billycraig Posted September 30, 2010 Report Share Posted September 30, 2010 Got a little debate going on and thought someone may be able to explain it well. I've been riding a 125 GG for a few years now and love it due to it FEELING lighter and more like a MTB. Back when I rode a 280 or 300 I always FELT that a 250 was heavier when I rode it but attributed that to having a little more power on the 280. Obviously this isn't the case with a 125 so can anyone technical explain why my 125 FELT lighter to ride. I rode a 250 in this years Scottish due to injury and again last weekend and both times It felt very heavy and cumbersome. I know the dry weights are similar so can anyone explain it so I can with my argument debate. Quote Link to comment Share on other sites More sharing options...
jordi Posted September 30, 2010 Report Share Posted September 30, 2010 Got a little debate going on and thought someone may be able to explain it well. I've been riding a 125 GG for a few years now and love it due to it FEELING lighter and more like a MTB. Back when I rode a 280 or 300 I always FELT that a 250 was heavier when I rode it but attributed that to having a little more power on the 280. Obviously this isn't the case with a 125 so can anyone technical explain why my 125 FELT lighter to ride. I rode a 250 in this years Scottish due to injury and again last weekend and both times It felt very heavy and cumbersome. I know the dry weights are similar so can anyone explain it so I can with my argument debate. Not really sure why, but I have heard that its something to do with the gryoscopic effect in the engine. But then what do I know Quote Link to comment Share on other sites More sharing options...
micm Posted September 30, 2010 Report Share Posted September 30, 2010 I was told the more torque the engine has, the heavier the bike feels, so I guess generally smaller engines will seem to make the same cycle parts feel lighter. However I did read a test of a 4RT Montesa sleeved down to 125 and it was said it felt as 'heavy' as the 250 original but maybe the torque was unaltered. Quote Link to comment Share on other sites More sharing options...
mikee Posted September 30, 2010 Report Share Posted September 30, 2010 jordi right in what hes say about gryoscopic effect in the engine....because the 250 uses heavy cranks and flywheel weight it has more gryoscopic weight than a 125... Still think the 225 with heavy flywheel weight is the best gasser going... Quote Link to comment Share on other sites More sharing options...
neils on wheels Posted September 30, 2010 Report Share Posted September 30, 2010 (edited) Billy, It's down angular momentum. The higher the rotating mass, the higher the rotation speed and the greater that mass is away from its axis, the more angular momentum there is in the engine. This translates to the bike being harder to deflect from its current direction and hence feeling heavier. Whilst the difference in overall weight of a 125 to a 250/280/300 is small, most of that weight difference rotates, i.e. the piston, con rod, crank & flywheel. Also the fact that a larger engine has a larger stroke increases its angular momentum & explains why a 225 (short stroke) will feel lighter than a 250 (long stroke) despite a small difference in engine size and overall weight. Regarding the future Trial 125 4-stroke. It has a relatively high rotating mass (there are lots more bits spinning round in a 4 stroke than a 2 stroke) plus as a sleeved down 250 it has a relatively long stroke, the same as the 250, & hence the crank, piston & con rod mass is spinning further away from its axis than in a 125 with a short stroke. Both of these factors increase angular momentum & hence the feeling that the bike is heavier when turning or moving it around. Edited September 30, 2010 by Neils on Wheels Quote Link to comment Share on other sites More sharing options...
mont Posted September 30, 2010 Report Share Posted September 30, 2010 Billy, It's down angular momentum. The higher the rotating mass, the higher the rotation speed and the greater that mass is away from its axis, the more angular momentum there is in the engine. This translates to the bike being harder to deflect from its current direction and hence feeling heavier. Whilst the difference in overall weight of a 125 to a 250/280/300 is small, most of that weight difference rotates, i.e. the piston, con rod, crank & flywheel. Also the fact that a larger engine has a larger stroke increases its angular momentum & explains why a 225 (short stroke) will feel lighter than a 250 (long stroke) despite a small difference in engine size and overall weight. Regarding the future Trial 125 4-stroke. It has a relatively high rotating mass (there are lots more bits spinning round in a 4 stroke than a 2 stroke) plus as a sleeved down 250 it has a relatively long stroke, the same as the 250, & hence the crank, piston & con rod mass is spinning further away from its axis than in a 125 with a short stroke. Both of these factors increase angular momentum & hence the feeling that the bike is heavier when turning or moving it around. Quote Link to comment Share on other sites More sharing options...
mont Posted September 30, 2010 Report Share Posted September 30, 2010 .yes I ride a 225 and it dose feel smaller and lighter than my 250 used to. Billy you will have to have a go next time I see you. Quote Link to comment Share on other sites More sharing options...
mikee Posted October 1, 2010 Report Share Posted October 1, 2010 Good write up there Neil ,, explains it very well... Quote Link to comment Share on other sites More sharing options...
billycraig Posted October 1, 2010 Author Report Share Posted October 1, 2010 Billy, It's down angular momentum. The higher the rotating mass, the higher the rotation speed and the greater that mass is away from its axis, the more angular momentum there is in the engine. This translates to the bike being harder to deflect from its current direction and hence feeling heavier. Whilst the difference in overall weight of a 125 to a 250/280/300 is small, most of that weight difference rotates, i.e. the piston, con rod, crank & flywheel. Also the fact that a larger engine has a larger stroke increases its angular momentum & explains why a 225 (short stroke) will feel lighter than a 250 (long stroke) despite a small difference in engine size and overall weight. Regarding the future Trial 125 4-stroke. It has a relatively high rotating mass (there are lots more bits spinning round in a 4 stroke than a 2 stroke) plus as a sleeved down 250 it has a relatively long stroke, the same as the 250, & hence the crank, piston & con rod mass is spinning further away from its axis than in a 125 with a short stroke. Both of these factors increase angular momentum & hence the feeling that the bike is heavier when turning or moving it around. Perfect answer thanks, I knew it was something along them lines but would of never been able to explain in. Thanks Mont, I'll always have a go on a 225, loved Kev's first one when I rode that and it does seem like the perfect combination of everything I like in a bike. Quote Link to comment Share on other sites More sharing options...
jordi Posted October 4, 2010 Report Share Posted October 4, 2010 (edited) Billy, It's down angular momentum. The higher the rotating mass, the higher the rotation speed and the greater that mass is away from its axis, the more angular momentum there is in the engine. This translates to the bike being harder to deflect from its current direction and hence feeling heavier. Whilst the difference in overall weight of a 125 to a 250/280/300 is small, most of that weight difference rotates, i.e. the piston, con rod, crank & flywheel. Also the fact that a larger engine has a larger stroke increases its angular momentum & explains why a 225 (short stroke) will feel lighter than a 250 (long stroke) despite a small difference in engine size and overall weight. I am sure thats what I meant to say Edited October 4, 2010 by jordi Quote Link to comment Share on other sites More sharing options...
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