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Dirt Bike Packing for Technical Terrain

Tour of Idaho folks - please listen up!

by Martin Hackworth

Photos: Martin Hackworth, J.R. Hackworth, Brian Horton

KLIM Arsenal - a great option for carrying items for day rides.

There are two things in the world that everyone knows: their own name and everything there is to know about the physics of motorcycles. Years of bitter experience have taught me that the latter is highly improbable (and the former is often suspect all by itself). Neither, however, slows down anyone on the Interwebs. It's called the Dunning-Kruger Effect. Check it out.

I generally don't like it when someone tosses out a resume as an introduction so I try not to do that myself. Nonetheless in this case it's appropriate to preface all of this with my credentials. I was a Lecturer in Physics at Idaho State University for about 25 years. Everything that I am about to tell you is completely true.

Let me start with a declarative statement - when riding your dirt bike in technical terrain it is almost always preferable to tote heavier items on your back rather than on your bike. There is a qualifier for this and that is that you are spending most of your time standing on your pegs in the rough. If you are sitting on your seat like a slug you can stop reading right here and go see Jimmy Lewis for some instruction. After that come back here and you can pick up where you left off.

When trying to explain this in various motorcycle forums I invariably get pounced on by someone who rode the entire "_________" uphill both ways in the dead of winter through six feet of snow carrying a horse in their hard bags. That's just fine. Most of the time those folks are ADVRider types who also wore out an extra layer of memory foam and the sheepskin seat cover on their seat. Nothing wrong with that, but if you are riding non-technical stuff it doesn't really matter where you pack your gear. 
Saw Pack
A bit of respectful advice. If your shoulders and back really do ache badly after carrying 25 lbs on your back for a few days perhaps you should consider patronizing a gym and working on those deltoids, pecs and lats. Planet Fitness is $10 a month. You can't beat that with a stick.

Now let's get after packing for performance in the rough. In order to understand why it is desirable to get as much weight as you can off of your bike and onto your back in technical terrain it's important to understand the concept of moment of inertia (MOI). Stated simply, MOI is the radial distribution of matter around an axis of rotation. The more matter there is farther away from the axis of rotation the greater the MOI and the more torque, or rotational force, it takes to get that system to move or to stop moving.

Consider a motorcycle wheel. The reason that motorcycle (and bicycle) wheels have lots of empty space between the axle and rim is to increase their MOI about the axle. A completely solid wheel (uncommon in inline two-wheeled vehicles) would have a smaller MOI about the same axle. The greater MOI makes the wheel more stable. Once you get it going it wants to keep going in a straight line at a constant speed. This is the essence of motorcycle stability.

If you ever watch someone riding a motorized scooter with small wheels down the street you'll notice that it's difficult for them to maintain a straight line. That's an example of an inline two-wheel system with a low MOI. It takes practice to ride a small-wheel scooter smoothly in a straight line. That is because not only does does a wheel with a high MOI resist changes in speed but it also resists any change in direction

Change in direction includes lean angle. A wheel that has a high MOI about it's axle also has a high MOI about an axis that connects the front and rear tire contact patches - something that we'll call the "roll axis" of a motorcycle. This means that it takes more effort to make the bike lean from side to side if the wheels have a high MOI than it does if the wheels have a smaller MOI.

or a motorcycle or bicycle to turn at anything above pushing speeds it must lean. Tricycle steering does not work well on motorcycles or bicycles. Motorcycle tires have "U" or "V" shaped profiles specifically to exploit a turning effect called camber thrust. A combination of lean produced by torques that you supply through the handlebars, seat and foot pegs, camber thrust, and terrain itself causes dirt bikes to turn.

Since a high MOI requires more torque to initiate lean, motorcycle wheels with relatively high moments of inertia are directionally very stable - you have to do some work to get them to change direction. When I was road racing I was always amazed at how far a riderless GSXR1000 would travel in a straight line across the Mojave Desert after it left the track. This is also why riding with no hands in smooth terrain is easy (especially if you stand up). Torque supplied by terrain and/or torque that you supply through the seat, handlebars or pegs is required to make a dirt bike lean and turn. The higher the MOI, the greater the torque required.

There are additional factors besides wheel MOI that contribute to the total MOI of a dirt bike about it's roll axis. A heavier bike, especially one with the weight up high, has a higher MOI about its roll axis. When you start mounting accessories, which tend to get placed up high on the bike, you increase this MOI even more. This is why some folks who ride large adventure bikes, especially festooned with baggage, develop a false sense of the capability of their bikes in gnarly terrain. A heavy bike with a lot of mass up high feels really stable and controllable up to a critical point - and if you never reach that point you are good to go. But that point invariably is found when the going gets rough enough and that's when things go south in a hurry on a large or heavily loaded bike.

About the worst thing that you can do in terms of controlling your dirt bike (much of the time anyway) is sit in technical terrain. A rider who sits all of the time increases the MOI of their motorcycle about its roll axis - more than just about anything else. Most of us have a mass that is a significant portion of the mass of the motorcycles that we ride. Glue your butt to the seat while you ride and your bike will want to do little other than go where it's pointed. Sometimes that's just the ticket - but not when you are riding trails that require hundreds of subtle steering inputs each minute.

When you stand on your pegs you do two things that help your dirt bike change direction rapidly. The first is that you supply a leaning torque through the foot pegs which is much more effective than through the seat or handlebars. The second is that you decouple your mass from that of the bike because your feet on the pegs act as sort of a hinge - thus reducing the bike's MOI about its roll axis making it easier to lean and turn. You increase the leaning torque you are supplying at the same time that you are reducing the relevant MOI. 

Any gear that you have strapped up high on your bike increases the MOI of the bike about its roll axis and partially defeats what you accomplish by standing on your pegs. But if you wear that gear in some sort of pack or vest, and you stand on your pegs, you decouple the gear's mass from the bike - again making direction changes much easier.

Ultimately it all comes down to energy. Gravity assists in supplying the torques required to lean your bike over from vertical - but you have to supply the torques to return the bike to vertical on your own, actually fighting gravity in the process. Gravity doesn't care about conserving energy but your body does. A high MOI doesn't affect gravity but it does require that you burn energy at a higher rate to counteract the gravity-assisted lean and return your bike to vertical. Reduce your bike's MOI and you burn less energy fighting gravity as you ride.

As with all things there are a few caveats to this. The first is that if you hate things on your back, the distraction and stress of a pack or vest will probably cause you to burn more energy than you save by reducing your MOI. The second is that there comes a point of diminishing returns in unweighting your bike and that occurs when you have so much weight on your back that you are wearing yourself out with just the weight. The third is that the placebo effect accounts for as much as 40% or more of what is perceived as success in clinical trials. If you are convinced that you are better off with weight on your bike than on your back no amount of physics is going to change your mind. 

I carry my chainsaws and Pulaski for trail maintenance in a specially made saw pack. When I ride the Tour of Idaho the only things that go in my Giant Loop Mojavi's are bulky but light items and fuel (for safety). And anything that's heavy goes in the bottom of the Mojavi's. Everything else goes on my back. It's important that you use purpose built powersports backpacks and vests to get a comfortable fit, proper weight distribution and support. 

I'm 100% sure that the physics component of all this is correct. It's sophomore level physics. I've discussed most of this with Jimmy Lewis, who knows as much as anyone about riding gargantuan motorcycles in technical terrain, and I'm 95% sure that the biomechanics and rider input parts of this are correct as well. 
I am confident that if you give this an open-minded try you'll find that your overall fatigue goes down at the end of a long day by packing as much weight as you can, without overdoing it, on your back instead of your bike. It sure can't hurt to try. 
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