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Post by paulincayman on Dec 17, 2017 1:52:12 GMT 1
Evening all...
Still collecting parts for my 385 hybrid..using a +4mm long stroke crank. From what I've read it seems that a change of primaries to 2.68( I think) ratio is desirable and as far as I can tell that would be straight cut gears. so I was wondering if the same effect could be achieved by tweaking front and/ or rear sprockets whilst using the stock primaries..? Or is it more complicated than that.. I've been advised that my motor will be in the 60- 70 hp range . Not racing , short circuit Sunday fun use. Trying spend my limited funds wisely. Ha ha.
Cheers
Paul
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Post by Tobyjugs on Dec 17, 2017 4:54:34 GMT 1
Before the big bore engines became popular the straight cut gears were used in racing more for keeping the crank shafts from spreading (i think) as there was less axial force exerted at high rpm. The gearing can be easily adjusted using the right combination of sprockets. I have one bike with 70 hp and that has standard primary gears. The condition of the crankshaft was not known and it did start to spread, i just pressed it back into place and welded it (maybe not the best thing to do). Since then it has had a hard life, so i need to open it up and check it, but i still enjoy riding it too much.
If you do decide to get some straight cut primaries Norbo does a good deal on them, don't forget you will need new rubbers for the clutch and maybe a new bush for the basket and gear wheels (strongly advised)
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Post by midlifecrisisrd on Dec 17, 2017 11:37:22 GMT 1
Don't think you need to change the primaries on a 385. I didn't
I thought changing the primary gearing was more due to lower revving on longer stroke engines and allowing top speed to be altered beyond a few teeth on a sprocket
Oh and as said to reduce side thrust on the crank.
I may do it eventually as I run a semi floating crank
Steve
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Post by paulincayman on Dec 17, 2017 16:31:00 GMT 1
Thanks lads ,
Also got some good insights from Martin at MBD, agrees with reduces side load and crank spreading and added: increases gear gear box rpm which reduces load on tranny ,, increase output speed corrected with sprocket change. Thats the jist if it as I understood him.
Also answered my question about ballrace bearings and straight cuts working fine together. Nice chap.
love this forum.
cheers
Paul
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Post by arrow on Dec 17, 2017 19:58:49 GMT 1
Are you still going to run with an oil pump? If you change the primaries then you will need a different oil pump drive gear, and calculations on oil output will need to be made.
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Post by muttsnuts on Dec 17, 2017 22:02:55 GMT 1
you can run upto around 75 to 80bhp with standard primaries (IMHO), after that the power starts giving the gears and the crank a seriously hard time, running 70bhp(ish) will be fine, I have alot of customers with bikes pushing high 60's and low 70's running stock primaries with no reported failures to date
HTH
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Post by paulincayman on Dec 18, 2017 14:51:08 GMT 1
Are you still going to run with an oil pump? If you change the primaries then you will need a different oil pump drive gear, and calculations on oil output will need to be made. Something to consider.. Thanks Arrow. |
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Post by paulincayman on Dec 18, 2017 14:55:20 GMT 1
you can run upto around 75 to 80bhp with standard primaries (IMHO), after that the power starts giving the gears and the crank a seriously hard time, running 70bhp(ish) will be fine, I have alot of customers with bikes pushing high 60's and low 70's running stock primaries with no reported failures to date HTH Thanks Dave , I'm starting to think the straight cuts might be overkill ..especially considering my tiny circuit of an island . |
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Post by bare on Dec 21, 2017 18:59:38 GMT 1
Don't be Forgetting Why Yama fitted Helical primaries in the first place (These cost More to mfg)
Noise! helicals are semi quiet, Straight cuts Howl.
Your choice.
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Post by bazzer5115 on Dec 22, 2017 8:57:33 GMT 1
Well,straight cuts suppose to be noisy,but as I have a tuned ypvs running helical gears and a 421 cheetah cub running straight cuts I must say there is HARDLY no difference in noise at all,just a very,very slight whine,but you got to try hard to hear it.
Speaking through experience!
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Post by wallcraft on Dec 22, 2017 12:37:20 GMT 1
Well,straight cuts suppose to be noisy,but as I have a tuned ypvs running helical gears and a 421 cheetah cub running straight cuts I must say there is HARDLY no difference in noise at all,just a very,very slight whine,but you got to try hard to hear it. Speaking through experience! Especially when the whole of the gearbox is straight cut, you’ll never going to think shit that’s noisy. |
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Post by paulincayman on Dec 24, 2017 1:04:33 GMT 1
grew up with rattly pushrod motors....Its going to fun either way,, Happy Ho Hos
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