Featherbed: Twin vs Manx, handling, weight, etc

I've read that they did a lot of of moving the engine forward and backwards before finding the optimal COG on the Manx.
Finest thing on two wheels? For its time it was the best roadracer available over the counter. And still competitive years after best before date.
Though I prefer riding it on track compared to my more modern race bikes, it has its drawbacks in acceleration and braking. But a Manx tries its best to stay on the wheels even when the rider does mistakes.
For comfortable touring I love the Vincent and for gravel I like the B44.
 
Fast Eddie said:
Yer think… I’d never thought of that… (I’m lying of course, I’ve obsessed over it in great detail) !

I‘d love a Manx. Just can’t decide which of the kids to sell first.

Personally, I have no interest in building a 500 twin Domiracer replica. But the cheaper alternative to a Manx that I am mulling over is a 920 twin Manx…

Obviously the 920 would have quite a lot more oomph. I believe the engines would way more or less the same. I had always assumed a twin would upset the COG and spoil the fine handling of a proper Manx. But having read through some of the recent Domiracer stuff I’m not so sure.

Trouble is I am yet to meet someone who owns / has owned a good Manx who doesn’t think it’s the finest thing on two wheels…!
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I have a couple of friends who have successfully run 920 featherbed road racers, and I ran a 920 in my featherbed at Bonneville, but there were some lessons learned in the process. First, they do shake a bit, and sometimes things fall off.o_O And in my case, I think the frame cracking was partly due to the higher horsepower. But they are rideable, as long as you get them right. If not, your hands will go numb after a few laps around a track. If I were building one for the street, I think I'd do my best to keep the piston and rod small end weights as low as possible, either with the JS Engineering kits, or something similar, not lighten the crankshaft, and make sure to have the engine balanced appropriately for the rigid mounting, whether vertical or with the Commando tilt. And I'd build it as a mid-range torque monster, not a high rpm screamer. And maybe rubber mounted foot pegs and handle bars, with bar end weights. But it could still be a challenge to make it comfortable for longer range riding. Still, it could be a great ride if you got it right.

Ken
 
Thanks Ken.

It would be my current 920 I’d use, which already has JS pistons and rods. The pistons are actually lighter than stock 750 pistons IIRC.

I‘ve tightened the iso’s up as an experiment (I run them much tighter than stock anyway) an it still surprised me how smooth it was (relatively speaking of course).

It wouldn’t be a road bike though Ken, it’d purely be a track toy.

We’ll see though… I’m only pondering…
 
I built my 850 for the Wideline way back in the early 80s very important to get the crank balanced for the hard mounts, just wish they had all the goodies they have today for the motor, but I built mine on the cheap when money was hard and was lucky I found a old English gentleman who knew all about balance factor for what I was building, I am still running the same set up today and my 850 is not to bad and have never had anything fall off it in the 39 years its been on the road and in all that time only lost one rear muffler bolt and one top gear box mount nut, not bad for 39 years, be good to run Jim's products in my Norton motor but the cost of things is way out of my budget, very important to get that balance factor right and the motor as low as possible and close to the front.

Ashley
 
Fast Eddie said:
That’s a good point, the twin had indeed had a decade of US driven race interest behind it.

’cam peelers’… I like that one !
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Lowboy frame in AMA races would only qualify if 200 or more were made in a single year,
so. . . . .
 
Fast Eddie said:
Inspired by the chat elsewhere about Rudi Thalhammer’s Domiracer, I did a bit of reading up on the Domiracers.

I confess, I was somewhat ignorant. I hadn‘t realised they were as good as they were.

In particular the commentators of the day noted the lighter overall weight of the engine, even without alloy barrels and use of exotic alloys for cases, and the lower centre of gravity. Both of these factors giving handling benefits.

I had always believed the twin could not sit low enough in the frame so has a higher COG. But, although it may sit higher, the lack of overhead cam box etc did give it an overall lower COG. Apparently.

Whilst it’s very interesting to read, has anyone ridden a Manx framed twin and a full Manx, and if so could you share your thoughts and comparisons please ?
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It is not the height of the motor in the frame but its distance from the front tyre contact patch which ii's important
 
Fast Eddie said:
Inspired by the chat elsewhere about Rudi Thalhammer’s Domiracer, I did a bit of reading up on the Domiracers.

I confess, I was somewhat ignorant. I hadn‘t realised they were as good as they were.

In particular the commentators of the day noted the lighter overall weight of the engine, even without alloy barrels and use of exotic alloys for cases, and the lower centre of gravity. Both of these factors giving handling benefits.

I had always believed the twin could not sit low enough in the frame so has a higher COG. But, although it may sit higher, the lack of overhead cam box etc did give it an overall lower COG. Apparently.

Whilst it’s very interesting to read, has anyone ridden a Manx framed twin and a full Manx, and if so could you share your thoughts and comparisons please ?
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Yes, I recall my weak attempt to convince you of this!
Good on you for reading up on it.

So vibration could be a problem with a 920 in a Manx frame. That makes sense.
What are the vibration levels like on a 500 Manx at high rpm?
I recall talking to an old time Westwood racer who held the lap record there with his 650ss engined Manx chassis bike. In 1962 He removed the Manx engine and sold it to install a new 650ss!
You wouldn't do that now, would you?
I asked him which version he preferred and the answer was, " I want to get one of those newer Nortons, the rubber mounted ones that don't vibrate"



Glen
 
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worntorn said:
Yes, I recall my weak attempt to convince you of this!
Good on you for reading up on it.

So vibration could be a problem with a 920 in a Manx frame. That makes sense.
What are the vibration levels like on a 500 Manx at high rpm?
I recall talking to an old time Westwood racer who held the lap record there with his 650ss engined Manx chassis bike. In 1962 He removed the Manx engine and sold it to install a new 650ss!
You wouldn't do that now, would you?
I asked him which version he preferred and the answer was, " I want to get one of those newer Nortons, the rubber mounted ones that don't vibrate"



Glen
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Well although I’ve never ridden a Norton twin engined fathered, I’ve ridden several Triumph / Nourish twin engined featherbeds inc one Manx rep frame. I’ve also ridden (what I like to think is) a well sorted Commando.

But having recently ridden a very well built, top spec modern Manx, I can confirm it feels totally different. Far sharper, far more precise. Basically, it felt like a thoroughbred race bike.
 
Mike40M said:
I've read that they did a lot of of moving the engine forward and backwards before finding the optimal COG on the Manx.
Finest thing on two wheels? For its time it was the best roadracer available over the counter. And still competitive years after best before date.
Though I prefer riding it on track compared to my more modern race bikes, it has its drawbacks in acceleration and braking. But a Manx tries its best to stay on the wheels even when the rider does mistakes.
For comfortable touring I love the Vincent and for gravel I like the B44.
Click to expand...
Braking can be improved, at a price ( whilst still retaining front drum brake) 🥁 with a upgraded Manx 4LS drum front.
 
Fast Eddie said:
Well although I’ve never ridden a Norton twin engined fathered, I’ve ridden several Triumph / Nourish twin engined featherbeds inc one Manx rep frame. I’ve also ridden (what I like to think is) a well sorted Commando.

But having recently ridden a very well built, top spec modern Manx, I can confirm it feels totally different. Far sharper, far more precise. Basically, it felt like a thoroughbred race bike.
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What where the vibration levels like?

Glen
 
t ingermanson said:
Would it be worth exploring a "LongandLow" boy? The front of the seat is much further forward on the Lowboy. Featherbed length would give a bit more knee and elbow room, not to mention long carb inlets. I certainly don't have much of a jockey build either. The reason I never went pro. Wink, wink...



They announced they were going to do a limited amount of race frames this year based off an original frame they have in their possession:

View attachment 82963
The "cam box" dent is an interesting feature.

I've got a few motors I could imagine slipping in there.
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I think the first featherbed Manx was SOHC
 
A good Manx is hard to beat on any circuit. A Seeley 920 should be faster and cheaper
 
acotrel said:
Buy yourself the 6 speed TTI box. Vibration is not a problem.
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The bike I rode had a magnesium 6 speed TTI. Just superb.

It also had drum brakes, and on this I agree with you that decent discs would be a big improvement.
 
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