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Post by mike1972 on May 19, 2021 17:31:13 GMT 1
It would interesting to see what offset other people have used on the front sprocket for this conversion... I know the rs250/rd350 hybred conversion kits sold by fast from the past and mark dent all come with a 18mm offset sprocket , But would be interested if anyone has done anything different, With the 18mm sprocket the engine is pretty much central and the chain run is perfect, same place as it would be with the original rs engine in
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Post by mike1972 on May 19, 2021 17:39:48 GMT 1
Just a thought perhaps you can bring the chainline closer to the tire by some hub adjustments. It is always convenient to have non offset stock sprockets. Don't think its possible without an offset sprocket if you want the engine central
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Post by marrcel on May 20, 2021 8:02:49 GMT 1
Just curious. What is the better central? Hanging points of engine? Statical mass center? Dynamical mass center? Such as Middle of crank Middle of flywheel and crank together?
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Post by mike1972 on May 20, 2021 17:18:18 GMT 1
Just curious. What is the better central? Hanging points of engine? Statical mass center? Dynamical mass center? Such as Middle of crank Middle of flywheel and crank together? For me its the barrels and head in a central position in the frame IMG-20210508-WA0007 by Mike Bowker, on Flickr
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Post by andy748 on May 20, 2021 18:50:49 GMT 1
Hi Mike, daft question, are you fitting a remote choke set up? Andy.
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Post by mike1972 on May 20, 2021 19:09:57 GMT 1
Yes I,m hoping to fit a cable choke with a lever on the the side of the frame, I already have a nsr250 choke lever that will work , just need the brass choke plungers with a cable fitting , going to give Allen's a call to see if they have choke plungers with a cable fitting for these carbs , otherwise its trial and error to find something or modify what I have ,
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Post by 4l04ever on May 20, 2021 23:11:11 GMT 1
You can use a stepped rear sprocket, so that could save just under 6mm, and chain will still clear tyre and frame. Should end up closer to 12mm offset then.
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Post by andy748 on May 21, 2021 6:38:35 GMT 1
Yes I,m hoping to fit a cable choke with a lever on the the side of the frame, I already have a nsr250 choke lever that will work , just need the brass choke plungers with a cable fitting , going to give Allen's a call to see if they have choke plungers with a cable fitting for these carbs , otherwise its trial and error to find something or modify what I have , If they are Mikuni Tzr250 ones should fit, if you need measurements give me a shout I have some. Andy.
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Post by dusty350 on May 21, 2021 7:20:18 GMT 1
I would be concerned with such a large offset front sprocket. Have you played around with the sprocket spacer behind the front cog ? Pv is approx 19mm deep, but an Lc is 14mm, losing you 5mm off the offset. Issue then is does the chain contact the edge of the tyre, or even the swingarm ? Likewise, milling back the rear sprocket hub would save mm's on the offset, but again you may have tyre issues. If those options dont work, you could fit one of the blingy side cases Norbo sells with the extra sprocket shaft support built in. I think it would save wear on the output shaft bearing. Dusty
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Post by 4l04ever on May 21, 2021 20:57:18 GMT 1
On the standard Aprilia RS250 back end, the chain runs a fair way from the tyre. Looked to be more than 10mm away from memory. I looked at a bike with an 18mm offset front sprocket and the chain was about 10mm from the frame on the right side, so could go over 6 mm and still be clear of the frame and tyre.
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Post by mike1972 on May 22, 2021 8:02:43 GMT 1
thats interesting about the sprocket spacer didn't know they was different , theres about 14mm of clearance from chain to the tyre and frame ,the problem would be the rear don't really want to machine anything off the rear sprocket carrier as they are not the strongest to begin with, and I can't find a stepped rear sprocket anywhere , I ve already been looking at getting one of norbo,s side cases with the bearing as it will be the easiest way to fix it
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Post by mike1972 on May 22, 2021 8:17:44 GMT 1
Yes I,m hoping to fit a cable choke with a lever on the the side of the frame, I already have a nsr250 choke lever that will work , just need the brass choke plungers with a cable fitting , going to give Allen's a call to see if they have choke plungers with a cable fitting for these carbs , otherwise its trial and error to find something or modify what I have , If they are Mikuni Tzr250 ones should fit, if you need measurements give me a shout I have some. Andy. yes there mikuni carbs , there an 8mm dia brass plunger
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Post by dusty350 on May 22, 2021 8:46:35 GMT 1
You could counter bore your existing rear sprocket - if it's a 520 it will be 6mm deep - you could counter bore 2mm off that where the sprocket meets the hub, leaving 6mm on the teeth. Fit an Lc spacer instead of the Pv spacer, and your offset drops to 11mm which would be less stress on the output shaft bearing. The spacer on the right is Lc, on the left is Pv and the middle one is a spacer I had made for my 400 build; 20200923_115011 by dusty miller, on Flickr And the 2mm counterbore on the Talon rear sprocket; 20200923_114802 by dusty miller, on Flickr I actually didn't fit the rear sprocket in the end as I got a good run on the chain with an unmodified sprocket, but I started out with offset numbers as high as yours, so it became a mission to get it more acceptable for the sake of the gearbox ! Also remember the sprocket spacers are hardened, so milling a few mm off isn't really an option ! There is also some side ways tolerance on the chain, so although you want to achieve as straight a run as possible, a mm out should be accommodated without any problems. Not sure if issues with extreme offset is the reasoning behind Talon only stocking front cogs up to 9mm ? Everything after that is special order, up to 25mm and £100 to do it ! Anyway, food for thought ! Dusty
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Post by mike1972 on May 22, 2021 10:44:43 GMT 1
thanks for that dusty, will get an lc spacer turned up and hardened , just had a good look and I could probably move the engine to the left a couple off mm, just need to turn up some new engine spacers on the lathe , could possibly get the offset down to around 10mm
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Post by dusty350 on May 22, 2021 15:14:12 GMT 1
That sounds good👍 I've run with 9mm offset front sprockets before with no issues and no outrigger casings, so I think you'll be fine. Talon normally have the 520 9mm offset in stock too.
Dusty🙂
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Post by bryan on May 22, 2021 15:23:04 GMT 1
That’s the sort of bike I would like to build, only built a couple of crappy standard framed specials so far! Steve’s Build by Bryan Haynes, on Flickr
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Post by mike1972 on May 22, 2021 20:34:13 GMT 1
That's not crappy Bryan, that looks a nice build
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Post by peddrotzr on May 22, 2021 21:15:15 GMT 1
Sorry if you already know this however, I would also look at the lower chain run as the Aprilia RS Ypvs conversion the chain is very close and will rub on the to lower cross member. A few options, either fit idler gear to lift the chain off the crossmember or modify it to allow clearance I have worked on 5 of these conversions and all suffered from this. Best regards Johnny C.
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Post by 4l04ever on May 22, 2021 21:30:19 GMT 1
I would rather modify the lower cross member, as not keen on the chain idlers. My mate will be doing this to one of his soon. When he does it, I will try to get some pics.
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Post by bryan on May 22, 2021 23:10:32 GMT 1
That's not crappy Bryan, that looks a nice build Crappy standard frame not a crappy bike!😁 I always wanted to put a big bore striker motor in an ally spar frame.
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Post by bryan on May 22, 2021 23:42:38 GMT 1
You could counter bore your existing rear sprocket - if it's a 520 it will be 6mm deep - you could counter bore 2mm off that where the sprocket meets the hub, leaving 6mm on the teeth. Fit an Lc spacer instead of the Pv spacer, and your offset drops to 11mm which would be less stress on the output shaft bearing. The spacer on the right is Lc, on the left is Pv and the middle one is a spacer I had made for my 400 build; 20200923_115011 by dusty miller, on Flickr And the 2mm counterbore on the Talon rear sprocket; 20200923_114802 by dusty miller, on Flickr I actually didn't fit the rear sprocket in the end as I got a good run on the chain with an unmodified sprocket, but I started out with offset numbers as high as yours, so it became a mission to get it more acceptable for the sake of the gearbox ! Also remember the sprocket spacers are hardened, so milling a few mm off isn't really an option ! There is also some side ways tolerance on the chain, so although you want to achieve as straight a run as possible, a mm out should be accommodated without any problems. Not sure if issues with extreme offset is the reasoning behind Talon only stocking front cogs up to 9mm ? Everything after that is special order, up to 25mm and £100 to do it ! Anyway, food for thought ! Dusty I spoke to Takin this week about my front sprocket, that day as long as you don’t go under 4mm it’s ok.
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Post by mike1972 on May 24, 2021 17:27:15 GMT 1
Sorry if you already know this however, I would also look at the lower chain run as the Aprilia RS Ypvs conversion the chain is very close and will rub on the to lower cross member. A few options, either fit idler gear to lift the chain off the crossmember or modify it to allow clearance I have worked on 5 of these conversions and all suffered from this. Best regards Johnny C. Yes I did know this , my engine is fitted slightly higher and backwards to hopefully cure this , it meant slightly modifying the right side of the frame to lift the engine higher , the centre of the sprocket is now in the same hieght position as the the rs250 engine sprocket
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Post by mike1972 on Sept 17, 2021 11:10:30 GMT 1
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Post by mouse on Sept 17, 2021 17:31:26 GMT 1
Sorry to hear about your 'off' sounds painful!
But the bike looks beautiful, if that is any consolation!
Mouse
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Post by Mr Kipling on Dec 23, 2021 15:29:49 GMT 1
Any updates on this....
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Post by mike1972 on Dec 26, 2021 12:12:48 GMT 1
yes its coming along nicely, finished all the wiring now, everything works fine and have a nice spark, front and rear brakes fitted, just need bleeding. next job now is the pipes, picked up some alonze mid range pipes, with a bit of modifying with the tig welder they will fit fine
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Post by mike1972 on Dec 26, 2021 12:13:22 GMT 1
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Post by mike1972 on Dec 26, 2021 12:14:15 GMT 1
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Post by mike1972 on Dec 26, 2021 12:15:05 GMT 1
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Post by mike1972 on Dec 26, 2021 12:16:01 GMT 1
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