Go Beyond

Written by Teran McKinney
/ About Me / Half-time Remote DevOps/Systems Engineer for $40,000 /

Motorcycling physics and thoughts

This needs serious updating. It's a little embarassing but not entirely. Don't take it too seriously.

Merry Christmas everyone! Hope you're having a good New Years. Although I have no idea when you're reading this. So Happy St. Patrick's day, Valentine's day, and the works while I'm at it. And then it just gets pointless.

So... anyways. This post is on motorcycles. About fast motorcycles. About nimble motorcycles. Ones that you can drift. Bikes lightweight enough to push up a hill. Bikes with a low enough first gear to climb, and a tall enough top gear to get on the highway.

I don't believe in motorcycles in the traditional sense. I don't understand all of that nonsense about needing shocks and swingarms to corner. And I don't believe in the electric starter.

Not to say, there is never a case for the things above. But, if you want a fire-breathing machine full of excitement and purpose-built for performance, those don't always come into play.

So to turn a bike at speed, you lean it. This means, to my limited understanding, changing the tread contact patch at the tire. It swoops a different way and encourages a non-straight direction. Cornering is generally measured in G-forces, or newtons. Most-strictly, on a theoretical model it has to work with the traction coefficient between the road surface and tire. In practice, there's a lot more to it than that.

Pneumatic (air-filled) tires have a suspension-like effect on their own. They conform to the surface. Sometimes too much.. and sometimes too little, depending on load, stiffness, and air pressure.

But let's look at a race track. Beautiful, smooth asphalt and swooping curves. No bumps. No rocks. Just sticky, black goodness.

And what races on this surface? Complicated machines with a high seat position and pretty involved suspension underneath. But why? I can understand on the street to some extent, with potholes and all. Or exceeding high speeds on the freeway. But a track is so smooth.. and perfect. And leaned over, suspension plays less and less. Twisting flex in the chassis probably matters more than those tires popping up and down in a vertical fashion, when you're leaned over at pi/4. And even, I guess a side-to-side flex can come into play on the axle areas.

In general, it is my belief that suspension, in the modern sense (two shocks in the front, swingarm in the rear with a single shock or dual), only performs handling performance in the vertical direction. Which generally, is going straight over bumps. And there is a huge use for suspension with going over bumps. On a bicycle, usually not. You're probably doing something crazy on a bike by the time you've maxed out what a rigid steel frame can do. But on a motorcycle, the speeds and weight are much higher. A rigid frame will go over quite a lot, but in general, even maximizing tire selecting and setup, struts become necessary on a variety of surfaces if the goal is survival and sustainability.

That isn't to say that you can't go off jumps and curves with a solid suspension. And on loose surfaces, all that energy that the suspension attains on compression is pushed out on rebound. That increases the load on the loose surface (loose, meaning softer than the tire) and pushes back, giving more potential to lose matter to grip on to. And building a bike without suspension is much simpler. Not drastically, but quite a bit.

Alright. So I'm leaned over at pi/4 in a wide bend, going 40m/s on this smooth asphault. There's a few things I'm concerned about. Rotational flex in the chassis, for one. I've heard that Ducattis are built to allow some rotational flex to have different angles on the front and rear tires. This slight flex is supposed to help improve the predictability of the bike. Predictability is good, especially when your life is on the line. I'm also concerned about the rubber. And of course, it's treadless rubber. In the rain, I might want grooves, still unsure on that. So it's treadless, sticky rubber. The kind you can run your thumb over and feel the adhesion/drag. I want the rake angle to be set properly, as well. A short rake angle is twitchy. Easier to steer at low speeds, but usually not a problem on a bike. A longer rake (and not like a ridiculous, stretched chopper!) generally offers more stability and better contact patch angles in the corners.

This surface does have minor, minor bumps. So I probably want some chassis flex. I'm not sure if aluminium will work for that. Even though it's much lighter, it doesn't flex as nicely as steel does. So my frame is probably steel. And of course, because I'm crazy and don't accept every modern notion given to me, there's no shocks or swingarm. This also, conveniently allows for a nice and tight chain since it doesn't have to be loose enough to allow for extension through swing-arm shock compression.

What it all comes down to, now, is traction of the tire versus the road, and the forces which suboptimally are not applied as downward as possible on the road. Sorry for my poor terminology, but I mean that the mass of the bike and rider should be as aptly directed to the contact as possible. Inertial force is largely responsible for this, but that has its limits. Of course, more so at sharper turns at slower speeds. These forces you don't want usually come from mass above the tire, which is just about all of it. Imagine a motorcycle with lifted a meter in this corner, and it seems silly and like it's going to low-side. We have this silliness with modern track bikes, putting the rider and weight up so high. Anyone can tell with the center-of-gravity weight just sitting on a bike. All of that comes into play. Now, in my example it may not be much of an issue, but in a field where technology is pushed to the limits, it could potentially mean another m/s or more of speed around the corner. Enough to win a race, given all other things equal. It also allows for a lighter bike, which means better acceleration, harder braking, or less wear on components if braked/accelerated like a heavier bike.

Also means no brake dive, nor rake angle changes through suspension compression on the front and/or rear.

Given all of this, I believe that a steel framed, fully rigid motorcycle given the right angles and thought, with the same engine, could beat any track motorcycle within certain parameters on a smooth track.

I'm not saying that suspension doesn't have its place, and there isn't much greater genius than my limited intelligence involved in suspension design and setup at tracks. And I don't believe you can merely set your struts to be solid and have an identical effect. But, if you built this frame from the ground up, or possibly rigidified a frame with very bad suspension, it would be noticably better.

But what about hardtails, and how everyone says they handle terribly? Well, in my personal experience, it's because people build them in stupid ways. Hardtails are usually stretched, raked, big Harleys with forward controls and high handlebars. And they're ridden on the road, usually between bars. So on the road, you want to be able to survive speed bumps and the like. And forward controls is ridiculous for that. I'm completely okay with forward pegs for the highway, but the controls should always be mid on a hardtail. With forward controls, the shock of bumps will go straight up your spine with little way to lift yourself up. With mid-pegs, you predict the bump, raise up a little, and ride over it. It's not a big deal.

I know this isn't ideal, but I put rigid struts on my 1996 Honda Nighthawk 250. Personal experience does show that it's a rougher ride, but not unbearable by any means. In fact, I think with a good seat and the right bike, anyone can handle a hardtail. The lower center of gravity and seat position makes it easier to handle at low speeds, especially on slick or rough surfaces. Now, it does jolt you around badly on the gravel, but it's predictable and you're probably not going to be riding over gravel more than 5% of the time on a hardtail.

But, on the street, I think the front struts may be necessary. I've yet to test without, so I'm not sure.

I'm just amazed at how everyone knocks hardtails. I've ridden one, and it's fine. I'm not going to say it handles better in every single possible scenario (namely, vertical repeated bumps), but it's more predictable, lighter, and fine on a sub-300lb bike. And please, if you make your bike a hardtail, put mid controls on it! And no ape hangers, seriously. I like my bikes to function, not be show pieces for bar hopping.

So, back to the suspension. I think the suspension damping acts like oscilation. It's a wave of up and down, in terms of forces and compression. Pneumatic tires have their own wave, depending on the tire, wheel, and air pressure. With suspension (or even possibly, flexing of the frame, though unlikely), it generates its own wave. Many times, the waves can "fight" each other. I think this would mean they are opposite, or partly opposite. One pushes up when the other pushes down, etc. I think this is why swingarm bikes with monoshocks and certain tires are so picky on tire pressure. They feel numb and untrustworthy with even a small percentage of pressure difference. I don't notice this nearly as much on my hardtail, but I like to run a lower tire pressure than I would with the tire.

In general, you'll need X compression/rebound effect for a given course. In theory, if you had a perfectly rigid tire, you -might- be able to have more optimal dampening than the pneumatic properties of a tire will allow. So, I can see certain cases where on that metric alone, a nearly massless suspension setup could act better. But still, center of gravity is an obvious case in physics. And track bikes are so high up that if the cog was lowered, could definitely perform better on the track, within certain constraints.

Back to hardtails. Hardtails with a suspended front can pivot around the front shocks, in part. So they act like they are suspened slightly as the rear is not an entirely seprate mass. And I'm more concerned about hitting my mass head on to something; less so towards the mid and trailing mass passing over it.

I'm sorry for the scatteredness of this post. I have no doubt my physics understanding is significantly off. And I'm sure there are limits to what I have proposed. However, no doubt, rigid suspension is sometimes the best in given constraints. Those contrains may just not be a reasonable track that exists today. But perhaps, slow speed saloam? Most definitely.

And all on this note, I'm actually considering getting a Suzuki DR400SM. Which, of course, is a fully suspended bike with a monoshock in the rear. I am starting to learn, that even if my engineering theories are right, countless hours of research and numerous positive reviews don't always go wrong. I'll probably end up with a better bike in the DR400SM than I would hacking away at my own for quite some time.

Yet, I haven't ridden the bike yet. So we'll see. I'm debating rigidifying my two stroke if I can resurrect it. Hoping the rake angle won't be too negaively effected with the shocks bottomed out, though.

I love removing parts and designing things. Every once in a while, I find use cases where something was not needed. But often, like with motorcycle seats, enough time without it makes me appreciate them so much more and have a greater understanding of why it was there in the first place.

Sincerely, Teran

PS: This is the coolest motorcycle blog of all time: Attack Choppers

PPS: Haven't re-read/edited this post at all, and it's really late. Sorry for the confusion, typos, and weird mistakes! Poor logic or seeming of insanity is probably unaffected by my lack of energy, however, and accurately assumed.

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