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Thread: Winter Layup - 1995 Ducati 900 Supersport

  1. #616
    Join Date
    4th October 2008 - 16:35
    Quote Originally Posted by OddDuck View Post
    A very quick note - no photos sorry - filled tank with fresh petrol, made sure sump was full and tried the first start on the engine since rebuild.

    No issues. Gave it a few squirts of the accelerator and then a few turns on the starting motor. It caught and ran. The engine made oil pressure after a few seconds (go moly-based assembly lube!) and after that it was good to give it some revs just to hear the sound again.

    Very pleased. Miss my bike.
    is there a more pleasing exhaust/inlet/engine sound than a ducati V twin?

  2. #617
    Join Date
    28th January 2015 - 16:17
    2000 Ducati ST2
    Lower Hutt

    yet more theoretical guff - this time it's dynos - nothing built though

    I've been laying low on things bike and garage for the last few months while clearing debt. It's been frustrating but necessary. Having time but little of anything else, I'd spent a few hours contemplating a home dyno setup and going through the details.

    Now, I want to be clear up front on this: I haven't built one, tested one, used one, hell I haven't even seen one properly. I saw a dyno setup being used once, on a car, from about twenty meters away. That's it, that's all the practical experience I've got here. Every word of what follows is old textbooks, pocket calculators, the internet, biros and refill, and more than a bit of OCD. I thought it was interesting and just possibly useful to someone, hence the post, but this really is just turning ideas over. The following is absolutely no substitute for practical experience.

    I'll also skip ahead to the conclusion, so we get the disappointment over with: it turned out that it really doesn't make sense to home build one, at least not for me.

    Estimated cost: $2000 and realistically my spare time for about half a year, that's if there aren't interruptions of course.

    I have one bike to tune. One. Once. And summer's coming. I could really use that $2K to actually ride with. It really does not make sense to take half a year and take on another project, to end up with a one-use tool that's been used and now clutters the place up, if this was successful at all anyway. It might make sense if I had many bikes to tune, or on going tuning, etc etc, but at this time I don't. Maybe later.

    So... the basics, apologies if I'm telling you stuff you already know:

    A dynamometer ('dyno') will let you take your engine to full throttle, while measuring output in terms of HP and torque, but also monitoring engine behaviours like air-fuel ratio, ignition timing and whatnot. It's basically the Holy Grail of things garage and engine tuning. This is not easy to do. As such, they aren't cheap.

    There are two basic types: Inertial and Brake.

    Inertial is simply a massive rotating mass. The bike accelerates the mass and by dint of cleverness, data can be extracted.

    Brake is the other type, capable of absorbing steady RPM and torque. It's way easier to read but much pricier.

    Some types of brake dyno:

    Friction - an axle with brake disc/s and brake caliper/s
    Eddy Current - an axle with discs and electromagnets
    Hydraulic Pump - a pump pushing oil through a needle valve
    Fan - basically the bike drives a giant air fan
    Water brake - a driven dished plate turns against a stationary dished plate, inside a flooded housing
    Force lubricated, oil shear friction brake - basically a wet multiplate clutch

    They've all got pros and cons but the most common is the Eddy Current.

    I became curious about the last type, the oil shear, so spent a while playing with the idea. The basic concept is that it's similar to a multiplate clutch: driven discs alternate with stationary discs. There's shear in a fluid wetting the disc surfaces. The assembly center is pressurised by an external pump so there's always a positive pressure gradient in the fluid between the center of the discs and the outer diameters.

    Basic formulae:

    For two plates, one moving relative to the other, with a fluid between them:

    shear stress = kinematic viscosity x plate velocity / plate separation

    Force between plates = shear stress x plate surface area

    Plates can be likened to discs, with an area and an average radius and velocity

    Torque per disc pair = force x radius

    Total torque = torque per surface pair x number of surface pairs

    and torque required to be absorbed, RPM, power etc can be readily determined via the bike's specifications for engine and gearbox.

    Of course it's basically just a clutch. I'd had the bright idea that I could make something that would replace the rear wheel. I'd counteract the torque via a handy through hole on the engine casings, with a spring balance measuring torque back to the body of the dyno. Presto, pop the bike up on a rear stand, remove rear wheel, fit dyno, take runs, success!

    Yeah... cutting a very long story short, after many, many calculations, it turned out that doing things this way means that the dyno has to cope with massive torque at full throttle. 540 Nm @ 1500 RPM, with my bike in 6th gear and the engine held at 6000 RPM. That's the lowest torque situation I could set up at peak output for the 900SS. There are space constraints of course, limiting the diameter and number of discs.

    The dyno's going to be absorbing between 60 to 80 kW of mechanical power. That's a lot of heating. I worked out that, to get around 10 C thermal regulation across the plate stack while under full load, I'd have to flow 2 litres of cooling water per second. That thermal control is needed because fluid viscosities tend to decrease significantly with temperature.

    After some work with disc diameters and spacings, the reason for the pump pressurising the center became clear: the only way to get acceptable torques is to run the discs at tiny gaps. 50 to 25 microns, or similar. They're basically in contact. The pump is necessary to make sure that discs stay lubricated and won't bind together, despite the conditions.

    Fluids: about the best I could find that was commonly available was 80-90W gear oil, with a viscosity of around 250 C.S. (centistokes), but it's rather temperature dependent. It gets slippy as things warm up. Golden syrup was contemplated (2000 C.S.) but again, same issue of temperature vs viscosity, and anyway I'd get a permanent ant infestation in the garage when the inevitable spills happened.

    Finally I did something I really should have done first: I took a pair of steel clutch plates and had a play, first with golden syrup, then with gear oil. It was messy but demonstrative.

    There is an effect. This idea does actually work. However it's a weak effect, even with something as heavy as room temperature golden syrup, the plates pretty much have to be ground into each other for there to be significant countertorque.

    Further calculation showed that the bike-mounted dyno is possible. With 21 plates around 500mm diameter, 80-90W gear oil, and cooling, this could (in theory) just work. However it's pretty marginal, the space constraints of replacing the rear wheel really limit what can be done (about 80 to maybe 100 HP). In the real world I'd say a roller system built into some kind of structure is far more realistic.

    Material: a full sheet each of steel and aluminium (as a bearing pair) would be needed for the plates, plus laser or water cutting, plus an axle, hub, housing, pressure plates at each end, seals, pressure pump, balance scale, RPM counter, etc etc... you'd be lucky to get away with $2K, or so I think.

    Noise: this is full throttle sustained running in suburbia. It's not going to happen for very long before I'd have conversations with officials, unless I can build some way to effectively muffle the bike's exhaust note while not changing the engine performance.

    Yep. Nice idea, good to play with, not going to happen. At least I didn't start spending money.

  3. #618
    Join Date
    14th July 2006 - 21:39
    2015, Ducati Streetfighter
    Blog Entries
    Pretty sure you could find a company with a Dyno and talk a deal well under that value to tune your bike.

    And the suburbia thing. Bugger em - run it during the week and make the bored housewives damp with the sound of a Ducati at revs

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