Constructing the Nemett Bobcat

[doubletopParticipant @doubletop]

My crankshaft dilema has been solved. I knuckled down and fabricated it and opted for loctite and pins.

 

As this forum is a bit of a PITA when you have lots of pictures I've posted my approach over here.

**LINK**

I'm pleased with the result. Hopefully it will be of use to somebody

Pete

Edited By Doubletop on 14/04/2013 08:28:41

[doubletopParticipant @doubletop]

A question if I can please?

Can anybody see the reason for the odd dimension for the rocker support mounting holes in the head being 6.75mm from the manifold face rather than 7mm? The only reason I can see is to clear the 2mm cap head screws to be used for the manifold as the cente of the tappet roller end up offset from the center of the valve stem?

Pete

[Sub MandrelParticipant @submandrel]

Hi Pete

I noticed that one of the rollers isn't aligned with the valve in the pictures of Bobcat. I assumed it was a machining error, not built in.

The tappets on my Suffolk will only overlap their cams by 2/3, due to the need for the cams to avoid the crankshaft, which would othersise need to be pretty flimsy! Luckily they are hidden away…

Neil

[doubletopParticipant @doubletop]

My next question is how do you balance the crank and pistons?

In Brian Rees article on Brenda II (issue 4455) he explans the Tom Pascoe method for a V twin is to balance with one complete conrod and piston on the common crank. Does the same method apply to an inline twin when the pistons are on a common crank?

Pete

[doubletopParticipant @doubletop]

My next question is how do you balance the crank and pistons?

In Brian Rees article on Brenda II (issue 4455) he explans the Tom Pascoe method for a V twin is to balance with one complete conrod and piston on the common crank. Does the same method apply to an inline twin when the pistons are on a common crank?

Pete

[doubletopParticipant @doubletop]

My next question is how do you balance the crank and pistons?

In Brian Rees article on Brenda II (issue 4455) he explans the Tom Pascoe method for a V twin is to balance with one complete conrod and piston on the common crank. Does the same method apply to an inline twin when the pistons are on a common crank?

Pete

[Sub MandrelParticipant @submandrel]

Hi Pete,

From reading ETW articles and other sources its normal to balance each reciprocating weight with a rotating counterweight of half its weight, so the answer is yes. Obviously a horizontally opposed twin needs no counterweight and is well balanced.

Here's an oddity – on a holiday in france I bought a model boat magazine and my french was just up to reading it (Icould guess what I couldn't translate!) There was a design for a two-cylinder single-acting steam engine in it with a 90-degree crank arrangement. Very odd.

Neil

[Sub MandrelParticipant @submandrel]

Hi Pete,

From reading ETW articles and other sources its normal to balance each reciprocating weight with a rotating counterweight of half its weight, so the answer is yes. Obviously a horizontally opposed twin needs no counterweight and is well balanced.

Here's an oddity – on a holiday in france I bought a model boat magazine and my french was just up to reading it (Icould guess what I couldn't translate!) There was a design for a two-cylinder single-acting steam engine in it with a 90-degree crank arrangement. Very odd.

Neil

[Sub MandrelParticipant @submandrel]

Hi Pete,

From reading ETW articles and other sources its normal to balance each reciprocating weight with a rotating counterweight of half its weight, so the answer is yes. Obviously a horizontally opposed twin needs no counterweight and is well balanced.

Here's an oddity – on a holiday in france I bought a model boat magazine and my french was just up to reading it (Icould guess what I couldn't translate!) There was a design for a two-cylinder single-acting steam engine in it with a 90-degree crank arrangement. Very odd.

Neil

[JasonBModerator @jasonb]

Is there any chance that Malcom did the balancing and sized the weights accordingly so there should be no further balancing needed.

J

[JasonBModerator @jasonb]

Is there any chance that Malcom did the balancing and sized the weights accordingly so there should be no further balancing needed.

J

[JasonBModerator @jasonb]

Is there any chance that Malcom did the balancing and sized the weights accordingly so there should be no further balancing needed.

J

[doubletopParticipant @doubletop]

Neil,

I can see the need to do this but I'm asking how would it be done. I can see why 90deg crank on a steam engine, but single acting would seem that it missed the point somewhat.

Jason,

I assumed that Malcolm would have had the balance weights right but you would have missed the PM dialogue I had with StichNZ after he showed us his video

**LINK**6

He said it ran with some vibration until he added some extra weight to the middle counter balance. So rather than run it and then have to take it apart again to fix the problem I thought I'd ask what the method for balancing was for an in line, same crank angle, twin.

Pete

[doubletopParticipant @doubletop]

Neil,

I can see the need to do this but I'm asking how would it be done. I can see why 90deg crank on a steam engine, but single acting would seem that it missed the point somewhat.

Jason,

I assumed that Malcolm would have had the balance weights right but you would have missed the PM dialogue I had with StichNZ after he showed us his video

**LINK**6

He said it ran with some vibration until he added some extra weight to the middle counter balance. So rather than run it and then have to take it apart again to fix the problem I thought I'd ask what the method for balancing was for an in line, same crank angle, twin.

Pete

[doubletopParticipant @doubletop]

Neil,

I can see the need to do this but I'm asking how would it be done. I can see why 90deg crank on a steam engine, but single acting would seem that it missed the point somewhat.

Jason,

I assumed that Malcolm would have had the balance weights right but you would have missed the PM dialogue I had with StichNZ after he showed us his video

**LINK**6

He said it ran with some vibration until he added some extra weight to the middle counter balance. So rather than run it and then have to take it apart again to fix the problem I thought I'd ask what the method for balancing was for an in line, same crank angle, twin.

Pete

[Sub MandrelParticipant @submandrel]

Sorry Pete,

Lost in there is the simple answer, hang one piston off the crankshaft (instead of two) and then statically balance it. For other configurations, you can use suspended 1/2 as heavy weights insetad of each piston/con rod. Of course Malcolm may have done this before he found he needed more mass to achieve dynamic balance…

(I think there may be a picture of this exercise in the latest ME).

Neil

[Sub MandrelParticipant @submandrel]

Sorry Pete,

Lost in there is the simple answer, hang one piston off the crankshaft (instead of two) and then statically balance it. For other configurations, you can use suspended 1/2 as heavy weights insetad of each piston/con rod. Of course Malcolm may have done this before he found he needed more mass to achieve dynamic balance…

(I think there may be a picture of this exercise in the latest ME).

Neil

[Sub MandrelParticipant @submandrel]

Sorry Pete,

Lost in there is the simple answer, hang one piston off the crankshaft (instead of two) and then statically balance it. For other configurations, you can use suspended 1/2 as heavy weights insetad of each piston/con rod. Of course Malcolm may have done this before he found he needed more mass to achieve dynamic balance…

(I think there may be a picture of this exercise in the latest ME).

Neil

[Martin-tygParticipant @martin-tyg]

Hi all,

balancing a parallel twin engine is done, by the motorbike manufacturers, by having balance shafts driven from the crank. Not really looked into it, but the basic 360 degree parallel twin is going to vibrate. Anyone who has ridden an early Triumph twin will go along with this. If you want smoothe then you need 3 cylinder two stroke or a six cylinder in line four stroke – BMW make one (in a bike) and they are very well balanced.

Just use nylocs to hold it down

martin

[Martin-tygParticipant @martin-tyg]

Hi all,

balancing a parallel twin engine is done, by the motorbike manufacturers, by having balance shafts driven from the crank. Not really looked into it, but the basic 360 degree parallel twin is going to vibrate. Anyone who has ridden an early Triumph twin will go along with this. If you want smoothe then you need 3 cylinder two stroke or a six cylinder in line four stroke – BMW make one (in a bike) and they are very well balanced.

Just use nylocs to hold it down

martin

[Martin-tygParticipant @martin-tyg]

Hi all,

balancing a parallel twin engine is done, by the motorbike manufacturers, by having balance shafts driven from the crank. Not really looked into it, but the basic 360 degree parallel twin is going to vibrate. Anyone who has ridden an early Triumph twin will go along with this. If you want smoothe then you need 3 cylinder two stroke or a six cylinder in line four stroke – BMW make one (in a bike) and they are very well balanced.

Just use nylocs to hold it down

martin

[doubletopParticipant @doubletop]

Neil

Thanks; I had been asking whether there was a difference between a in line or V configuration, and I can't see why only one piston conrod assembly would be used for static balancing. Anyway I thought if thats the way its done I'll give it a try.

Suspended between two darning needles in my watchmakers lathe and clearly out of balance

A couple of washers hung on to get an idea what was required

A pair of brass weights attached to the middle counterbalance weight and now in balance

It needed balancing so don't assume Malcoms counterbalance weights are correct.

Pete

[doubletopParticipant @doubletop]

Neil

Thanks; I had been asking whether there was a difference between a in line or V configuration, and I can't see why only one piston conrod assembly would be used for static balancing. Anyway I thought if thats the way its done I'll give it a try.

Suspended between two darning needles in my watchmakers lathe and clearly out of balance

A couple of washers hung on to get an idea what was required

A pair of brass weights attached to the middle counterbalance weight and now in balance

It needed balancing so don't assume Malcoms counterbalance weights are correct.

Pete

[JasonBModerator @jasonb]

I was just looking at the video of Malcoms first run of the Bobcat, can't see much in the way of vibration

It will be interesting to see if the additional weights are needed

[doubletopParticipant @doubletop]

Jason

Thanks for the link, encouraging and now i know what I'm shooting for. I only asked about this as StichNZ had said his was out of balance and needed additional weight. If you go back to the other thread and find StichNZ's details you see he's an engine person. Also the latest ME showed balancing the crank for the V twin Brenda II. Based on the advice I thought I'd see if I needed additional weight.

We'll see and now I've made provision for the additional weights it will be easy to add subtract weights

Pete

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