XS650: (Un)Sprung Weight   Leave a comment

Some would say I’m coming unsprung. Some that I’m unhinged. Others don’t even see me. Me? I can get along with myself most of the time. And that’s all that really matters. In my own way I’ve found my balance. The theory of multiple intelligences makes sense to me.

It’s not so clear cut however when it comes to my bike.

My grandfather always quoted the 4 B rule. You don’t lend your Bundle, your Better-half, your Bike, or your Bum. As for my Bundle? What the Better-half doesn’t get the Bike does. And my Bum spends a lot of time on the Bike.

Which brings me closer to my point. Your bike is basically for you. You ride her. You put your trust into the bond you have with her every time you ride. You. Your bike. The road. The conditions.

Really, she is set-up for you.

Parts of the matrix making up your ride experience. Your mass. Your type and style of riding. Determiners of basic geometry:

  • Rake. Neck height. Trail. Wheel-base. Fork length. Centre of gravity…All factors in how she handles.
  • Tyre size. Rear shock angle. Spring rates front and rear. Preload. Compression dampening. Rebound. Oil weight….factors relating to how well she handles surface variations and cornering.

Major considerations are Sprung and Unsprung Weight.

Force the wheels up and down. What moves is Unsprung weight. Your suspension forces this into contact with the road. Reduce this weight and your suspension has less work to do keeping it all in contact with the ground. Releases the suspension for the other part of its’ job. Dealing with the G-forces experienced in cornering, acceleration and deceleration. Giving you , the rider, a softer ride:…

  • Front Wheel:  tyre, tube, valve, rim, spokes, hub, wheel bearings…
  • Brakes: rotors, calipers, caliper mounts, pads, hoses … drum, shoes, linkages…
  • Forks: lower legs, guard, brace, damper rods, fluid, axle, speedo drive…
  • Rear: tyre, tube, valve, rim spokes, hub, sprocket, cush drive, axle, adjusters, rear of chain, rear of swing arm, brakes-disc or drum, drum plate lock, end of chain guard…

Sprung weight moves with the frame. Held by your suspension, isolated by it from the road. Includes: motor, transmission, you-the rider. With gear. Mass. Means energy.

Some are both, one end fixed to the frame, the other tracking the wheels:…

  • fork spring, swing-arm, shock absorber, coil spring, suspension linkages, some rear guards, chain guard, rear chain or drive-shaft,…
  • Weight split isn’t 50/50, division has less to do with center of gravity than centerline from the pivot point…
  • unsprung swing-arm rears are lighter than the sprung front.

All of it you need to push.

Then you have Rotational Weight, which can be either sprung or unsprung.

Why is this important? Well, if you’re my dog, and something moves, you nail it or bite it. Even better, both.

There’s inertia. The resistance of any physical object to a change in its state of motion or rest. Want to move something from rest? Change its’ velocity? Or direction? Stop it? Acceleration and deceleration. Find some energy.

Do this around a pivot. Angular Momentum.

Things that go round are kind of a double whammy. They need to be accelerated forward, and rotated.

Every gram of mass in the tyre or rim has the same effect as two grams in the frame. The tyre’s angular intertia is approximately equal to its linear inertia.


Cruising? Weight is pretty much irrelevant. Air drag followed by frictional forces due to rolling resistance of tyres, bearings, etc.are your primary concerns.

Hardtails…no suspension, zero effect.

  • Rotating mass affects acceleration.
  • Total mass requires work to accelerate, decelerate, change direction or elevation.
  • Extra unsprung mass has a negative effect on a bikes’ ride.

That is why rotating weight is an issue. Rotating weight includes anything that turns: .. wheels, tyres, brakes. Also, engine parts: the flywheel, crank, clutch, cam, transmission, chain, …..

When accelerating you put energy into two things beyond that needed for a constant speed for the given riding conditions…

  • acceleration, F=ma. (linear momentum). Mass of the bike, wheels and rider.Rectilliner acceleration, Increasing kinetic energy
  • rotational acceleration- (angular momentum).extra to the energy required to accelerate in direction of travel.

Though static weight matters, pursuing rotating weight matters more.

Minimum weight is just that. Minimum. Should your bike be lighter, add ballast. Where you want it. Low.

Light has it’s advantages. Rotating weight has more.

There are practical advantages aside from better acceleration and deceleration.

Look at the variables. Type of track. Your style. How much brake you use. How much the course will require. What and where are the G-forces. Reduce rotating mass, you could use smaller brakes. Lightness means the whole range of components get an easier time. Tyres. Shocks. Springs. Brakes. Rider.

Smaller brakes are easier to manage, generate less heat. The weight can go elsewhere.

You could go to the effort of laser cutting new crank discs from modern lightweight materials. Expensive. While you’re at it do it right. 270°. 256 crank discs were commonly drilled and filled with lead, from factory. When these plugs started falling out they stopped. Cranks are finely engineered. Be careful. A well balanced crank is what you really want. Drilling for lightness is a compromise between structural integrity and performance. Crossing the finish line last is better than not at all. Cheaper too.

Running total-loss? Have a range of rotors, drilled, for different weights. Swap them in and out to suit. Could even have several sets of wheels, having different diameters and weight distributions. For that matter, if sharing a bike it could be a good idea to have several sets of forks, and rear shocks, all having different spring ratings, dampening profiles, preloads, actions. Even different length swingarms. If your forks are a little longer, moving them up and down in the triples trees gives easy geometry adjustment. Several sets of triple trees-predrilled for different rakes, trails, wheelbases. Doesn’t take long to swap any of these in and out for individual riders.

Short-tracks are easier. Narrower rev range. Cornering is more predictable. Easier set-up. Larger tracks, are a little different. Corner and straight combinations. Rpm. Velocity, Cornering dynamics. All more complex.

With lighter rotating components, you get up to speed at a faster rate.

Start at the front. Reducing weight with the largest parts first. Drilling shafts is expensive and the gain is very close to the centre of rotation. Little gain for the cost.

Lower friction bearings. Lighter grade oils changed frequently. These may give better results than taking components to their structural limits.

How far do you go? There are breaks. And there are breaks waiting to happen. Always at the wrong moment. Strain, power loss, or vibration. Vibration will literally eat horsepower. Rear wheels never see it.

Power to the wheels is always very important. The more power getting to your tyres, the faster you’ll go. Assuming you and your bike can handle it.

Unsprung vs sprung weight have the same efect on acceleration and top speed.

Lighter wheels, tyres, brakes, rotors have gains due to the lower ammount of power required to rotate them. Lighter springs can be used. Frames are less disturbed by surface irregularities. Gains frequently, improperly attributed to unsprung weight.

Weight removed from unsprung components such as rear wheels may affect traction if the wheel is not under control.

Require precise control of wheel movement? A low percentage-45:55-of unsprung weight is an advantage, giving faster lap times, higher top speeds. Because traction is improved.

Unsprung:sprung ratios affect wheel control. Especially on uneven and road racing surfaces. Of lesser importance for dirt track and little value for drag racing or hardtails.

The rest is up to you. How much are you prepared to spend? Reliability vs performance. Strictly racing? Maybe you sacrifice reliability for performance.

The lightest and fastest bike is only as good as it’s rider’s training and technique.

Together you move this.

You. Your body weight. Your motor. United. With machine.

Bio-mechanic body language.

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