The Worlds Fastest NIMBUS

[Peter Jack]

Lars, I can tell already this build is going to be at least as entertaining as your Indian build was. Maybe you can expand the crate slightly and bring them both over. Just don't mix up the parts during reassembly! 

I look forward to seeing the completed bike. From what you've shown us it looks like another WINNER.

Pete

[roygoodwin]

Lars,

I figured that adding one or more bearings was going to require a billet crank or a LOT of welding and enough RE-machining that it would nearly be as much if not more work than a billet.  The billet might not be an economical solution.  I can't tell for sure from the pictures, but perhaps there's enough material on the crank between cylinders 1 & 2 and 3 & 4 to add bearings there by machining a journal into the original crank.  that would reduce the maximum unsupported section to 2 cylinders.  I *think* that adding a counterweight between cylinders 2 and 3 might help, but am not finding my engineering books that might support that & evidently I'm not asking Mr google the right questions.  The crankcase might not have enough material to let you support additional bearings, again it's hard to tell from the pictures.  I'm sure you'll come up with something.  I'm looking forward to the build progress reports.

roy

[wobblywalrus]

The idea of using the same approach as you did with the scout is a good one.  Streamlining will give you speed without overstressing the engine.

[charlie101]

An assembled crank, tack welded wouldn't be very expensive to make. What's the bearing size, stroke and bore spacing on a Nimbus? There is a lot of 2 and 3 bearing crankshaft specialists in the classic Fiat and BMC world.

[octane]

Thank you Pete and thank you Bo the Walrus !

Lars,

I figured that adding one or more bearings was going to require a billet crank or a LOT of welding and enough RE-machining that it would nearly be as much if not more work than a billet.  The billet might not be an economical solution.  I can't tell for sure from the pictures, but perhaps there's enough material on the crank between cylinders 1 & 2 and 3 & 4 to add bearings there by machining a journal into the original crank.  that would reduce the maximum unsupported section to 2 cylinders.  I *think* that adding a counterweight between cylinders 2 and 3 might help, but am not finding my engineering books that might support that & evidently I'm not asking Mr google the right questions.  The crankcase might not have enough material to let you support additional bearings, again it's hard to tell from the pictures.  I'm sure you'll come up with something.  I'm looking forward to the build progress reports.

roy

Roy: thanks for taking an interests in my silly project.
Unfortunately I'm too dumb to quite get it.
Maybe ( .. er ... probably ) I misunderstand things, but again: as I see it there's no way
I can ad extra main-bearings to the existing crank.
Please take a look at this pic of the original crank.
Yellow line = line of rotation



...or ?

An assembled crank, tack welded wouldn't be very expensive to make. What's the bearing size, stroke and bore spacing on a Nimbus? There is a lot of 2 and 3 bearing crankshaft specialists in the classic Fiat and BMC world.

Thanks Charlie, but again; this is beyond me, and BTW anything outside of 'not expensive' is
beyond my financial capabilities.
I'm afraid ( well, not really ) that I'll have to do with what I've got



Well: time to get this build-diary started.


I'm not posting all this to tell anyone how to do things. That would be preposterous.
I'm posting all this for two reasons:
One; hoping that all you smarter-than-me guys will give me some good advise along the way
as you , so generously, have done before.
Two: showing them dreamers out there , who haven't dared doing these kind of things,
that if a daft bloke like me can do it...you can do it !
You can move those dreams from the impossible-box to the possible-box.


Now why on earth would anyone want to race a Nimbus .. of all things..at Bonneville ?
It's not exactly the obvious choice, is it ?

No..but let me give you a bit of background;
these machines holds a certain affection in this country. Everyone knows them, even
those who know nothing about motorcycles. There's a kind of national 'pride'
( for lack of better term ) for these machines...like a combined HarleyDavidson/Indian in the USA.
For anyone over a certain age they played a part of daily life and history.
Due to import-restrictions for a long time after the war, it was almost impossible
to import motorcycles , so the Nimbus was used for ... everything.
The military had a huge number of them





...which btw is the reason for the relative abundance of spare-parts.
The military had a HUUUUGE stock of parts.

The postal service used them:



...so I have fond memories of the Nimbus buzzz as they came twice every day
to empty the mail-box across the road from my childhood home.

The police used them





...as I said: they were used for everything, even for freakin' animal-ambulances (!)



...and I had one as a young man. It was my second bike ever, and the first one I really rebuild.
I've told this before but it's silly enough to be repeated:
that's me on the right ( ... yes, yes yes ; the beard is false .. ha ha),



...during a shooting of a TV-movie. We persuaded a cop to chase me down the streets, blaring horns,
flashing light and all in his cruiser. You should have seen the faces of the drivers of the cars passing us..priceless !

Had to sell it. That was thirty years ago. I've missed it ever since.

[octane]

...so I decided to build one again, and having a certain affection
infatuation for Bonneville, why not make a Nimbus Bonneville Racer.
( In fact I could come up with a lot of good reasons not to, but being an unreasonable
person I just disregard those ).

Initially I wanted to take a standard Nimbus, frame and all, and go from there,
so I started making dodgy drafts ( pretending to work )





...but it soon became apparent that it wasn't such a good idea for several reasons,
mainly that the frame is made from 'spring-steel' which is kind'of impossible to weld,
( every part of the frame is rivited (!) together, like a frekin' old bridge )
so I was VERY limited when it came to modifications, like if I wanted to alter the
angle of the front-fork.
At this time I had borrowed a clapped-out frame to fool around with



Then I took one good long hard look at the frame



...and thought to myself ; I have always had a dream of making my own motorcycle-frame,
( and filed it under "Impossible Dreams" )
and even if I have never ever so much as bend a single pipe/tube in my entire life,
and never seriously welded two pieces of steel-tubing together....if I ever should do a frame,
this is IT... a motorcycle-frame doesn't come much simpler that this one.
So back to the drawing board for some more drafts, now with the freedom to
change things around as I bloody well pleased





...and sure: there's a blower there. If it ain't blown..it sucks. Everybody knows that, ei ?!

Borrowed a few Nimbus engine-parts and stumbled into an old Danish-made fairing for a MZ bike,
and did some mock-ups to get the proportions right



and figuring out how, and where, to mount the blower



..more drawings ( still pretending to do actual work )





..decided to ditch the original tank. Didn't fit anymore after I altered the angles of the frame
and besides: it look like one of them things the old folks had under their bed
so they didn't have to go all the way to toilet at night.
Remembered that I had a tank somewhere. Came from an old Danish moped.
Lo and behold : it fitted as if this was meant to be



Heeeeey : I'm on a roll here !

Time to bribe my Mr. Computernerd colleague into making something a lot more
interesting that what he do for actual work.
In fact it didn't take any persuasion AT ALL, so a BIG thank you to Rasmus Hansen !!!
for helping me



Here's Rasmus hard at work



That went surprisingly fast. Maybe due to the fact that he has tree hands and extra joints on his fingers...dunno .





( For any co-nerds out there : I think he used a combination of Auto-Cad and Rhino .... not sure )


The drawing went up on the wall for some additional contemplation and evaluation



and after a while we did some modifications and thought on how to put the darn thing together




There was just NO way that I was going to bend those tubes myself.
As I said I'd never done it before, and I wasn't going to loose my virginity
doing it for a bike that will rush over the big white dyno at enormous speed

 ( yeah...right )

..so I looked up the nearest place ( didn't know where else to start ) who do CNC tube-bending.
One: it was like five minutes away..Two: they guy who picked up the phone
turned out to be an old-bike nut. Not a bad start !
He was just the nicest and most helpful guy I could ever dream of bumping into.

At this point I had read everything I could possibly find concerning the choice of
tubing for building a motorcycle-frame, including every post on that here on the forum.
I soon realized that asking about it is like asking; "What engine-oil should I use for my XXXwhatever".
NOone seamed to agree on anything, so I listened but in the end I went with what the
CNC-guy recommended:

"..32 mm / 4 mm . ( That's: 1¼ ”  / 1/6 ” )
seamless cold drawn hydraulic line pipe according to DIN 2391/C...EN 10305-4...St 37.4 NBK

Carbon steel tubes for machine structural purposes.  
Application: motor-car construction, pneumatic and hydraulic operating mechanisms.."

One of several things that persuaded me was the fact that that particular type
is the one that the Swedish authorities accept for home-made motorcycle frames.
( Forget about doing such a thing here in Denmark... no way ).
I does not have to be 4 mm , but I went for that, hoping to make it strong enough
so I didn't need the bird-nest of tubing I see on other bike.
( I do understand that a thicker tube does not necessarily compensate for less tubing,
but we'll see how it works out )

It quite beefy:



OK then: I'm building a tractor...ODINs TRACTOR,
but heeey: the weight 'penalty' for going from 3mm ( 1/8 " ) to 4 mm was around 7,5 kg / 15 pound,
 and who cares...not me.

So the guy had 12 meters ( 36 feet ) brought over from Sweden ( needed two 'lengths' @ 18 feet ),
CNC bend the whole darn thing for very little more than 300 US$ .. ( including the price of the tubes )
cash .. out the back-door.
There are still nice people out there.

[joea]

im just in "awe"...and feel so dang priveledged to bear witness to what you are doing and the story of it "all"...!

THANK YOU immensely for sharing what you are doing....!

Joe Amo

side bar: this is a genuine response of love and appreciation, not one of my normal harsh-sarcastic-near derogatory posts..

[Tman]

Very cool! Love the story so far!

[Old Scrambler]

I especially appreciate the 'pretend-work' that needs to be done before bending the first piece of steel. Your mock-up pic shows your feet behind the rear axle..........is that OK with the rules?  I was told at BUB that the rear axle/chain-adjustment nuts and front axle pinch-nuts need to be safety-wired for next year.

Great start...........

[roygoodwin]

Lars,

I've made a VERY POOR modification to the drawing of the nimbus crank you provided, in an attempt to clarify what I was talking about.  what I'm talking about would be to machine an additional main journal into the crank in the web that connects the throw(s) for cylinders 1 & 2 as well as 3 & 4.  In the drawing I've just hacked up one web.  The journal would have to be quite narrow to leave enough material to connect the throws.   You would also have to make a bearing carrier that would hold the additional main bearing shell and main cap.  the carrier would fit in the crankcase between cylinders 1 & 2 as well as 3 & 4 and be attached to the crankcase in some way.  There may not be enough material in the crankcase casting at the appropriate location to adequately support the bearing carrier or the shape may not allow the addition of the bearing carrier at all.

[DND]

Hi Lars

Just tuned into your thread, wow that is one neat looking bike

You have the eye for what looks right, if the right engine plan came up do you have the money for a new crank & stronger rods to keep the engine together

What makes me nervous is the way it is the crank could start to flex a bit and get into a harmonic in a long hi gear run on the salt and bang

If you make some new parts the crank could have a thinner rod bearing a make room for the extra mains , and a stronger rod and have the crank drilled for some oil pressure with a pump you could mount in the engine or outside

Is there a crank builder in you neck of the woods you could visit and talk over a game plan, lots of rod builders but need a well thought out crank

You could make a main bearing girdle that bolts to your pan rail and the case to hold the extra mains and  space down the pan or notch it to fit the girdle

It looks like it is a do able up grade to your neat salt racer

G Don

[octane]

Thank you VERY much Joe and Tman.
Encouraging and kind words from you guys here,
means a lot to me and are truly appreciated !

... Your mock-up pic shows your feet behind the rear axle..........is that OK with the rules?

Yes. This being an "A" class build ( Special Construction ) , the rules goes like this:
7.G.1 Foot Rests:
Footrests shall be provided and the location is optional.

The rest of the SCTA Special Construction rules were really hard for me to comprehend
so just to be on the safe side I send a photo to Tom Evans ( Head motorcycle tech )
to ask if I was OK:
( Note how the crank-case was kindly lend to me by the 'Danish Con-rod Throwing Champion' )



...and I was kindly given the go-ahead.
The AMA/BUB rules are somewhat clearer in that respect.

Great start...........

Thank you Old S !

Lars,

I've made a VERY POOR modification to the drawing of the nimbus crank you provided, in an attempt to clarify what I was talking about.  what I'm talking about would be to machine an additional main journal into the crank in the web that connects the throw(s) for cylinders 1 & 2 as well as 3 & 4.  In the drawing I've just hacked up one web.  The journal would have to be quite narrow to leave enough material to connect the throws.   You would also have to make a bearing carrier that would hold the additional main bearing shell and main cap.  the carrier would fit in the crankcase between cylinders 1 & 2 as well as 3 & 4 and be attached to the crankcase in some way.

Thank you Roy. The drawing is fine ....NOW I get it...I told you: I'm a bit daft

There may not be enough material in the crankcase casting at the appropriate location to adequately support the bearing carrier or the shape may not allow the addition of the bearing carrier at all.

I'm afraid you're right. Both on the question of there being enough material on the crank to do the modification (there isn't )
and on the subject of mounting support/bearing carrier in the crank-case. There is no room for it,
and neither is it something I can see as doable, as what you (maybe) see as 'the crankcase'.
in fact is the upper part of engine, which is (for lack of better term) a combined 'upper engine block and cylinders'.
It's made of cast iron , and it's not of very substantial thickness.

Also: welding ( a bearing carrier ) onto cast-iron is really not something I'd like to go into.
That would also mean that the carrier would have to be made of cast-iron. ( In order to weld it onto the 'block' )

Thanks again for your input, but I just can not see this happening.

Hi Lars

Just tuned into your thread, wow that is one neat looking bike
You have the eye for what looks right,

Thanks Don !

if the right engine plan came up do you have the money for a new crank & stronger rods to keep the engine together

That's easy to answer: ... no

For a couple of reason:
 One; this country is a Third World country when it comes to engine building, tuning, Hot Rod'ding, building bikes
building cars....anything like that , which is part of American culture is just not ( well: almost ) present here.
There is no tradition for that kind of stuff, partly because the rules, regulations and laws here
are prohibitive at a Stalinist level....Really !

( And btw; I do not, by any means have the money for it. This is a low-budget build.
'Low' defined as 'the height of an average freckle' )

As I mentioned: there is NO way one can scratch-build a bike here ( and run it on the streets )
and the rules for any kind of modifications are absurd.



Two; having any kind of crank or rod ( or anything else for that matter ) build here is insanely expensive.
Here's an example: I needed a head-stock for this build.
Instead of having that relatively simple thing made here, it was cheaper for me to buy it from USA,
having it put on a plane and flown over the Atlantic and pay the import-tax on it.
( With a bit of un-luck that happens to hit me once in a while, the darn thing is now on it's way back
over the big pond because the postal-service here f****d up.......auuuuurgh !!!)

What makes me nervous is the way it is the crank could start to flex a bit
 and get into a harmonic in a long hi gear run on the salt and bang

I don't like that last word one bit.
.-)
but you may very well be right.
On the other hand: I don't worry too much about it.
When I build the Indian engine I was told that it would never hold up,
taking an ancient 12-13 hp engine and tune it to it's present almost 40 hp ( at the rear-wheel)
what with those con-rods that look like something out of a moped , and all:



...but heeey: it's still here, in one piece.

Now on the Nimbus they're this small:



.

.

.

.

.Naaa....just kidding !

If you make some new parts the crank could have a thinner rod bearing a make room for the extra mains , and a stronger rod and have the crank drilled for some oil pressure with a pump you could mount in the engine or outside

Now the oil-pressure thing had me slightly worried too, but I've contacted someone who makes a modification to
the Nimbus engine, so as to get like double the oil-pressure.
More later on that.

Thanks Don !

[octane]

Now I had a lot of tubing to practice and do tests on.
Basically I wanted to check how it works with TIG-welding a tube this size,
penetration etc. and to test the impact on the strength of the tubing from the welding.
A lot of welding....and cutting up, to check.

The welds were done with stainless steel 'filler', type 316 Lsi.

The pretty welds are not done by me. The dodgy ones are.
I have no experience TIG-welding steel....and it shows.

Therefore the final welding will be done by a highly qualified friend.

Here are some of the tests:

















...and send, and showed, the pics to folks with experience on these matters.

One test was to really really really put on a lot of heat and move it slowly
and repeatedly.
That gave a 100% penetration:



...but applying that technique to weld two pieces of tubing together



...wasn't such a good idea.
I put it in my tube-bender ( more on that later ) but without the
'mandrel' ( or whatever it's called ) to concentrate the pressure at one point
and gave it the full 12 ton workout



...and it was obviously clear that the amount of heat had made the
tubing weak and brittle next to the welding ... the 'heat-zone' one might say



Lesson learned.

[octane]

...now actually before the above tests, I practiced doing 'fish-mouths',
to prepare for putting the frame together
As I mentioned , I have VERY little practice or skills, fabricating or working
with steel, and none whatsoever on working with tubing.

I don't have a fancy tube-cutter tool for this stuff.
..I can't afford it
..I would be too dumb to figure out the exact angles with which to do the cutting
so it was all done 'free-hand' using first and angle-grinder and then one of my
favorite tools, this one
( I don't even know what such a thing is called. I call it a trunk-sander ( like in elephant-trunk ))



...quite a challenge, I'd say, but it went surprisingly well,
if I may say so ( I just did...didn't I ).
Some are kind'a good...some not so much, but I'm getting there















Obviously the trick was to grind/sand off just a little at the time
...check it up against the tube that it's supposed to match...repeat, repeat..etc...
so as not to remove too much material....if you do, you're screwed and
can dump that fancy just-CNC-bend tube.

[octane]

And now a short intermission in the build-thread to let me invite you to visit the
nicest motorcycle workshop in town;

the Nimbus-only 'J.C. NIMBUS' run by the ever-so-kind John
and located 10 minutes drive from me:

It's a busy place:



..he's got the tools:



Cylinder-head/valve job:



Care for a cylinder, Sir ? :



Then how about a crank-shaft :



I nearly fell over backwards when I walked in and saw this one.
It's exactly like the one I had some 30 years ago



Nimbus engine test-stand:



Nimbus frame check/repair jig:



Nimbus-parts galore:





Cut-up engine for educational purposes:





Note the rather nifty, and for the time very advanced, combined dynamo
and overhead-cam bevel-gear-drive






Nice place, oy ?!
I hope you enjoyed the visit.