collapse

www.dimecitycycles.com


www.restocycle.com

www.CITYLIMITMOTO.com

www.jadusmotorcycleparts.com

www.bisonmotorsports.com

www.speedmotoco.com

www.cognitomoto.com

www.townmoto.com

www.sparckmoto.com

www.Moto-Madness.com

www.pistonsociety.com

www.steeltowngarage.com


Author Topic: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp  (Read 2718 times)

Offline JadusMotorcycleParts

  • DTT SUPPORTER
  • *
  • Posts: 776
    • Jadus Motorcycle Parts
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

  • DTT SUPPORTER
  • *
  • Posts: 1515
  • Careful With That Axe Eugene
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

  • DTT SUPPORTER
  • *
  • Posts: 776
    • Jadus Motorcycle Parts
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

  • DTT SUPPORTER
  • *
  • Posts: 3628
  • crazy as a fox
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

  • DTT SUPPORTER
  • *
  • Posts: 776
    • Jadus Motorcycle Parts
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

  • DTT SUPPORTER
  • *
  • Posts: 776
    • Jadus Motorcycle Parts
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

Offline killerx

  • Posts: 41
Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #36 on: Sep 26, 2017, 16:09:41 »
You should be able to drill some holes in the webbing of the rockers.
05 Victory Hammer(110hp 115tq @ the wheel)

73 Honda Cl350(Haven't decided what to do with it)

Too much of everything is just enough.

Offline JadusMotorcycleParts

  • DTT SUPPORTER
  • *
  • Posts: 776
    • Jadus Motorcycle Parts
Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #37 on: Sep 27, 2017, 03:13:54 »
You should be able to drill some holes in the webbing of the rockers.

Thanks man, this is the kinda advice/encouragment I was looking for  ;D  I am so sure I saw a great post about someone doing just that over on a Yamaha quad forum, but can't seem to find it now.  The only gains through this method though would be smaller holes right out towards the ends of the rocker.  Because drilling holes close to the fulcrum may make it lighter, but won't do anything for lightening the load on the cam or the valve spring.  Worth investigating on these scrap rockers though for sure!

Offline jpmobius

  • DTT BOTM WINNER
  • *
  • Posts: 1073
  • where does this go?
Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #38 on: Sep 27, 2017, 11:01:41 »
Reducing weight is almost always a good thing as long as strength is not compromised.  However, it is worth keeping in mind your end goal.  What do you achieve?  Do you have in mind increasing safe operating rpm?  Generally higher revs make possible higher output.  Or do you plan on more aggressive cam profiles which will increase valve train accelerations? (same consequences valve train wise as increased rpm) Valve trains are often the limiting problem in output because the natural frequency of the springs needed to control the valve train components becomes too close to the engine rpm.  Drop a spring on the floor and it will "ring" or oscillate for a bit at its natural frequency.  If you "excite" it by driving it to this natural rate (or a harmonic of the natural rate) it will try to continue this rate of oscillation instead of following the camshafts intent.  Commonly called valve float and can be very destructive.  The solution is to require a lighter spring (or stiffer)(lighter meaning lower in mass in this context) that has a higher natural frequency so as not to be excited by the available rpm.  A lighter spring  will of course require lighter valve train parts.  Valves, pushrods, lifters, spring retainers, etc all move (mostly) linearly so simply making them lighter overall works toward this goal.  Rocker arms on the other hand rotate, so how you lighten them is less straight forward.  Mass at the center of rotation is of little concern as it moves very little, where mass at the end of the arm(s) is very important.  However, loads on the arm(s) generally become higher with stiffer springs and/or increased rpm, so one should be careful when lightening these parts so as not to compromise the beam strength of the arm.  Likely you would prefer a stronger rocker than a lighter rocker - which is not to say you still don't want the lightest you can get!  Anyway, the point is to be careful and thoughtful when altering your parts.  I'd say the lightening you have done so far has no down side, but potentially reducing the beam strength will provide negligible gains at best and potentially unnecessary troubles.  Having your valve train disassemble itself at max revs can be unpleasant!
« Last Edit: Sep 27, 2017, 11:04:31 by jpmobius »
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

  • DTT SUPPORTER
  • *
  • Posts: 776
    • Jadus Motorcycle Parts
Re: Ton up SR250 - a cafe racer by the numbers: 100mph, 100kg, 30hp
« Reply #39 on: Sep 27, 2017, 14:08:47 »
Thanks for the inout jp!  I'll refer you to Reply #34 in this thread - just a few posts up on this page.  There I state all the engine mods I plan on doing with all the goals and some of the reasons.  Yes, lift the rpm ceiling to 10,000 rpm by putting the whole valve train on a diet (hence the whole rocker arm exercise) and using modern beehive valve springs - to avoid many of the problems you mention. 

Good point about the rocker mods though, this was my gut feeling...  Further mods might risk strength or may even result in flex at high rpm?