Wednesday, October 29, 2014
Motorcycle Suspension Springs
Most of you have looked at your suspension components and understand that with springs, the suspension allows the wheel to travel up and down over the bumps on the roads that we travel. The front and rear springs support the bike. One of the ways I like to get customers to visualize what the spring is actually doing on their motorcycle is to imagine the bike sprung far too stiff to the point where it is basically solid as a rock. In this case there really is no suspension and it is not hard to imagine how rough this ride would be. Any bumps encountered would cause a lack of traction as the wheel would be kicked up off the ground at anything but a snails pace. Next, picture the bike with a very soft suspension where it would crash through the travel easily and wallow. Sometimes this is harder to picture because unlike the first example the suspension is actually working, just not very well. This soft setup creates quite a bit of instability and allows the chassis to pitch for and aft when on and off the gas or when hitting bumps or inconsistencies in the road surface. Instability on a motorcycle is a bad thing and should be avoided at all costs as it causes loss of traction and/or control. The only way to avoid loss of traction on either of these examples, over stiff and over soft, is to back the pace way off and hope we don’t need to maneuver, brake, or roll on the throttle. This would be basically no fun at all, and dangerous. A better option is to find springs that are suited to the rider and bike so we can really get the most out of our bikes and be safer.
The diameter of the coils and number of coils defines the spring’s rate. To make things confusing they are measured in standard and metric so to simplify I will be using metric rates. For a 1.0kg/mm linear rate spring it takes 1.0 kilogram(2.2lbs) to move the spring into its travel 1mm and it would take 2.0kg to move it 2 mm. In the shop we use conversion rates when we are sourcing springs from American companies like Penske but we normally use kg/mm as the unit of measurement for fork and shock springs. I would also like to point out that Ohlins ships their springs to us in Newton Meters(NM) and then we do a quick calculation back to kg/mm. The calculator and/or spring charts are in use quite a bit back in the shop and at the parts counter. For instance a linear rate fork spring that measures .95kg/mm is equal to 53.2lbs/inch. Because there are 2 forks it takes 106.2lbs to move the fork one inch. Another 106.2 or 212.4 pounds total would move the forks 2 inches into their travel because the fork springs are linear.
When we take apart a telescopic fork the springs are usually in the range of .80kg/mm to 1.05kg/mm and shock springs range from about 7.0kg/mm to 14kg/mm. Whether the springs are on the front or the rear they need to be selected to match the rider’s weight, skill level, and intended use. We have charts and data from the shop and the track that help us select these springs but the bike has to be on the lift to verify that the correct spring has been selected.
Not only are the rear springs heavier but they also have a wider range and this is because force transmitted into a shock is done with mechanical leverage and not a simple telescoping action like the two forks on the front end. To apply this leverage to the rear shock most current motorcycles have a linkage that attaches to the swingarm and the frame and allows the wheel travel to be less than the shock shaft travel which makes for a smaller shock. But not all bikes have this linkage. The new Monster821 shock is bolted to the swingarm and then directly to the rear cylinder head. This is not only cool but it provides an excellent leverage ratio for great handling. Come on by the shop and take a look at the different suspension designs on the current models and used bikes.
From the desks of Steve, Matt, and Nate.