OzVMX Forum
Marque Remarks => Yamaha => Topic started by: jimg1au on October 28, 2007, 04:09:13 pm
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HI
do you need to put the balance shaft into the motor of 360 or SC500 yamahas.the later model dt yzs don't have them and i was wondering if there is a need for it to be there.the motor is going in a pre 75 antig dirt tracker.
thanks
jimg
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good question jim,
a little further down the track i'll be starting on my DT360a conversion to an MX copy and was thinking of doing away with the balancer too.
i've read of it being done before on the 360 but unfortunately it wasn't a very technical description.
i think i'll be happy to leave mine on the bench when the motor goes back together.
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Jim,
I don't know much about counterbalancers in two strokes, but it's a ripper mod for fourstrokes that have them fitted, much better acceleration and a freer? reving motor. Just a little vibration, mainly at idle.
Although it may prove deadly to carry out such a mod on a sc500, I beleive they have a fairly explosive motor WITH the balancer fitted, could prove to be a fire cracker without it.
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Love it ;D ;D
Fella's single cylinder engine balancing keeps engineers in a job please remember that.
Some background.
Unlike twins, triples, fours (in-line and V config), V8's etc etc all have some parts going up and some going down thus the resultant forces tend to cancel each other out(Ducati have exploited this to it's potential). Singles however only have one part going up/down and one going round and round. If a graph is made of the forces made by the crankshaft assembly(on an X and Y axis) the line looks like a wave, going positive and negative.Balance in a single is expressed as a %, so a 0% balance factor may have the engine jumping up and down on the spot like a jackhammer and a 100% balance factor will have it moving backwards and forward.I hope I explained that well enough. There is a fairly complicated measuring/weighing procedure as well as the maths but most engines run around a 60% balance factor(reciprocating/rotating mass), An engine with a balance shaft has a 50% BF so when the forces can be cancelled buy running the balance shaft '50% out of phase' IE crank has -50newtons balance shaft +50 Newtons= 0 Newtons.
So having got all the BS out of the way Jim if your at all fussy and want the free revving engine a re balance is the order of the day.It will usually involve drilling 2 8-10mm holes on the same PCD as the crankpin. Job jobbed. I have removed balance shafts before but re-balancing the crank was 20 steps ahead of the cruder methods.I also think it contributed to some gearbox problems.At least with a re-balance you can determine where the worst vibes will occur(usually outside the rpm zone where the engine spends most of it's life)
HTH
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i 90% followed you Lozza, great stuff.
one more bit of clarifiaction please,
rebalance the crank with the balancer still fitted?
or,
rebalance the crank after the balancer has been removed?
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thanks for the infomation.i was going to use the crank out of my dt400 engine that i have in bits in a box.they don't have a balance shaft and are the same stroke as the dt360a.will check all this out later as i am going to be out of action for 6 weeks after shoulder reco
cheers
jimg
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I'm pretty sure that the DT360 was never sold with the 'Ohmiphase' balancer. It is in the PR release, press release etc but never saw the light of day.
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HI HAVE BEEN RIDING MY MX360A WITH NO BALANCE SHAFT IN IT FOR THE LAST 3 YRS WITH NO PROBLEMS, LEAVE IT OUT, PARTS MAY BE HARD TO TRACK DOWN FOR THE BALANCER, THE DT360A COME STANDARD WITHOUT THE B/SHAFT, EVEN THOUGH THERE IS A PROVISION FOR THE SHAFT AS THE CASES ARE THE SAME BETWEEN THE MX AND DT,
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Omniphase love that one.
Rebalance after the shaft is removed, post a pic of the 2 cranks and it will be easy to see how the balance factor has been put into practice.
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i also have a 360 with no balancer, thing is the best runing bike i have off the line, never ever noticed it shake on the line or in transit.
on the other hand the TM lumps up and down like a pig at idle and wobbles Back and forth on the rev, as far as i know its standard ( not sure if they have a balancer or not) either way the MXA 360 is smooth as silk with it out. I also ran a superlight back in yesteryear on the flat stuff revved its 100cc tits over 11,000 and over 100kmhour, in a little bike you counldnt pick it either, smooth as silk. It might be a model/brand /size specific thing.
As a rule of thumb id leave it alone until you had somthing apart then maybe not put it back and see, if its ruff - stick it back in, if its not leave it out.
And in your case if your bike never had it - probally dont need it ......hehehe
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I wasn't aware that the 360 had a balancer, in fact I'm pretty sure they don't because on the SC500 4 speed the balancer resides where fifth gear lives on a 250/360 5 speed. I removed it along with the 4 speed gears when I fitted the DT250 5 speeder to my SC500 back in the day. I didn't see any need to rebalance the engine and it's still running like a Swiss watch in my old Champion frame 30 years later.
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Actually Freaky H100's have a balance shaft and you can easily pick it when the balance shaft is not installed.Rebalancing the H100 crank is where I was forced to learn about it.I had that baby to 15,500 on the dyno one day!
A balancer residing in the gear set is fairly unusual, as they have to be timed with the crank and that's hard when you can't see the marks.Would love to see any pics.
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Actually Freaky H100's have a balance shaft and you can easily pick it when the balance shaft is not installed.Rebalancing the H100 crank is where I was forced to learn about it.I had that baby to 15,500 on the dyno one day!
Soz read back Loz, what im saying is it also HAD the balancer in it but we pulled it OUT and never looked back, if anything on that little motor we found it drew power, we got to the stage we used 5w fork style oil in the tranny to cut drag, we just started getting stupid, i only ever over revved it once and it shot the top end out, i used a KArt digital tacho on the plug and while tuning and racing we found the power actually dropped off dead after 12K , the way it was tuned, so keeping it below that band it was a very stable almost bullet proof bike , infact it only every let me down once in years of racing it, and to be fair to it, the over-rev incedent was infact that single event, and it happened on the main sweeper at mallala when i was dragging with a 4 stroke at a SA titles, i couldnt think for the life of me how that bike was staying with me so i kept it pinned > then poof we rolled across the line and spent the lunch break swapping to the 2nd motor, after spending that final round in the pack i noticed some other strange occurances, such as a bike that for some reason could 5 click gears instead of the standard 4 ? WTF.... another cheater had stuck a CR gearbox in the cases, after we finished the event, i cornered the ealier bike and found out the prick had stuck an illegal 150cc motor in a 125cc MA classed event bike. That was the last round of the 99 state titles, we won the series outright, but after the dust had settled i thought about the cheating little worms, then later decided that was the last road race i would do.
Since then i started to mess a little with DB on farms and as in the other thread 'Re: where have the old dudes' gone fell out with the competition idea as i felt it was tainted. Last year i joined a Dt club bought some old bikes and went racing again, and that guy with the dodgey gearbox bike turned up in the same Dt club, after a big fall out all those years ago, it seemed time does heals all wounds, he now has his kid riding and swings in a side car, he even came up to congrate me on the classes we took out this year, and it seems when you take a step back and think about all the time and money you spent to chase a little plastic trophy, in hindsight, once you have it, you realise how insignificant it is, this year i found all the compeditors in pre 75 riders did what they could to keep each others bikes on the track. in years of competing its the first time i have seen that work. it was a pleasure to compete and meet new freinds. ( sorry to digress) so in closing dont bother putting the balancer back in............ ;)
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Never said the balance shafts don't sap power, first thing to be ditched when I put a CR gearbox in.I ran without the shaft for a season then I got a series of problems with 5th gear(jumping out) eventually traced to the shift drum(despite no visible signs of wear).While it was in bits I rebalanced the crank, vibes were not existant as were the gearbox woes,performance was big jump up compared to the unbalanced jobbie.The 15K was with an RS 125 pipe it just kept revving, peak was at about 12,500 but it wanted to rev an rev felt like it would have done that all day(it would not have handled the heat) but that was all the convincing I needed, as far as paying attention to crank balance factors.
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well if you have the tools lozza use em :O)
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(http://i96.photobucket.com/albums/l187/mx250a/balance.gif)
A lot of weight and complexity there. Probably a good thing not to have.