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Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 1920 times)

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #30 on: Sep 11, 2017, 17:12:18 »
Haha, yes, considered having it bump start only actually.  But with previous experience bump starting a high comp thumper (on my own), no thanks.  I'll keep some small practicalities on the bike  ;D

Offline CrabsAndCylinders

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #31 on: Sep 11, 2017, 17:58:43 »
Dont install the kicker, start it with a roller!

I did that on a couple bikes, it was fun and cool, I thought, except when stalling going up a steep hill during rush hour.
Lighter, Quicker, Faster.
ZX-14, 900F x 2, 1100F, R100, CBR600, SR500, GT500, RZ350, KZ1000 x 2, Moto Guzzi Lemans lll, CBX550, RD 350, 750 SOHC police special, RG250, TL1000R, GT750, KTM Super Duke 1290 R, Harris/Z-1, Norton 750 Commando, Green 77 KZ650

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #32 on: Sep 17, 2017, 10:44:52 »
It may sound like a boring start, and it isn't where a lot of other people start but perhaps should...  Tyre choice.

For this lightweight racer I will matching tyres to rims first.  Then re-lacing them to the stock hubs with the Jadus 18" stainless spoke sets.

The tyre of choice is the Avon road rider, for many reasons.  It is tried and true, can handle the speed and handles really well - from both reviews and personal experience with one up front on the other Jadus SR.

http://www.avon-tyres.co.uk/motorcycle/roadrider

The size of choice will be a 90/90-18 up front and a 100/90-18 at the rear - will be plenty wide enough for this kind of power and will keep things light (always good with less rotating mass).

This means that rim choice will be aluminium for light weight and a size 2.15"-18 rim for the front, 2.50"-18 for the rear.  Here are some guides I used:
https://ridewrightwheels.com/pages/motorcycle-tire-wheel-fitment-chart
https://www.scribd.com/doc/253007466/Motorcycle-Rim-Width-Tire-Size-Chart
http://dropbears.com/motorcycles/utilities/tyrerim.htm

So next pay check, wheels will be the first purchase. 

Offline stroker crazy

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #33 on: Sep 17, 2017, 18:51:51 »
If you haven't already chosen the rims, you could try Morad TC:
http://www.central-wheel.co.uk/rims/morad_rim/morad_rim.html

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 #34 on: Sep 18, 2017, 17:22:54 »
If you haven't already chosen the rims, you could try Morad TC:
http://www.central-wheel.co.uk/rims/morad_rim/morad_rim.html

Crazy

Good advice, have used their services many times and been very satisfied.  But this time I will go with a Swedish supplier who manufacture their own spokes and are very helpful - and also local!  They are a reseller for Excel rims too, so will go with them this time.  Will report if any major difference in quality to the Morad's.

Offline JadusMotorcycleParts

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Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #35 on: Sep 22, 2017, 03:31:31 »
Figured I may as well introduce some of the planned engine mods (at this stage - can always change).  So far no plans to do much with the bottom end, it'll all be improving gas flow, capacity and compression.

_ Custom made high comp 77mm piston (263cc, retains stock sleeve, no case boring required) - piston already measured up and sent to Special Piston Services in Australia - they specialise in one offs and a re super friendly with a lot of experience:  http://www.specialpistonservices.com  will try raise cr from 8.9:1 to 9.5:1 or even 10:1 if we can work out a good dome.

_  Hot cam, the most aggressive profile, regrind by Tighecams in Australia: http://tighecams.com.au

_ Ported head to allow better airflow - or, to allow the cam to do its thing

_ Custom titanium intake valve, SS exhaust valve, both with narrower stems and both much lighter than stock - will maintain same valve size though (more on that later)

_ Valve seats custom made from NBC to wick away heat from the ti valve better

_ Bronze valve guides - also for better heat transfer to the head

_ Single beehive valve springs

_ Titanium keepers, seats and retainers

_ Lightened rockers and tappets

Then I'll be doing a few things on the intake and exhaust side too - as mentioned previously some kind of crank case ventilation scavenging system and probably a harmonic intake.

As an experiment, I already tried lightening some rockers I had from a spare engine.  Good and worth while result.  Especially becuase the weight shaved is at the points furthest from the fulcrum - where they 'weigh' the most to the valve train. Those small tabs at the end of the rocker are for the decomp lever on the XT and TT model engines and because the rocker is a symmetrical part, Yamaha have used it on the intake side as well - where it is not needed (where neither are needed in the case of the electric start SR).  I will be converting to kick yes, but am hoping with the right technique, won't need the decomp set up from the XT engine.

Hoping to lift the rpm ceiling to 10,000 by putting the whole valve train on a diet with the aforementioned mods  ;D