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Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 14610 times)

Offline Pete12

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #50 on: Dec 16, 2017, 18:22:07 »
Following this thread with a lot of interest, you're certainly analytical in your approach, very impressive.
I know that you really know your stuff with these little motors, but as you are chasing horsepower have you given any thought to removing the balancer shaft and then rebalancing the crank accordingly? Apologies if you've already covered this.

Offline hooligan998

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #51 on: Dec 17, 2017, 01:30:54 »
Great info in this thread.  In.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #52 on: Dec 18, 2017, 17:14:08 »
Following this thread with a lot of interest, you're certainly analytical in your approach, very impressive.
I know that you really know your stuff with these little motors, but as you are chasing horsepower have you given any thought to removing the balancer shaft and then rebalancing the crank accordingly? Apologies if you've already covered this.

No, not covered yet and very good point, glad you brought it up.  I did look into this and read up on it.  Basically, vibration is a horsepower sapper.  Plus it is incredibly destructive on engine components.  I could have balanced the crank shaft with the new piston, but you can only balance for first order vibrations in a certain rpm range - say It was balanced for 10,000rpm, it would be horrible through the rest of the range.  I would still like a smooth spread of power through the rpm range.  Most modern 4 stroke singles (if not all?) have a balancer shaft, and those engineers are chasing power too.  It's just better to get gains else where.  Like for example by removing the electric start starter clutch, sprocket and chain, I will be removing a lot of rotational mass right there  :)

Offline stroker crazy

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #53 on: Dec 18, 2017, 23:46:34 »
vibration is a horsepower sapper.

It also makes a machine unpleasant and tiring to ride.

Crazy

“Ride like the Wind” W.H.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #54 on: Dec 19, 2017, 13:31:09 »
Here are the items I will do away with.  I will also have the crank either welded or plugged at these holes - the oil delivery to the starter clutch will no longer be needed, so they can be blocked off (will double and triple check this).  When taking apart the engine I cable tie assemblies together in the order they came off and then place them in ziplock bags.  Along with a bunch of photos too!  I also take photos of any special orientations or bolts that are longer than usual - to remember their placings.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #55 on: Dec 25, 2017, 13:15:54 »
Merry Xmas all, hope you had a good one.  I was pretty stoked because the custom piston I ordered arrived the day before the weekend.  More about that later.

I have thought through (checked and double checked) the starter gear thing.  I have decided I will not be blocking any oiling galleys. The centre part of the gear rotates on a bushing.  The bushing has oiling holes very similar to other parts in the engine - the small end, the rocker arms, the clutch basket etc.  This rotates on a plain machined bearing inside the starter gear - it even has golf ball drilling dimples to encourage better oil retention.  On each side of this starter gear are some slots/details that indicate oil distribution.  So I decided to retain it so that the other engine components around it - mostly the drive clutch drive gear and the counter balancer drive gear, receive oil the way they are supposed to.  However, because not using the actual gear, I cut that part of it off and will have it turned down on a lathe to match the outer diameter of the core.  This will make sure it is balanced - don't want any minute wobbling things at high rpm!

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #56 on: Dec 25, 2017, 13:18:21 »
And on a completely different note, I reluctantly took on a commissioned build for a Danish customer.  I am now super stoked about it and I am glad he convinced me to take it on - it'll be fun.  But this adds another distraction to this project!  If you're interested, keep up with the build on Instagram and I'll keep posting progress there.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #57 on: Dec 27, 2017, 16:32:29 »
And here is the awesome custom piston from http://www.specialpistonservices.com in Australia.  I went to them because of their reputation and ability to produce one offs.  Plus they were great to deal with - Colin has a wealth of knowledge and is very friendly to have contact with.  They have stock of many different ring sizes and this sort of determines their piston diameter capabilities/offering.  In this case, it was a 77mm (3.5mm dia increase from stock) piston which equates to 263cc, or roughly a 10% displacement and compression increase.  It weighs in at about 330g's (without the packaging) which is roughly 10g (3%) heavier than stock.  I think this will be fine and hopefully the balancer shaft will take up the diff/vibes.

I have touched on this previously but will mention it again here because I am getting a tonne of questions on social media about it...  People ask 'why not use a warrior piston'?!  Or, why not go big and bore up the crankcases?  Almost every piece of engine tuning literature I have read advises against any one big change to the engine without compensating in other areas.  The engine will work much better as a package if each components is improved slightly and incrementally.  So, a bit more displacement, a bit more comp, a bit more cam (both duration and lift), a few more rpm (lighter valve train), a bit better flowing head, a bit better exhaust, intake etc etc.  Using a stock warrior piston requires both a crank case bore plus a new liner and increase displacement by a whopping 30% - making the engine way out of balance not just in respects to engine vibrations, but also the ability of the other components - how could you expect the stock SR head to flow 30% more air to match that new piston area?  And the crank to cope with that extra weight?  Although, these are just my interpretations and my decided direction.  I might end up being very wrong come testing  ;D




Offline jpmobius

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #58 on: Dec 27, 2017, 20:05:44 »
You are headed in the right direction!  Hold your course!
Mobius


On a long enough timeline, the survival rate for everyone drops to zero.

1973 RD350 Yamaha build  http://www.dotheton.com/forum/index.php?topic=66498.0

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #59 on: Dec 31, 2017, 09:59:37 »
I have now packed up the items that will be taken to the engine rebuilders.  I will have the crank rebuilt with new components and have the SR sleeve bored up to match the piston.  Should I provide the piston clearance spec or will they have a good guide for that based on bore, cylinder construction, piston and sleeve material and the fact that it's air cooled?  The workshop manual has a spec of 0.035 - 0.055mm.  Is this good?

Hope you all have a great New Years!