Quote:
Originally Posted by oldbmw
This is not a fixed thing Margus. On the diesel bike forum where bikes often run with a max of 9hp, transmission loss can be substantial. For instance the old Enfield gearbox when filled with grease as recommended consumes over 25% of the crankshaft power. Switching to 90 grade oil halves this, with a further decrease of power absorbtion when using 50 grade oil. The new (since 1995 or so) 5 speed gearbox on 50 grade oil will often allow a bike to go an extra 10mph, as it absorbs very little power.
Many modern bikes run the gears in the very thin synthetic engine oils which reduce drag to a minimum, wheras the bmw's use separate gear oil of 75/80 weight. Worse for BMW's is that the gearbox is directly coupled to the engine, so there is no primary reduction. This means the input shaft turns 2-3 times faster. to compound this problem, the output shaft is offset to drive the shaft. This forced BMW to use a gearbox that not only spins fast but final drive is via transverse cogs, all of which conspire to drain power. Old Gearboxes and most car gearboxes usually lock the input to the output on the same shaft, so in top gear power is not trasferred by energy wasting gears at all. This normally can easily be checked by looking to see if top gear is shown as a 1:1 ratio. Final drive ratio should always use the least amount of intermediaries for highest efficiency ( and probably component life).
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Hmmm...
I did dynoed my GS and guys who've dynoed most of the bikes and have extensive experiences on this field said the BMW-spec gearbox/paralever drive-train is within the power loss spec of most of other modern shaft-drived systems (or my over 60Kkms done GS performed well above factory performance specs to "cheat" the dyno machine results calculation), including japanese bikes with wet clutches and gearboxes floating in the crankcases in a very thin engine oil. I use spec oils only: usually 75W-90 GL5 both in gear- and bevel box.
The pro for BMW engine+gearbox system is all the shafts rotate parallel, which, at least in theory, should give the best mechanical efficiency and longevity - i.e. lot of unusually high mileage BMWs, Honda STs & Goldwings, some Guzzis. Engines where 90-degree turns are done have their own "uneffective" losses. So maybe that's why on dyno the BMW specs the same as other shaft-drived bikes that have all the friction going on in the thin engine oil, but with 90 degree turn at one point before it's transferred into shaft(?) I.e. bikes like Yamaha Diversion, Suzuki GS or VX, Honda XLV etc.
I agree the power loss ratio does vary, that's why I use the word "approximately" anyways. Depends on the temperatures of oils, mechanical details (i.e. Klingelnberg-Polloid spiral pattern in BMW paralevers), ratios, oil viscosity etc. But with modern shaft drived bikes the power loss is around 18% range running in common transmission oils in bevel boxes. I'm sure Enfields etc with different oils (even full of grease variant as you said) have different results and probably have more power loss than the modern siblings.
Last edited by Margus; 17 Jan 2008 at 08:18.
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