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Four Stroke engine build?

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Kind of off the wall question about how the current Thumpers are designed and working for Trials. I have built a number of off road bike engines and lots of competition exhaust systems. The only actual "Trials" bike that I ever rode was a run down TL125 that I compared to my "Race 125 4-stroke" that weighed 20+ lbs less than the TL and put out twice the hp. The "OLD" style Thumper Trials bikes were all low rpm power but slow to rev and heavy flywheels for slow difficult sections. The newer bikes like the Montessa seem to be really snappy and quick throttle and I understand that they are really short stroke and big bore for their displacement.

If you were to start to modify one of the older engines (basic design like an XR200 modified to a 250 like the TLR trials bikes) or (vintage XL250 modified to 300-360cc with big bore/short stroke) would the desired result be a heavy flywheel or a fast snappy acceleration. Just how much flywheel would a "New Era" homebuilt Trials bike want/need. I understand that a CRF150R engine isn't quite right for Trials work! HA!HA! But what kind of trade-offs do you need? I watch the trick boulder jumping and Pro's climbing verticle hills and don't for a minute think that a TL250 type engine can handle that. I know that I need to get some seat time on a more current 4-stroke Trials bike but right now I am just working different ideas!

Thanks for any comments.

Swiss

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The Scorpa use Yamaha YZ/WR motors in their four stroke version. I imagine at a minimum they have carby, flywheel weight and exhaust mods. Probably ignition and maybe cams too.

I’ve ridden a late model four stroke Beta, briefly. It coughed a bit:p but was quite smooth.

I think that like 250F MX bikes, four strokes are “cheaters”. :bonk: I.E. Easiest bike to find your limits on. Also easiest to ride consistently near your limit for longer.

As an aside which doesn’t apply to an old bastard like me with no kids. I think young riders are best to learn trials or MX on 2 strokes.:banghead:

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let me let you guys on a little secret .i rode a box stock yamaha ttr125 and rode a bunch of sections and do you know that it is perfect for a box stock bike other than i would go smaller on the front sprocket!thats why i am a scorpa fan as far as 4-strokes go.

Kind of off the wall question about how the current Thumpers are designed and working for Trials. I have built a number of off road bike engines and lots of competition exhaust systems. The only actual "Trials" bike that I ever rode was a run down TL125 that I compared to my "Race 125 4-stroke" that weighed 20+ lbs less than the TL and put out twice the hp. The "OLD" style Thumper Trials bikes were all low rpm power but slow to rev and heavy flywheels for slow difficult sections. The newer bikes like the Montessa seem to be really snappy and quick throttle and I understand that they are really short stroke and big bore for their displacement.

If you were to start to modify one of the older engines (basic design like an XR200 modified to a 250 like the TLR trials bikes) or (vintage XL250 modified to 300-360cc with big bore/short stroke) would the desired result be a heavy flywheel or a fast snappy acceleration. Just how much flywheel would a "New Era" homebuilt Trials bike want/need. I understand that a CRF150R engine isn't quite right for Trials work! HA!HA! But what kind of trade-offs do you need? I watch the trick boulder jumping and Pro's climbing verticle hills and don't for a minute think that a TL250 type engine can handle that. I know that I need to get some seat time on a more current 4-stroke Trials bike but right now I am just working different ideas!

Thanks for any comments.

Swiss

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The flywheel on my old Beta weighs about 7lbs and is about 5 inches in diameter. I added another lb just for kicks. Man, you couldn't stall that thing.

I haven't weighed my Sherco flywheel but at a glance it looks a tad lighter but about the same diameter.

Those XR200's were kind of a mountain goat to begin with. A little more flywheel, very low gearing and some trials tyres would make it a fun crawler.

Have fun and let us see what you create.

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u do know there are two flywheels on a beta right? there is another on the left that is about as big as a normal bikes flywheel.

Hmmmm,, even on the Zero? If so, no wonder it's stump puller.

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ok... can't find an engine diagram... if ur bored pull the left side engine cover and take a look. u can almost see it in the casting of the rev engine cover right behind the water pump.

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The flywheel weight you talking about is on the 250 only I believe. I know my 270 doesn't have one, but I have seen a breakdown on a 250 and sure enough there was a weight behind the counter sprocket.

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OK Guys to get back to the question. IF you were to build a 190-200cc mini-Thumper using one of the 125-200cc Honda engines, would you currently build it with LESS flywheel so that it had more snap which the old Honda engines lacked? Or would you build it as a Plonker with lots of flywheel but little of the modern snap to the engine? I think that the OLD design was fine for its day of putting up sloppy hills and around big rocks but can't compare to the newer designs that let you literally jump from boulder to boulder and agressively attack a difficult hill.

It seems like a night and day difference between how the bikes work. Much more 2-stroke like and this doesn't mean that there is no flywheel because the OLD design mini-Thumpers had a lot of crank flywheel compared to the newer designs, not talking about added on weight or heavy magneto rotors.

Swiss

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Actually more flywheel does not mean more power, the HP and Torque numbers remain basically the same. A heavier flywheel will generate more Momentum which makes it harder to slow down or stop, but on the opposite side it takes just a bit longer to get that heavier flywheel moving also. So you sacrifice some throttle "snap" for some momentum which will keep the engine turning over when some force tries to stop it.

Same kind of confusion goes with the old wive's tale about Stroker cranks mean more Torque. Simply not true. For a given displacement engine the Torque can be optimized for either long stroke or big bore. You need to tune the engine to match what you want it to do.

How much flywheel does the Montesa/Honda 4T have?

Swiss

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If i had to guess not a whole lot as far as trials bikes go if it is anything like their 2-stroke... that thing snapped. first gear would rev out instantly... kinda annoying, like the chassis though.

Anyway, in the trials world that explosiveness is done by getting the engine spinning first then using the clutch to take all the stored energy in the flywheel to launch the bike.

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Actually more flywheel does not mean more power, the HP and Torque numbers remain basically the same. A heavier flywheel will generate more Momentum which makes it harder to slow down or stop, but on the opposite side it takes just a bit longer to get that heavier flywheel moving also. So you sacrifice some throttle "snap" for some momentum which will keep the engine turning over when some force tries to stop it.

Swiss

More flywheel means more stored energy. Rev the bike, deal out power with the clutch. That's how virtually everything in trials is done. The only snappiness comes from a quick clutch finger.

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