Author Topic: 2T supermono  (Read 1008 times)

casal-fan

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2T supermono
« on: April 10, 2017, 08:56:41 PM »
This idea has been brewing for a long time.
Started finding parts to build it for a couple of years now, and since topend work is almost finished. I thought I could post here. Something different.

The doner bike will be a cagiva Mito 125.
Great little bike with chassi capabilities that go wayyyy further and cope with much more then the 125cc engine can deliver.
Furthermore it will be upgrated with full WP suspension set, as well as 3.5-4.5 rims

The engine choice fell on a spanish engine.
A gasgas blok with balance shaft - and those 2 last words were the key for the decision. But also parts availability, price, etc...
Cylinder, head, etc were sent to the UK to someone that (otherwise then me) knows what heīs doing, and working together and exchanging ideas we got to a good compromise on how to go about working on the engine (he did the work - I mostly talked nonsens ;D)

A great deal of carving in the cylinder, a great deal of attention going into transfers ducts, exhaust ducts, time consuming for the tuner to make molds while porting to get things exactly right (thanks tuner (-P)) etc all this helped along with engmod simulations, wich was showing mid 60s at very low 9500 RPM with good overrev capabilities (the low RPM was my choice... a 72mm piston is... big. Inercia forces calculations sugest that the stress will be... a lot, even at only 9500 rpm.)
But even if the simulations translate into midt 50s at the rear wheel, another key factor for going fast is massive... torq availability.

Next step, do the very last finish work on the cyl. (chamfer, polish) and then store everything until assembly time comes.
Engine is at moment dissasembled, and crank has been done with a 129mm rod instead of 125.5 (no pics of that sorry...)
No breath holding here... it will take time.
Hope youguys like the pictures.

James P

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Re: 2T supermono
« Reply #1 on: April 11, 2017, 10:16:41 AM »
That looks like a great project Rui. Keep us updated on your progress!

A few years ago, I met someone who had built a Mito with a fuel-injected CR500 motor :o . It was in one piece, but wasn't running and needed more work. I wonder if he is still in one piece...  ;D .

Regards,
James

EEKNOWS

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Re: 2T supermono
« Reply #2 on: April 11, 2017, 03:46:00 PM »
Gas Gas is a good engine they will rev to 10,000 no problems all day. Over 10,500 for extended periods will be 1 meeting/track day for a big end. Mito frame is good enough. Keep the standard footpegs with the rubber mounts  ;)

casal-fan

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Re: 2T supermono
« Reply #3 on: April 11, 2017, 11:43:54 PM »
Thanks James.
Hope it runs good... when the time comes. Gonna do my best.
Vibes seem of course to be the great prob here. Hope the counter shaft will do good here.
Eeknows, good tip on the pegs ;D The idea is also to mount the front of the engine on vibedampers, guess where that thought came from ;D
The RPM  numbers you mention above. Are those for the 66 or 72 piston.

Another very important aspect on choosing the engine was the gearing.
Being a trail oriented engine, the scaling is not too bad. Actually is the nearest to a road race engine I could find, out of a few engines I looked into.
Secondary gearing was also limited by space between the engine output shaft and the swingarm pivot, so the engine would have to be kept within std. size sprockets. Here, the Gas was the only engine I found that could satisfy this. Remember that CR250 would have to carry very large front sprocket if it was to go somewhere...

Gearing chart with 14/37 sprockets 196km/h (wishfull thinking ;D) with around 1350 RPM drop 5th-6th when shifting at 9750RPM.
Added bonus - those sprockets sizes or if required for lower top speed are of the shelv items (-P)




EEKNOWS

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Re: 2T supermono
« Reply #4 on: April 12, 2017, 10:35:21 AM »
Rui, thats with the 72mm stroke (in a superkart) 68mm will do 10,500-11,000. Your top gear (0.78) is very high that might want addressing in the future as with high gearing it might not want to accelerate. Won't know that until you give it a run  :D

casal-fan

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Re: 2T supermono
« Reply #5 on: January 07, 2018, 01:36:10 PM »
Some more research yesterday evening.
This time, trying to visualise the differences on chassis setups between the chassis to be used wich will be the cagiva mito, and TZ 5KE

Interesting stuff, I think... while miles apart in one way, quite similar anyways.
Longer wheelbase on the mito, and visibily an outrageous effort from Mr. yamaha to bring weight to the front wheel on the 5KE.
Still, generally, its not bad at all... lifting the mito rear a bit (easily done by the damper to be installed wich has 10mm height adjustment buit in) would bring steering angle and swingarm angle to closer 5KE values...
The Mito tank is a bit of a problem.
While early racebikes had long tanks, look at the 5KE, its short (NXA is also quite short), the mitos is long and humpy... not so good for riding the thing on track.
Hope to be able to do something about this, maybe a 50cc tank? Input/suggestions are welcome here... as always.
I got hold of a NF5 or NX5 tank (93 RS - never know if those are F or X), but that ainīt gonna work... shame itīs aluminium, I even thought if it would be a good idea to strap it to the ground as it might just rise to the sky... it weights nothing.

Something new in the powerplant department also... but that is another story... will update soon.




casal-fan

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Re: 2T supermono
« Reply #6 on: May 11, 2018, 10:51:44 PM »
Long time since Iīve written here...
Mainly decisions on what to use.
Well, I have this very neat tigcraft chassis with a rotax mono 4stroke, but since I donīt like to ride 4trokes, decided this chassi would be better for a 250 supermono 2stroke.
The tigcraft chassis is designed for a mono 4strokes that does vibrate a lot. Solutions in this chassi like f. eksempel the swingarm pivot not bearing the engine (where a lot of 4stroke monos do) is a great designwise start point to mount a single cylinder 250cc engine.
The chassis is also shorter in wheelbase then the mito, while still boosting a fairly long swingarm, 1355mm while 1375 for the mito chassis.
Also have some goodies for the tigcraft frame, WP stuff all round, 40mm front, trying to achive something very light in apearence. The tigcraft frame is sturdy and small in apearence.
Powerplant will be a gasgas 300 botom end from the quad, wich is equiped with balance shaft, coupled to a cylinder kit from Wiwa Rennsporttechnik in Germany specific for this engine blok. A 72mm stroke, 66.4 bore setup, capable of mid 60s horse power I supose.
Got it as a kit, with recommended ignition timings as well as pipe design, head volume design for 102 RON, all this delivered from Wiwa, superkart stuff.
An interesting mix...

Still have the engine with the topend that Martin prepared, with powervalve wich is a bit more tricky, but always a possibility to use that engine in this frame as well, as the engine bloks are the same.
Building something like this bike, very simple and yet very efficient (hopefully :)) 2stroke has been in my mind for a long time... hope I can do it.

Some pics:


thump566

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Re: 2T supermono
« Reply #7 on: May 12, 2018, 08:27:31 AM »
Stunning, still have the urge to build a supermono although yours is looking far better already than I could achieve. look forward to more updates as they happen  :)  (-P)
Still firing on two.