excellent triangulation this is nearly possible.

In the next post I will tell more about, example why tubes 9, 10, 11 and 13 go to that node and why is the node where it is. These things are hard to clarify clearly so if you have something to comment please do so.