| Author | Message | ||
     Ray_Maines |
More specifically, other things being equal, would a third gear roll on produce the same Horsepower & Torque numbers and curves as a fourth gear roll on? How about first gear, fifth gear, Euro pulleys, smaller wheels, flat tires, whatever, you get the idea. | ||
| Mikej |
I think the answer is no because a dyno measures engine rpm and rear tire speed and how fast the drum spins up. A*B*C type of thing, so I think theoretically it shouldn't really matter much what gear you dyno in, but you get better results and more accurate readings in some gears than others. I think of it in terms of scale and somehow it sort of makes sense to me. Okay, I'll step aside and let the experts tell me where I'm right or wrong with the appropriate documentation and math and such and whatnot and whatever.  | ||
| Blake |
Mike forgot that it takes considerable power to quickly spin up a flywheel and that 5th gear avoids two pairs of geared power transmission and their associated parasitic inefficiencies. | ||
| Ray_Maines |
Mike: You're saying that smaller or less powerful motors would produce a better looking graph if they were measured in a lower gear? Maybe a Suzuki Habachi would make a better looking graph in 6th gear while a M2 should be run up in 4th? Blake: Most of the Dyno's we have access to are Inertia type Dynamometers and things like light weight wheels / tires and direct drive 5th gear transmissions would indeed, produce higher numbers. But I suspect that would be a small difference. I was wondering if a first gear run up would produce seriously different max. power number numbers than a fourth gear run up? Where’s Aaron when you need him?  | ||
| Aaron |
Horsepower is torque x rpm (divided by 5252 but that's a nit, just a scaling factor to make it match what some horse could do a long time ago). Gearing changes the torque & rpm makeup of the horsepower, but the product of the two, i.e. the power, doesn't change. For example, say I've got a motor spinning 7000rpm and making 75 ft/lbs of torque. That's 100hp. Run that through a 1.6 primary (35 teeth front, 56 teeth rear). The clutch will see 7000/1.6 = 4375rpm and 75 * 1.6 = 120ft/lbs. 4375rpm and 120ft/lbs is still 100hp, it's just made up differently. Same thing happens in the gearbox and secondary drive. The torque & rpm makeup changes, but not the hp itself. In other words, if you had a way of tapping into the crank to make your measurement instead of the rear wheel, you'd still measure the same horsepower. It'd just be made up of a different combination of torque & rpm than the rear wheel has. The Dynojet inertia dyno just looks at the acceleration rate of the drum, and the rpm it's spinning at, to determine horsepower. Obviously it accelerates faster in a lower gear, but it's also not spinning nearly as fast, so it sees a high torque/low rpm composition of horsepower. In top gear, it accelerates slowly but it's doing so at a much higher rpm, so the dyno sees a relatively low torque and high rpm composition of horsepower. But it's the same horsepower. Roughly. Now, if you plot torque & rpm with the Dynojet, 1st gear and 5th gear will look much alike. How can that be if the torque & rpm makeup are so different? Simple, it doesn't attempt to show rear wheel torque or rpm, it only shows engine torque and rpm. It measures engine rpm directly, and it extracts engine torque from hp and engine rpm. It literally won't even plot torque for you unless you use the tach pickup. Now, Blake is right too, there are frictional losses in the powertrain, and being in different gears can cause different amounts of losses. In particular, in 5th gear none of the power has to go through the countershaft at all, the transmission mainshaft just gets connected directly to the final drive gear, so that's the gear you tend to get the highest horsepower reading in. Finally, there's an effect that I don't fully understand that comes from having less gear reduction between the crank and drum. Blake explained this once, I'll let him elaborate. Last time he explained it, someone else on the board made the claim that what Blake was really describing was that less gear reduction in that path gives the drum more leverage against the flywheels and this somehow makes the mass from them have less effect on the drum's acceleration rate. So the bottom line is that you do tend to get slightly higher numbers as you go higher in the gears. But the bike isn't making any more power, the creeping up of the results is an artifact of the measurement method. BTW, every 6-speed XL equipped bike I've dyno'ed showed more power in 5th than 6th. So apparently the power path losses are more significant than the effect of taller gearing. | ||
| Timbo |
Cool. That's one of those questions I've wondered about but never asked. Thanks for asking Ray, and thanks for explaining Aaron. Timbo | ||
| Jim_Witt |
Aaron wrote: Now, Blake is right too, there are frictional losses in the powertrain, and being in different gears can cause different amounts of losses. That being the case (meaning frictional losses) wouldn't micropolising those components pickup addition ponies. Ya, I know I'm off topic, but I have this thing for micropolising and electropolishing. Cheers, -JW:> | ||
| Blake |
Jim, Racers often run a 10W30 weight oil to significantly reduce parasitic losses. Viscosity is one of the main factors affecting parasitic power loss. The losses associated with viscous friction ...
However, keep in mind that lower viscosity lubricants also reduce film strength and thickness thus reducing protection to highly stressed wear points. In the transmission there are also hydrodynamic losses (stirring of the lubricant). Back cutting of gears helps reduce inertia, but also it reduces surface area and the amount of hydrodynamic stirring losses. Polishing would help to promote an overall increase in film thickness since it would remove the little peaks and valleys that act to reduce the effective average film thickness. So polishing would have the effect of increasing average/effective film thickness. The better the surface and precision of a mechanical assembly the lower the viscosity of the lubricant required to protect it. One of the unavoidable parasitic losses is due to the heat of flexure. As metallic parts are loaded, they flex elastically, like a spring. As the work is performed to cause the flexure, some of it is lost due to the imperfect elasticity of the material. Some of the work in compressing a spring, or flexing a gear tooth is transformed into heat and thus lost. So as a flexed part is allowed to return to it's natural unstressed/unflexed state, it does not return all the energy which it absorbed under flexure. Some is lost and dissipated as heat. If you can stiffen highly stressed cyclically loaded components, you can reduce the parasitic losses of flexure. You might recognnize that there lies a catch. Why not make all drivetrain parts super big and stiff? Then we end up with increased viscous, hydrodynamic, and inertial losses. As with most things, it is a case of balance. Generally if mechanical components are sized for durability, their stiffness will be sufficient to reduce parasitic losses to acceptable levels. Viscous losses are generally more significant than those of flexure, especially at low load levels where flexure is significantly reduced. There are very few metalic alloys with nearer pefect elasticity (stiffness) than steel. Berrylium-copper is one. Unfortunately they are VERY expensive and/or are unsuitable for use in highly loaded drivetrain components. Advanced composites like metal matrix formulation may offer improved stiffness for future drivetrain components if their costs can be reduced. Optimum mechanical design is a very interesting subject of study that encompasses a wide range of engineering sciences. Sorry, I got carried away there. ![:]](http://www.badweatherbikers.com/buell/clipart/proud.gif) Yeah, polishing might help, it might not show up on a dyno WOT run though. | ||
| Blake |
Aaron, Think of your flywheel as a little dyno drum. The only way to more quickly spin up (accelerate) a dyno's drum is to apply more power (more energy over a shorter duration). In lower gears, the flywheel spins up more quickly. Just like the dyno drum at the rear wheel, it takes more power to accelerate the flywheel more quickly. That additional power absorbed by the flywheel never makes it to the dyno drum. The lower the flywheel inertia and the higher the dyno drum inertia, the lesser this effect. | ||
| Aaron |
So basically what you're saying is that reducing the total load on the motor allows the mass of the flywheels to become a higher percentage of the load, and therefore they have a greater effect on the results? Kinda makes sense I guess. I still think the parasitic losses of the gearbox's indirect paths are the bigger effect, as evidenced by the way my 6th gear results have always come out lower than my 5th gear results. I've seen a couple bikes that showed better results in 4th than 5th, too, I still don't understand that. My Blast is that way, so is Dale A's LSR Sportster. | ||
| Blake |
Aaron, I wish I could diagram the analogous circuit for you. I think it would be clear to you then. Sometimes it helps to exagerate a thing to help imagine the physics of it. Imagine the flywheel had a mongo huge inertia equal to that of the dyno drum. Sitting in neutral, just to rev the engine to redline would take over three times as long as it would normally take to hit the rev limiter on the dyno in 5th gear (overall drive ratio of 3.366). What would the dyno report for RWHP if you put the mongo flywheel bike on the pump? Answer... about 1/4th of what you would normally expect. Better results in 4th than in 5th? That is a puzzler. Maybe something to do with the type of wheels or tires? If you get a chance, try running the blast in 1st, 2nd, 3rd, 4th. I'd be surprised if the trend doesn't hold where higher gears yield more RWHP. Blake (lemme see, capacitors in series with a current limit and a step transformer in between...) | ||
| Ray_Maines |
Yeah, either that or a Turbo Charger............ | ||
| Buellnuts |
I guess I am quite possibly the stupidest Bastard on the planet and can not be taught anything new and wonderfull. I guess it's gunna take a crayon and a coloring book to teach me anything. Dummer-n-astump In The Rain Forest, Bob | ||
| Ray_Maines |
Thank You Aaron. What a guy! You too Blake. Second Stupidest bastard on the planet, RaY2K (aka Ol' Dirt Beard, Bone Crushers M/C) |
