ESE's works engine tuner

[peewee]

If the longer rod has consistently yielded more power, why have some manufacturers gone to shorter rods?

KTM used to use a 132 Rod with a 29.7 compression height piston, then a 129 Rod & now a 125 with a 38 compression height. I would think case volume (and cylinder height) would be the same on these motors based on these numbers. What would be positive about the shorter rod length?

have you ever tried to ride a 60hp dirtbike around a motox track ? i really doubt even the top pro riders had that much power. the japanese and ktm have been perfectly content for many years with about 45hp, which is quit a bit, considering the new bikes are barely 210lb. the main focus is not at all about power, but rather further development in chassis, suspension and weight loss.

how all the manufacturrs ended up at 66x72 with 125mm rods is a mystery to me. maybe it lends itself well to good packaging and handling.

think about this. for a common 250 dirtbike engine to reach 2.2 rod ratio like the rsa, would require 160mm conrod

[SwePatrick]

Breathing changes also with different rodlengths.
One can optimize certain areas with different rodlengths.

Short rods tend to breath better in lower rpm areas.

[Peter1962]

Wobbly, would the use of a longer rod + spacer plate be high on your priority list for tuning a ktm 250 MX bike ? What rod lenght en which brand would you choose ?

[Frits Overmars]

Wobbly, would the use of a longer rod + spacer plate be high on your priority list for tuning a ktm 250 MX bike ? What rod lenght en which brand would you choose ?

Peter, you might also ask Wob about the combination of a longer rod and a higher-placed gudgeon pin. That's what I would be looking for.

[Matt@TYGA]

Hi Guys,

Playing with a 110cc boxer twin with shared crankcase. Measured the case compression ratio at 1.45, which seems a little small to me. But not having much experience with the boxer doesn't help.

For the record; running straight pipes (not expansion chambers) as room is extremely limited, and the owner insists on using straight pipes so that he doesn't have to redesign his board to get them to fit.

The engine's fitted to a surf board. Propulsion is through a jetski style prop etc.

Without messing with the engine geometry it's gone from 35kmh up to 46kmh, mostly by trying many pipe lengths and a bit of reed valve and petal work, and several prop designs and pump diameters.

Anyway, the question I have is regarding the case volume. Anyone have an opinion on the ratio? I'm tempted to try something very different and see what happens.

[Peter1962]

Peter, you might also ask Wob about the combination of a longer rod and a higher-placed gudgeon pin. That's what I would be looking for.

Thank you for the suggestion, Frits !
(I did not even know that pistons for a ktm 250 were available with a higher placed gudgeon pin).

[Frits Overmars]

Neither do I, Peter. But most 250 singles use a 66,4 mm bore nowadays, so if KTM doesn't have them, chances are that you'll find suitable pistons elsewhere.

[wobbly]

The case volume would only need changing alot if you are looking for a big top end boost as I was looking for using a KTM for kart roadracing.
One model of KTM 250 has a 130 rod and I used a Vertex CR250 single ring race piston.
Yes,Frits is right ,all the 250 now have the same bore and stroke so anything will fit.
This combo needed an 8mm plate, and that gave a good case increase along with a plate behind the reed so I could fit in a VF4 from a later model KTM.

[Muhr]

Thanks you guys for valuable comments. I did some computer modeling and realyze that classical V "common finger" good enough and my idea of 90 degree pin offset require more bearing supports on shaft. Smaller torque variation not enough reason then.


Anyone know if intermediate part of jbb crank were one solid part or welding (press fit) of finger and disk?

Finally I am going to make 140cc V with 9500 rpm on max power which should weight below 3000 grammes including fuel injection.
It will be rather long ~4 month project. I think if I will make crancase precisiously then will dont need sealing ring on crankshaft.

This is my take on it.

[speedpro]

I took the RS50 out to Levels on the weekend for the Southern Classic and beat up some 250 4ts, 150 2ts and 150 4ts.
In race 2 I set the 4th fastest time of a grid of 35 bikes. Also got it up to 143km/hr with a wind break, ~136ish without. Pretty chuffed about... until I seized it at the fastest bit of the track and it put me in hospital with a bruised lung and coughing blood...

Well, your 50 is being well used Dave and even looked beautiful until we both ended up in the gravel.
http://www.grandprixmachine.com/blog...uthern-classic

141kmh in 1984 on a Suzuki GT50 bucket. Not much progress speed-wise really

[jbiplane]

This is my take on it.

Great Muhr!!! Me make wery close to your design, just medium part a bit wider. I did some 3d modelling and realyze V twin with common finger can be balanced perfectly. When I virtually rotate crank I achieved 0 centre of gravity fluctuation. Just balance should be relatively heavy compare opposites. May be
The main question now how sturdly fix medium part.

Could you show your assembly "exploded"? Seems pin need step in the center.

[ken seeber]

These look cool..

http://www.cyclenews.com/…/250cc-two...reetbike-bac…/

Nice colour too.

[tjbw]

These look cool..

http://www.cyclenews.com/…/250cc-two...reetbike-bac…/

Nice colour too.

Broken link ken?

http://www.cyclenews.com/2017/12/art...ike-back-baby/

More info on the engine would be good.

[Grumph]

141kmh in 1984 on a Suzuki GT50 bucket. Not much progress speed-wise really

I would suspect that Chris could do a bit better in a proper record attempt - Levels back straight is approached via a tightish 180 degree sweeper - and it ain't very long either...

[F5 Dave]

I considered it, although a smaller pilot would be ideal. Sadly it occurred to me bucket rules allow oversize which i had, where speed records won't.