DSL Posted August 30, 2007 Share Posted August 30, 2007 When up on axle stands engine running in neutral the wheels slowly rotate, not enough drive to move the car when on the ground, infact you can stop it with a light application of a single finger Link to comment Share on other sites More sharing options...
Tony C Posted August 30, 2007 Share Posted August 30, 2007 The oil in the gearbox BRG Brooklands SV 😬 It seems that perfection is achieved not when there is nothing more to add, but when there is nothing more to take away. (Antoine de Saint-Exupery) Link to comment Share on other sites More sharing options...
old captain slow Posted August 30, 2007 Share Posted August 30, 2007 No idea really, but maybe it's oil circulation from the rotation of the 1st motion shaft in the gearbox just enough to turn the output shaft. Seems a bit unlikely though. C7 CDW Link to comment Share on other sites More sharing options...
Alex Wong1697456877 Posted August 30, 2007 Share Posted August 30, 2007 Friction? Even though the dogs are not engaged, ther gears are still rubbing on the shaft and enough to transmit to the lay shaft, and to the output shaft? Just guessing. Link to comment Share on other sites More sharing options...
Nifty Posted August 30, 2007 Share Posted August 30, 2007 Answer A Keep off the straight and narrow 😬 Edited by - Nifty on 30 Aug 2007 14:28:47 Link to comment Share on other sites More sharing options...
k80rum Posted August 30, 2007 Share Posted August 30, 2007 Indeed... the paddle-wheel effect. Darren E Website and Emerald maps library Superlight R #54 Link to comment Share on other sites More sharing options...
DaveK Posted August 30, 2007 Share Posted August 30, 2007 Internal friction in the gearbox When you turn the input shaft by hand in neutral its still stiff to turn due to the friction of the gears etc on the shaft. When the weight of the car is on the wheels this overcomes the friction in the box, but when you lift the car its enaough to turn the wheels Dave Link to comment Share on other sites More sharing options...
Nifty Posted August 31, 2007 Share Posted August 31, 2007 When in neutral there is no drive connection, the only think that connects the input drive to the output shaft is the oil. Under the conditions described by DSL the gearbox is behaving in a manner like a torque converter. Since it is not designed for this purpose it is exceedingly inefficient and hence the output shaft can be braked with minimal effort, the weight of the car on the driven axle being far greater than that braking effort and hence it does not turn the axle when the car is on it's wheels. The efficiency decreases yet further as the oil viscosity decreases and hence when the gearbox oil is hot the wheels often don't turn with the car on stands. The extra drivetrain resistance of an LSD can also prevent this habit from being exhibited. Keep off the straight and narrow 😬 Link to comment Share on other sites More sharing options...
Stuart Faulds Posted August 31, 2007 Share Posted August 31, 2007 Nifty, you've got a *BIG* problem in your gearbox by the sounds of it Link to comment Share on other sites More sharing options...
Alex Wong1697456877 Posted August 31, 2007 Share Posted August 31, 2007 only think that connects the input drive to the output shaft is the oil. The gears are all in connection, it's just the gears are not engaged onto the shaft by the dogs, but they can still turn on them if there's little enough resistance (ie wheels off the ground). Bear in mind the main shaft is spinning at 1000 rpm ish at idle, it's not suprising the gears are turning even if not engaged. To get paddle effect from oil, the two shafts would have to be separated and the gears on the main shaft would have to be turning. As they are engaged all the time (unless your layshaft parts company with your gearbox casing), it cannot be oil paddle effect and must be friction. Edited by - Alex Wong on 31 Aug 2007 09:04:00 Link to comment Share on other sites More sharing options...
Nifty Posted August 31, 2007 Share Posted August 31, 2007 Alex, A little research suggests the following: My analogy to a torque converter is a little incorrrect, a viscious coupling would be a little more appropriate. As you say the gears are all rotating around the shaft on bearings but not directly locked to it. However, the drag (or should I say higher resistance to shearing) of the cold oil around the bearing transmits a certain amount of rotational energy to the shaft and thus it turns at a speed considerably slower than the input shaft. As the gear oil warms up the viscosity and shearing resistance decreases, and thus the transmission of rotational energy decreases resulting in a slower or stopped wheel. When cold the oil is able to transmit a force sufficiently great to overcome the inertia of the drivetrain and thus the wheels turn when off the ground. However, it is a weak force that is readily overcome by applying a small braking force to the wheel, like your hand. Keep off the straight and narrow 😬 Link to comment Share on other sites More sharing options...
Nifty Posted August 31, 2007 Share Posted August 31, 2007 Must go to bed, I'm working parsley night shifts at the moment ❗ Keep off the straight and narrow 😬 Link to comment Share on other sites More sharing options...
Frying Pan Posted August 31, 2007 Share Posted August 31, 2007 I always thought it was magic pixies See some pictures of the build here. 12000 miles completed! Link to comment Share on other sites More sharing options...
Alex Wong1697456877 Posted August 31, 2007 Share Posted August 31, 2007 But is it that the cold oil is transmitting the force, or is it unable to reduce the friction enough to prevent it? the friction from metal on metal? BTW, top job last weekend. Forgive me if I looked blankly at you but I was only 95% sure as to who you were and didn't want to make an arse of myself? Link to comment Share on other sites More sharing options...
Nifty Posted August 31, 2007 Share Posted August 31, 2007 But is it that the cold oil is transmitting the force, or is it unable to reduce the friction enough to prevent it? The friction from metal on metal? Apparently a gearbox filled with water does not exhibit the same characteristics, so it is the drag/shearing resistance/surface tension/internal friction etc of the oil ... but we're now getting into the semantics of physics and chemistry .. so I think I'll bail out. Lets just agree that we were both right, since the drag of the cold oil can probably be explained by friction 😬 BTW, top job last weekend. Forgive me if I looked blankly at you but I was only 95% sure as to who you were and didn't want to make an arse of myself?I didn't notice that you did, in fact I seem to remember you called me by name at one point Anyway, I shouldn't worry about making an 🙆🏻 of yourself. I think I successfully secured that prize by taking the p155 out of big boy Lyall, only to spin right in front of the commentary box ... I think I handed him the whole of his afternoon's commentary on a plate. I also woefully underperformed for the engine spec so I think I win that prize too (though I was pleased to be consistently improving over the 5 runs). Keep off the straight and narrow 😬 Edited by - Nifty on 31 Aug 2007 11:20:40 Link to comment Share on other sites More sharing options...
Myles Posted August 31, 2007 Share Posted August 31, 2007 TBH, you can observe this effect on Scalextric cars etc. if you lift the rear wheels -the fronts will (commonly) rotate slowly - presumably due to some resonance. I'm not suggesting that this can explain why several kilos of wheel/tyre/diff etc. do the same thing btw... 🙆🏻 🙆🏻 🙆🏻 🙆🏻 🙆🏻 Alcester Racing 7s Equipe™ 🙆🏻 🙆🏻 🙆🏻 🙆🏻 🙆🏻 Alcester-Racing-Sevens.com Link to comment Share on other sites More sharing options...
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