- Toe Rag
- Aug 29, 2005
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Oh yeah I meant to find these earlier.
Damping rod v cartridge fork
Ok yeah it's almost certainly cartridge in the one, damping rod/nothing at all in the other one.
While I'm here I might as well elaborate on the aforementioned harshness of damping rod forks and why cartridges are functionally superior, for anyone interested. Effort post incoming:
Basically what a DR fork does is force fork oil through an orifice to bleed energy out of the system and slow the spring's oscillation. The problem with this system is that the size of the hole has to be a compromise between the demands of low and high speed damping.
Low speed damping pertains to the relatively slow, gradual movements in the fork caused by braking, turning, acceleration and rider body movements. High speed damping pertains to the rapid movement of the wheel as it judders and bounces over road irregularities. The faster and heavier the bike is, the further apart these two are.
The ideal fork has relatively heavy low speed damping so that the bike feels controlled and stable, but relatively plush high speed damping so that the tyres can snake and slither over bumps without bouncing off or skidding.
Unfortunately the damping rod fork is basically the opposite of this; a simple hole offers very low resistance at slow speeds, so the bike feels wobbly and soggy, but it also has an upper flow limit above which the fork effectively hydrolocks because the hole can't possibly flow enough oil in the brief time available, so the bike feels crashy over sudden bumps and vague over ripples.
Different brands judge this compromise differently eg Yamaha tend toward plushness, Kawasaki toward stability. Smaller, lighter bikes that don't go very fast can make do with relatively soft low speed damping, so they tend to have supple grippy forks and can use their tyres very effectively even with crude suspension. Faster bikes, like the various midsized 650's, have to lean more toward stability and mass control at high speeds, so you get the crashing sensation more often and the ability to fully exploit the tire is compromised.
A cartridge fork effectively consists of the above, but with added one-way valves that are forced open by sudden jolts, which let extra fluid bypass the hole. The force needed to open them is carefully calibrated by stacks of flexing shim washers; the other, related problem of needing to separate the compression and rebound damping is taken care of by having valves in either direction. This whole assembly together is the proverbial cartridge; functionally it's closer to the firm low speed, plush high speed ideal, and the overall effect is a drastic broadening of the bike's ideal operating range aka plusher ride, better grip and improved stability all at once.
Torque curves
Marty the sensation you're referring to is a result of the different torque curves. It's not possible to make an engine equally efficient at all rpm so torque output swells until a certain rpm then drops off as the engine revs past is sweet spot. Because power = torque x rpm, and torque is directly correlated to cc, small bikes tend to concentrate their torque toward the top of the rev range to maximize power. From the rider's perspective this feels like a smooth and gradual increase until you hit the redline, and having to change down for a sudden burst of acceleration in order to access the high rpm torque.
A 650 has a much 'lazier' engine because the torque is concentrated in the mid-range for maximum ease of use. From the rider's perspective this feels like like a sudden, immediate thrust that tapers off surprisingly quickly, followed by a wheezy extra couple of thousand revs before the redline which may as well be a formality because you've changed up ages ago.
Behold:
The important line is the blue one. You can see the Yamaha only really starts coming on song around 6-7000rpm, builds to a peak at ~9500, then gently tapers to the redline. The power curve in that tapering area stays mostly steady because even though tq is dropping, it's dropping slowly enough that increasing rpm makes up the difference. The area between ~9000 and ~11000 is what is sometimes called the 'power band', it's basically where you want to keep the rpm if you want to go fast. The power is concentrated up top to maximize hp:cc, this is a 'peaky' engine.
The 650 on the other hand has an almighty horny bulge starting at 3000rpm, a brief emissions dip around 5000 as usual, then builds to a peak at 7,000 before dropping off a cliff at 9,000. You can see that revving past ~9500 is mechanically possible but basically pointless because torque has dropped below what you get at even 3000 rpm, the power curve dropping off a cliff there is a reflection of that; the increasing rpm can't make up for the drastic loss of tq. The power is spread out in the middle so it's easily accessible regardless of gear and rpm. This is a punchy, mid-range engine.
And because no discussion of torque curves is complete without these, here is a Harley:
A wall of torque starting basically from idle, collapsing into an inefficient wheeze at 5,000rpm. The engine feels perfectly linear and you get the same urge for a given throttle percentage at basically every rpm, this is what's colloquially called a torque monster or tractor depending on your view.
Bonus big two stroke:
Two fifths of gently caress all until 7000, then a tyre-melting, chassis pretzeling, rear end-puckering sudden rush. This is what's known as putting hairs on your chest.
ABS and Traction Control
Ok so for anyone reading this post: coydog is responsible for the wall of text that follows, but the information is aimed at everyone because it's clear there's a general lack of understanding on the topic here.
If you want to understand traction control, first you must understand anti lock brakes, both because of principle and because TC builds on already-existing ABS hardware.
Firstly: what does ABS do? It allows the braking wheel to keep rotating in a situation where, without intervention, it would lock up and stop spinning. We want the wheel to keep spinning firstly because a turning wheel can stop much harder than a skidding one (normally much less effort is needed to keep something sliding than to get it sliding in the first place so it follows the there is more friction before you start to slide), and secondly because a turning wheel can still steer and stop the bike from falling over/crashing.
How do abs do what it do, shaggy? The most basic systems have wheel speed sensors at either end connected to an ecu controlling a series of solenoids linked to a hydraulic pump. All of that besides the sensors is packaged in one box called the ABS module, which the brake lines run into. The ecu simply looks at the difference between two wheel speeds - if one wheel is turning significantly slower/not at all, it's safe to assume that wheel is locked or about to lock. If a wheel is locked, the appropriate solenoid moves and releases the brake on that wheel. This has to happen so the wheel can start turning again, it only takes a split second. As soon as the wheel starts to rotate, braking pressure is restored with the aid of the pump and solenoids. This usually results in rapid lock-unlock oscillation that leads to a very unpleasant sensation, especially if it's the front wheel, but it's better than crashing. More sophisticated systems (cornering abs) integrate lean angle and pitch data from an IMU and thus work better at a lean, and can to some degree anticipate locks before they happen, but they still operate on identical principles.
Crucially, the ABS can't do anything if there's no mechanical traction; it can unlock a locked wheel, but if there's no friction available it'll just lock up instantly the moment brake pressure is restored. What this feels like IRL, in the rain for example, is the ABS only being able to apply very light braking pressure and the bike only barely slowing down even though you're pulling it back to the grip; on gravel and other loose surfaces, it is actually better to lock up and drag the wheels like a ground anchor than trying to parcel out the tiny amount of grip traction available, and this is usually the situation terrible internet people complain about when mandatory ABS comes up.
Tl;dr locking a wheel happens because brake force exceeds friction and ABS can only affect the first half of the equation by taking away brake pressure; it can't help you if the tyre itself lacks grip for whatever reason, but it can make those situations more manageable.
OK but what about TC? Well, those wheel speed sensors work both ways, and traction is just braking in the other direction. What if we connected them to the ECU and told it to reduce power when the back one starts spinning faster than the front? In a nutshell, that's what TC is. It's important to understand at this point that TC wasn't developed as a safety feature, it was developed as a way for GP racers to extract 100% of the available drive traction without constantly risking orbital insertion and paralysis. So in the same vein as ABS: a skidding wheel has less grip and control, so the goal is to reduce power just enough to restore grip.
How to reduce power? There are a number of ways and their effectiveness neatly correlates to how much money it takes to build the system.
The absolute crudest method is an ignition/fuel cut - rear wheel starts to spin, ecu immediately cuts power to the coils or injectors or both, engine immediately becomes dead weight. This is a crap method for two reasons: 1. shutting off all power is very sudden and aggressive and usually destabilizes the bike unless you're basically upright 2. it does nothing to account for throttle position, and therefore has no effect on engine braking or the flywheel effect (more on this later). Otoh it is cheap and works on any bike with minimum effort and development.
A better way is to reduce power without shutting off completely. There are myriad strategies out there for doing this but they all boil down to a mixture of retarding spark timing and reducing throttle angle, while accounting for lean angle. To do this you need some extra gizmos: fly by wire throttle to overrule the meatsack's hamfisting, IMU to establish lean angle and pitch, both of which cost money so are only becoming common now.
The nitty gritty or, why TCS isn't magic. TC principles are always the same, but engines are all different, so we need to talk about power pulses and the flywheel effect.
The ICE doesn't make smooth, continuous power like an electric motor, but rather produces a series of jackhammer blows with not much in-between. A thumper does one hit per two crank rotations, a 180° parallel twin does two, with a short gap between the pulses and a long one on either side. An inline four is a relentless series of even blows every 180° of rotation, a v-twin is two quick hammer blows followed by a long pause, a v4 or crossplane i4 is groups of hammer blows with long pauses in-between.
Why does this matter? If traction is like braking in the opposite direction, then ABS principles can be applied:
Imagine a thumper that has just exceeded the traction limit, so the most recent hammer blow has made the wheel spin faster than road speed. There is now a lengthy pause before the next hammer blow, during which the tyre has the chance to take a breather and re-grip. This means that when wheel spin starts, the rider or TC system has absolutely loads of time to reduce torque before the next hammer blow keeps the wheel spinning - just like ABS pulsing the brake on and off. It's not coincidence that dirt bikes, where dealing with traction loss is routine, are all thumpers or twins at most.
Imagine the same thing with an inline four: the first hammer blow gets things spinning, and then there's another and another and another with very little pause in-between. This gives a very smooth sensation but it's not good for the tyre, which can't catch a break. Anyone who has tried riding an i4 down a gravel road has felt the sheer terror of shutting the throttle only to find the bike just keeps spinning and going sideways. This is partly the seamless power, partly flywheel effect. This is why Yamaha bothered to make the crossplane crank, why anyone bothers with v4's, why v-twins can keep up with fours despite the lack of outright power - when the pulses are clumped together with long pauses between, managing traction is easier.
What is the flywheel effect? Imagine our thumper idling. There is a pulse every two rpm, but in-between those pulses there's a whole lot of friction, pumping losses and valve spring weight to contend with that are all trying to bring the engine to a stop. If you have no flywheel, that is exactly what will happen, because the one of the flywheel's many purposes is to build up momentum to both smooth out the power pulses and keep the engine spinning against it's own drag when rpm is low.
If you think about it, you'll realize that the more cylinders you have, the smaller the flywheel needs to be, because there's less time for the crank to slow down between pulses. Great! More cylinders = more power, lighter flywheel = more power! But there's a flip side to the flywheel effect that flat track racers are intimately familiar with - it is a form of built-in traction control. If the wheel starts to spin and you/tc don't reduce throttle in time, engine rpm will increase and make the slide impossible to control. How quickly this happens depends on the flywheel - a big heavy flywheel won't just instantly accelerate, it'll take time to come up to speed, time that the rider can use to throttle back. A big thumper or v-twin with heavy flywheels gives you loads of time to react - Harleys are some of the easiest bikes to control wheel spin on for this reason. An i4 with emaciated high-revving flywheels gives you no time at all.
Combine this principle with the power pulsation effect and you can see that different engines have drastically different 'native' TC that is then overlayed with your riding and suspension and tyres, and then electronic TC. In some situations it is literally impossible to react fast enough because the engine is gonna do what it's gonna do and electricity can't stop that any more than a human can and it feels like you've teleported to the ground, in other situations it is trivial and feels like spreading hot mamba butter on an eagle's testes.
Big i4 sportbikes are designed to have manageable traction at the very limit - the engine is spinning really hard, the throttle is fairly open, the rear suspension and tyre are heavily loaded, the tyre itself is very sticky and has very progressive grip loss when loaded. At this point a skilled rider can 'peek' over the limit by a couple of percent and manage the slide with his body and skill - the TC just gives a safety net in case he fucks up, and it only needs to contain a little bit of power on top of what the bike is already containing mechanically.
Compare this to the 'dr650 on the road in the rain' example - traction loss is very sudden, the suspension and tyre arent heavily loaded at all, the engine is barely spinning. The system has to react very quickly but it also has to do a huge job with limited resources; the bike's native TC is barely loaded so it all has to happen by just cutting power and hoping for the best, and this is on a bike designed for easy to control traction loss with a slow thumper, heavy flywheel, soggy suspension and lazy geometry. The upshot is that you can tip the balance very easily with your weight and skill, but you need to be expecting traction loss in the first place for that to work. Hence git gud by riding a dirt bike everywhere, hence another reason to learn on an enduro.
Now imagine the sportbike in the same example. Sudden spin, no tyre or suspension loading, steep geometry and a zipping i4 desperate to rev - you're going down, TC or no TC.
I leave it to the reader to draw their own conclusions about the relative weighting of skill, machine and electronics when it comes to not-crashing from wheel spin.
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Mar 21, 2021 13:41
