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KTM 350exc engine spins backwards

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Hi guys,

Last month I fitted a new piston to my mates 13' 350exc and confirmed that the engine spins in the opposite direction to the wheels. I always suspected this due to the position of the cam chain tensioner but with the cam cover off and kicking the engine over it does go backwards!

 

I find this very interesting and wonder why - anyone have any thoughts on this.

 

I think the 450 and 500 KTM are conventional, I have no experience on the KTM250 or SX models.

 

Cheers

Aussie

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Think about it. Engine rotation to clutch (trans main shaft) to countershaft. That is three direction changes of rotation. The chain/sprockests do not change the direction of rotation, only gears.

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Yes Dirtrider, I thought that also, not sure.

William I realise that the covention is crank spins the same as the wheels, the clutch shaft spins opposite and the sprocket shaft the same as the wheels.

The question is, why would KTM spin the crank opposite to the wheels? There must be a very good reason. I have not seen it before and there does not seem to be alot of discussion.

 

Thoughts....

 

Aussie

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I don't know what the gearbox looks like on those, but they may have eliminated 1 stage of gearing to improve the packaging (and therefore need to spin the motor the opposite way).

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There are no bikes I know of that have an intermediary gear (simply takes up too much space) that will allow a crank rotation direction change. Crank direction of rotation is based on which side of the engine the chain is driven.

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All the bikes spin the same direction.

 

Not true.  The BMW 450 rotates "backwards" due to its use of a chain drive primary.

 

You have 4 shafts; rear wheel, output, main, and crank.  The wheel and output are chained, so they have to rotate the same way; forward.  The rest are geared to each other, so, the main rotates backward, and the crank forward.  Unless it uses a primary chain, or there is an idler gear somewhere on its own shaft between the engine and trans, or it has a coaxial input/output trans shaft arrangement like a typical automotive trans (like old British bikes and Harleys) a backwards rotating crank will rotate the wheel backwards.

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Hi Lowmass,

Posted Today, 12:20 AM

 

All the bikes spin the same direction.

 

Thanks mate for the discussion! I'm sure thats in the spirit Thumper Talk!

Shoot me down in flames, do you have any experience with this bike, if so you should know better, if not go away.

 

Back to the topic, I removed the cam cover off my mates 350exc and hit the starter button. The cam shafts definitely spin in the opposite direction to the wheels. Mabey the lower cam sprocket is driven via an idler gear thus reversing the direction of the cam chain and shafts but the crank does indeed spin in the conventional direction. I did not spin the crank when the cylinder was removed because KTM use a crank lock bolt to simplify timing. If someone has split the cases on a 350exc they might know and continue the discussion....

 

Cheers

Aussie

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 KTM use a crank lock bolt to simplify timing.

 

 

Excellent feature, BTW.

 

You hit on the cause of your confusion when you raised the question of how the cams are actually driven.  The lower sprocket is an extension of a cam drive gear that also drives the counter balancer.  Being geared to the crank, it turns opposite the crank rotation, and thus, so do the cams.

 

There have been relatively few bikes built over the last 50 years that rotate the crank opposite the wheels, the BMW 450 being the most recent one I can think of.  As to why they might have chosen to, there are a lot of folks who look at the whole BMW product line and think that the company is just philosophically driven to do things the way nobody else does.  But, it may also have been an attempt to have the torque reaction forces of the crankshaft and rear wheel run counter to each other, so as to partially obviate both.

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Yes Grey I think you have cleared up my confusion. Thankyou.

 

KTM do have some excellent deign features often aimed at simplifying things. I don't own one yet but am leaning more and more towards them. They do seem a little mysterious in some ways, as it seems there is not as much technical discussion as the Japanese bikes. The KTM riders are also amazingly loyal, this often does not help with discussion.

 

Cheers

Aussie

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Aussie

 

Umm did I miss something??  your response was a bit punchy

 

I was in no way trying to start trouble. Simply chiming in with a basic fact. 

 

BTW Grey I understand the  the word "All" was not technically accurate. I was simply responding within the context of the vast majority of MX bikes.

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I understand the  the word "All" was not technically accurate. I was simply responding within the context of the vast majority of MX bikes.

Words mean things.  Technical accuracy in a technical discussion is actually a fairly important element.  If you don't want to be misunderstood, you should avoid such generalities.  "All", in this case was not just technically inaccurate, it was flat out wrong in a black and white kind of way.

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I assure you I understand that words mean things. I lead a group of designers and engineers in the high energy Physics realm. I also am a good judge of character wich is why I will leave it there and say good luck with yours ;)

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I see. Because you were called out for making a fundamentally inaccurate statement in a technical forum, I therefore have a character flaw.

Very well. You enjoy the rest of your day, won't you?

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Hi Lowmass,

I do not want trouble either, I was hoping for a discussion on the topic and your blunt reply did not help. Mabey I should have put a question mark after my topic headline?

I am interested in KTM's and their design characteristics and would like to learn more about them.

 

Cheers

Aussie.

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Aussie,

 

I too was hoping for more discussion. I do a lot of mechanical design work and it is a passion.  It was not meant as a "blunt" arrogant , disrespectful, or flippant remark. It was meant to be simple to the point and as a lever to initiate more discussion. Apologies for offending but honestly not the intention. The topic seamed easy going as I read through the posts and I responded with that spirit. I was on the fly at work between various fires, that was all it was.

Also I too am very interested in KTMs design particulars. In fact about 6 or 7 years ago that interest started as I could clearly see that although they had a way to go with their understanding , the bikes were showing truly great design decision. I often study closley the design differences between jap and ktm. In my work we often are designing and building prototypes that have to perform first time around. This makes building a good product difficult at best. I have a room full of well educated engineers and Phd's. Their smart, and they know their math and science , BUT , that in no way produces a GOOD product. I use the design particulars of the MX bikes often in meetings to show how to do it. In the last few years KTM is becoming more and more the showcase in these meetings. 

 I would say the most impressive thing about KTM is their willingness to design apart from market pressure. A perfect example is their sticking to steel frames. To stay their was a risk. It was a decision guided more by solid engineering rather than bowing to the whims of the masses. In other areas they are cutting edge when compared to the rest. take apart their motors and you see attention to detail that you dont see in the jap bikes etc etc.

 

As well I am looking to buy new in the next yr or so. At the moment KTM is at the top of the list and its not because I like orange;)

Edited by lowmass
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A backwards rotating mass will reduce some of the gyroscopic effect of the wheels. Whether this improves handling is probably up for debate. However it WILL change handling. Two identical gyros spinning the same direction add angular momentum. Two identical gyros spinning opposite directions cancel angular momentum.

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yep gyros all over the bike influencing handling. it gets complicated by the fact that these gyros are spread out at different points relative to the center of mass. As well some are accelerating /decelerating at varied rates. whats interesting to me with the 250 and 350 sxf's is that the cam chain tensioner is on the tight side of the cam chain. Typically cam chains are tensioned on the loose side of the chain taking up slack on the run of chain that is not being pulled tight by the drive force. Im struggling to understand why they would do this as it seems you would have to build a very robust structure to handle the forces this way adding unnecessarily to the weight of the mechanism. However that chain mass whipping around in there is very dynamic. Perhaps the forces are not as they seem.

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