Fitting Jawa engine into featherbed frame

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Charkmandler

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I've bought a DOHC Jawa engine from the late 70's to replace my pushrod 350 engine until I get a chance to build a 500 for racing. I'm working on positioning the engine in the frame (wideline) and my idea position of forward and low is not possible because the bottom engine lug for the front engine plate is too wide for the frame rails. Pulling the engine back and raising (as can be seen in the photo) does not look right to me with the weight high up, any lower and there is room for the engine plate but not enough for a nut each side
A compromise can be reached if I put some flats on the inside of the lower frame rails, this will allow the engine to move down a little.
Would it be acceptable to heat the low rails and put flats on them?
I've read some people advise but I'm not convinced - any advice?

Thanks, Mark

Fitting Jawa engine into featherbed frame
Fitting Jawa engine into featherbed frame
 
I though brazed welding was safe on Norton frames. I'm not a welder nor a frame builder though.

Just wanted to say that looks like a lightweight fun ride project. :cool:
 
When Vincent motors are put into featherbeds, the bottom rails are sometimes replaced by plates to get the motor lower.

If you go the route of denting the rails, do it with the tubing cold. Cold working steel is less traumatic to the grain structure. As long as you don't create any sharp corners in the tubing, you should be ok. Preferably, support the back of the tube with a block with a matching radius cut into it, and do the denting all at once on a press or in a vise, rather than beating it with a hammer.

Unless it's a Manx frame, the steel is just an unsophisticated hi-tensile steel, and relatively soft. If it's a Manx frame, the tubing will be have a thinner wall and will be harder. The same techniques will work, just require more care. You should not have any problems if you've done anything like this before. If not, go to the metal supply shop, get some tubing and practice! Best not to be surprised.
 
t ingermanson said:
When Vincent motors are put into featherbeds, the bottom rails are sometimes replaced by plates to get the motor lower.

If you go the route of denting the rails, do it with the tubing cold. Cold working steel is less traumatic to the grain structure. As long as you don't create any sharp corners in the tubing, you should be ok. Preferably, support the back of the tube with a block with a matching radius cut into it, and do the denting all at once on a press or in a vise, rather than beating it with a hammer.

Unless it's a Manx frame, the steel is just an unsophisticated hi-tensile steel, and relatively soft. If it's a Manx frame, the tubing will be have a thinner wall and will be harder. The same techniques will work, just require more care. You should not have any problems if you've done anything like this before. If not, go to the metal supply shop, get some tubing and practice! Best not to be surprised.
Click to expand...
Thanks, thats good information. Not sure I want to go the route of replacing the lower rails with plates. No problem making backings for the tube with a radius.
Just for interest would you have a picture of the rails replaced with plates - I can't find one.
 
If you are serious about constructing a race bike, you should redesign the frame, making it shorter by cutting the top and lower rails. Featherbed frame geometry is odd at the outset! At least this gives you the opportunity splaying out the lower rails as needed. By cutting the downward tubes below the crossing with the upper rails, you have an ample length of tube to accommodate the change in angles. It goes without saying this has to be carefully planned and executed, preferably using 3D CAD. Tubes are rejoined by using inner tubes in an overlap fashion. Welding or brazing should be outsourced to a professional welder.

- Knut
 
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mdt-son said:
If you are serious about constructing a race bike, you should redesign the frame, making it shorter by cutting the top and lower rails. Featherbed frame geometry is odd at the outset! At least this gives you the opportunity splaying out the lower rails as needed. By cutting the downward tubes below the crossing with the upper rails, you have an ample length of tube to accommodate the change in angles. It goes without saying this has to be carefully planned and executed, preferably using 3D CAD. Tubes are rejoined by using inner tubes in an overlap fashion. Welding or brazing should be outsourced to a professional welder.

- Knut
Click to expand...
I don't really want to go that far and when the Norton engine was in I couldn't fault the handling.
 
What would be the opinion if I added a lower bracket to the frame at the front and had the front engine plates connected to the two brackets on the frame and just the top lug on he engine. On the speedway bike the top and bottom lugs are used as the engine plates use the engine as a stress member. In the featherbed case it won't and I'm thinking it would be ok.
 
Charkmandler said:
Just for interest would you have a picture of the rails replaced with plates - I can't find one.
Click to expand...
Fitting Jawa engine into featherbed frame


It seems the only real hard and fast rule of featherbeds is not to leave out the headsteady.

Slimlines had two tabs for the front engine plates. Does it make a difference? No idea. Give it go. Seems like it might allow the motor to float with only one attachment point on the front of the motor, but you might run into the fatigue limit on the plates sooner than later. Worst that happens is they break and you make new engine plates for both engine studs. Perhaps make the plates out of 6mm or 1/4"aluminum, so they are the "fuse" and break before the frame does. I always advocate (to whoever is willing to listen to my BS) for building the risk and failure point into the cheap and easy part to replace.

JAP single in a double cradle (Phoenix?) frame:

Fitting Jawa engine into featherbed frame


Looks to be solving for same problem as you're running into. Manxes were run upright though, so must be something there.
 
t ingermanson said:
View attachment 116781

It seems the only real hard and fast rule of featherbeds is not to leave out the headsteady.

Slimlines had two tabs for the front engine plates. Does it make a difference? No idea. Give it go. Seems like it might allow the motor to float with only one attachment point on the front of the motor, but you might run into the fatigue limit on the plates sooner than later. Worst that happens is they break and you make new engine plates for both engine studs. Perhaps make the plates out of 6mm or 1/4"aluminum, so they are the "fuse" and break before the frame does. I always advocate (to whoever is willing to listen to my BS) for building the risk and failure point into the cheap and easy part to replace.

JAP single in a double cradle (Phoenix?) frame:

View attachment 116782

Looks to be solving for same problem as you're running into. Manxes were run upright though, so must be something there.
Click to expand...
If I use one mounting point on the engine at the front but triangulate plate with two mounting points on the frame, the engine is held solid at the back so should not float.
Just found a photo of a Seeley Mk3 and the Jawa engine is just held by the rear mounting points.
 

Attachments

  • Fitting Jawa engine into featherbed frame
    Seeley Jawa.webp
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Charkmandler said:
Just found a photo of a Seeley Mk3 and the Jawa engine is just held by the rear mounting points.
Click to expand...
Seeleys have a much different head (head cylinder and head stock) mounting though, particularly the Mk3. The Mk3 also often times uses the "ladder" which is not fitted here. Perhaps the ladder is only desirable on twins? Don't know.

I'm certainly not saying what you want to do wouldn't work, just that there's a lot of variables going on. The closest thing is a Manx, so my inclination would be to follow that as your example. I think you ought to go for it and see what happens.
 
What has this guy done (see pics below, I realise it’s a different engine variant)?
Did he widen the frame?
He’s got the engine so low that the bottom fasteners are below the frame rail.

Maybe get the engine mount bosses milled, so they are narrower ?? There seems to be plenty of width there that’s perhaps not strictly necessary?

When you look at a Manx, they obviously got the engine as low as possible. To my eye your engine looks too high, and it looks like a top heavy engine already, but that’s just a layman’s observation.

IMG_3583.jpeg


IMG_3582.jpeg


IMG_3581.jpeg


HOWEVER…

This guy clearly thought it would be fine to mount ‘high’ in the frame… and it makes me wonder how the other guy made space for the engine sprocket / pulley… so maybe the ‘high’ engine is a none issue ??

IMG_3580.jpeg
 
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bottom front lug: how much could you narrow it for it to fit between rails as shown in original pictures? And also would the rest of the crankcase then also clear the rails further rearwards? IMHO, I would go for further forward engine position, rather than ultra low. And gearbox not too far forward relative to swing arm.
 
I think I'm pretty well decided to add an extra frame mount similar to the photo posted. This will put the engine low and forward with no issues for chain line etc and it looks right mechanically compared with mounting rearward and higher. Some of the wideline frames have a lower lug so I feel happy this should work. This leaves the bottom front engine lug redundant. Thinning the lugs was something I thought about but won't work. There is no issue with the rear lugs.

There is one remaining issue that is the head steady. The Jawa engine has a very light and thin cam cover that is held on with 6mm studs that extend from each cam bearing holder so will not be strong enough to support a head steady. I can't see anywhere else a head steady could be attached to. The Frame is gusseted at the front. Are there any alternative locations for the head steady - maybe a removable cross brace between the top frame rails?
 
mdt-son said:
If you are serious about constructing a race bike, you should redesign the frame, making it shorter by cutting the top and lower rails. Featherbed frame geometry is odd at the outset! At least this gives you the opportunity splaying out the lower rails as needed. By cutting the downward tubes below the crossing with the upper rails, you have an ample length of tube to accommodate the change in angles. It goes without saying this has to be carefully planned and executed, preferably using 3D CAD. Tubes are rejoined by using inner tubes in an overlap fashion. Welding or brazing should be outsourced to a professional welder.

- Knut
Click to expand...
Knut -
I would be interested in hearing ( in layman’s terms ) your thoughts on the oddities of Featherbed frame geometry. Seems to me ( an amateur) that it is one of the most successful and long lived frame designs in history. Keep it simple as I am the son of a Norwegian woman …
😉
 
If it were me, I'd get in place all the other components that might create an interference, i.e., oil tank, intake/carb/bellmouth, primary gear or pulley, etc. From there, see what fits where. Often times these decisions make themselves.

I'd also be inclined to spread the lower rails to make room, as Knut mentioned, by cutting the front cross brace, spreading that front lower corner, and replacing or patching the cross brace. Less fiddly than denting.
 

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