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Dave Martin
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3D printers
#747352 08/31/18 1:00 am
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i have a race bike with keihin FCR carburetors. the carbs have no filters, just a plastic air horn:

[Linked Image]

the air horn is short, and i want to test different intake tract lengths on my bike. easy to do with mikunis, because the air horn flange is round, and you just get velocity stacks of various lengthes and tie them on with a hose clamp. but i can't do that with the keihin, because the stacks are held on by two allen bolts 180 degrees apart, clamping down on a flange. air horns of different lengths are available from keihin for around $175 for four of the same length, and only two lengths are available.

what i've tried so far is just hanging the carbs out on the ends of aluminum tubes, with rubber couplers. not real good, because the carbs are heavy and the tubes are straight and get in the way.

i've been considering cobbling up extended velocity stacks from fiberglass, or something else. awkward.

then i looked at this guys puch moped, which currently does 61 mph. i met him two years ago at wilmington, when this machine was slower:

[Linked Image]

i was looking over his bike, and noticed the velocity stacks:

[Linked Image][Linked Image][Linked Image]

i asked him about them. he printed them up on a $200 3D printer he bought from amazon (dunno yet what the software costs . . . ). the material was about twenty five cents.

i had no idea that 3D printers had come down that far. the ones amazon sells for $250 generally have the problems that anything cheap from china has, but making one pair of velocity stacks for my bike would pay for the cost of the machine.

anything on the bike made of plastic can be designed and machined on the printer for a fraction of what it might cost any other way:

spacers
chain tensioner slippers
instrument mounts
light fixtures
throttle assemply parts
cable guides
side covers?
switch knobs
enrichener knobs


who knows?

i'm looking at them on amazon right now:

https://www.amazon.com/dp/B07BR3F9N...d_r=3d3fbd12-acb7-11e8-8e51-357518f34f72


Last edited by kevin roberts; 08/31/18 1:03 am.

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Re: 3D printers
kevin #747411 08/31/18 1:05 pm
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Autodesk Fusion 360 software is available for free as long as you don't make a lot of money with it. Windows 7 is the minimum OS version - I think. The more RAM the better. High learning curve if you have no experience with 3D design software. It has all the features necessary to easily create a complete velocity stack or extension and output a file for your printer.

Another cool thing about Fusion is that all files are stored in the "cloud". Workgroups can be set up to enable collaboration online.

Xometry and Shapeways are two service bureaus that I have some experience with. Using a service bureau provides the benefit of printing with a variety of materials. With Xometry, you can upload a CAD file and get a price quote instantly for many options of printing / machining / materials.

In any case, a cheap printer would be nice for proof of concept if not output of the final product. Now I want one.

Last edited by henryanthony; 08/31/18 1:24 pm. Reason: Additional information
Re: 3D printers
kevin #747421 08/31/18 3:05 pm
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The printer that I got will do a 500mm cube, cost under $800, from China, of course. Most of them are made there so why pay the middle man? Software for the printer is freely downloadable (Cuda). You can find free CAD software but it has to put out .stl files for the printer. My Acad2000 does not but Gibbs, what I use for my CNC machines, does.
If you look on the Triumph forum for electric start and Triple forum for my 12 valve head, you can see parts that I have printed.

Re: 3D printers
kevin #747438 08/31/18 5:18 pm
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I have been told by a manufacturer in my area that, with the proper calculations for metal expansion, a 3-D printer can make MASTERS for molds to be used to cast new parts in iron or aluminum.

If you can find a mold-maker to provide the figures for those calculations, you are on your way.

Re: 3D printers
kevin #747486 09/01/18 12:30 am
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i'm just at the beginning of this, but it looks very useful to me.

i was under the impression that the cheapest 3D printers were still well over a thousand dollars.


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Re: 3D printers
kevin #747540 09/01/18 1:29 pm
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I have an Imagemaker that cost $1,500 a couple of years ago. It's very nice, about 10x10x10


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Re: 3D printers
kevin #747597 09/02/18 12:55 am
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this entry-level printer was the one the guy used for his velocity stack:

https://www.amazon.com/Monoprice-Pr...mp;keywords=maker+select#customerReviews

but he liked this one better. $750 now.

https://shop.prusa3d.com/en/3d-prin...-1Y2b3QIVh4KzCh0YWg2WEAQYASABEgLfzvD_BwE

you can buy it either as a do-it-yourself assembly kit, or pre-built and tested. he said the instructions that came with it are superb, and the DIY would be the way to go, if you are mechanically inclined. get the one with the controls in front, he says.


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Re: 3D printers
kevin #752104 10/09/18 8:10 pm
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now im looking at using one to make a prototype sprocket carrier for a japanese wheel on my race bike. the 1983 GS 1100E rear wheel is 17 x 2.5, which is what i want, but the whole assembly needs to be about 2 inches narrower to get everything to line up.


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Re: 3D printers
kevin #752809 10/15/18 2:35 am
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Some models of Ducs have electrically adjustable-length air trumpets,
you could dial in a length on the fly.

Assuming they are in your ballpark for length to begin with.
Not exactly a low tech solution though....

Re: 3D printers
kevin #752945 10/15/18 10:35 pm
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with this single-purpose machine, flexibility isn't important. once it's finally in the power band it never leaves it, so a single-length tube would work fine.

just need to decide what the shape needs to be and how long.

old panic had some interesting observations about how the airhorn ought to be shaped. and lots of people seem to discover that no velocity stack is faster than any velocity stack . . . which i think means more than likely that people are trying to use the wrong shape.

[Linked Image]

Last edited by kevin roberts; 10/15/18 10:52 pm.

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Re: 3D printers
kevin #752977 10/16/18 12:59 am
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Kevin,

I found the same page you posted on the Shop Talk Forum. Page 2 on the link below.

http://shoptalkforums.com/viewtopic.php?t=122047

The discussion also said that shorter stacks improve high-rpm torque while longer stacks improve low-rpm torque.

One guy stated that the ideal shape is that of a nuclear reactor cooling tower from the narrow point to the top.

The thing about the elliptical flare is interesting. Probably need a dyno in a wind tunnel to sort this out. Or many iterations tested on the track.

3D printing certainly opens up what is possible regarding velocity stack design. Pipes that are straight, flared, hyperbolic, curved, like an accordian, a spring or, what have you.

After reading the article on Shop Talk I fooled around with Autodesk Fusion and its function to create hyperbolas. Here is an image of what I came up with. Not sure if this link will work though.

[img]https://drive.google.com/open?id=1q5PMM6FZJmGMDgXyX7TLt5OOcNTi-Nzn[/img]


Last edited by henryanthony; 10/16/18 1:36 am. Reason: Added image
Re: 3D printers
kevin #752978 10/16/18 1:26 am
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i think the best entry shape is prettywell established as elliptical, or close to that.

beyond that, i dont believe in anything but testing. ive had good results testing my stuff in the real world. but still lots to look at.


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Re: 3D printers
kevin #752998 10/16/18 10:25 am
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ive just started messing with cadcam

kid's stuff, like tinkercad.


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Re: 3D printers
kevin #752999 10/16/18 11:30 am
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Kevin, It'll be interesting to see if velocity stacks work on your engine....On my race engine, on the chassis dyno ,I tried stacks, longer intakes, and the best power and torque was made with no velocity stacks and the carbs mounted close to head...I have seen small high rpm engines taking advantage of velocity stacks...


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Re: 3D printers
kevin #753069 10/17/18 12:13 am
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i don't know. there are all sorts of formulas for these things, just like exhaust pipes, but they all are chock full of constants that come from nowhere, assumptions about ambient air temperature, density, exhaust temperature, charge density, fuel temperature, and on and on . . .

the speed of sound varies with air temperature, for just one.

[Linked Image]

but i can't remember seeing a formula take that into account. we're talking just inches, for an intake tract. if all these neglected variables don't matter, then neither does the length. i genuinely suspect the big difference in airflow is in the shape of the entry, and that's the easiest thing to test.

no way to tell except by trying it out.


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Re: 3D printers
kevin #753078 10/17/18 2:53 am
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Constants do not come from nowhere.
Speed of sound in a gas = SQRT( gamma * r * T )
gamma - ratio of the specific heat of the gas at constant temperature to that at a constant volume
r - gas constant 1.402 at 20C
T - absolute temperature
Or the velocity at any temperature relative to 0 C
c = c0 * SQRT (1 + t / 273)
t - temperature in deg C

The fundamental resonance of a pipe is
f = c / (2 * ( l + 8 * a / (3 * PI)))
c - speed of sound
l - length
a - radius

The second term is a correction for an infinite flange. However, putting nothing, flare or any thing else changes the correction.
You are juggling two design parameters, flow and resonance.

Re: 3D printers
kevin #753201 10/17/18 11:55 pm
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Quote
Constants do not come from nowhere.


you're right, dave. physical constants have physical reasons, and physics doesn't lie.

but the constants in the pipe formulas seem more corrections, trying to lump a bunch of unmeasured parameters together, and they're not explained. fuel/air emulsions are not ideal gases, and vary in density along the pipe, and change unpredictably due to reversion. even the initial composition and temperature of the gas varies with the jetting and the cam timing, and is not included in the formulas.

i can't measure the specific heat of my mixture at 66 F, RAD 108, 140 main jets, and then correct it when i switch to 145 main jets.

with pipe resonance, the harmonic waves in a perfectly straight tube may or may not correspond to the passage of mixture through a slide carburetor and port of differing radius over a few inches.

i'm not disagreeing with science, i'm just pointing out that perfect predictions require perfect conditions. i'm much more comfortable using science only for a first approximation, and then just testing my way through. i don't have to understand what works, just that it does.


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Bellmouth design article
kevin #753844 10/24/18 2:37 pm
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I was flipping through my archives looking for something else and found a link to an article about bellmouth design that may be of interest to any engineers here. Way above my head though.

www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf

The site also includes several articles regarding race engine design.

http://www.profblairandassociates.com/RET_Articles.html

Re: Bellmouth design article
kevin #753856 10/24/18 5:00 pm
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Seems to me ...
The physical constant , at full throttle , is the throat diameter
all you can do at the entry ... is relieve the nozzle entry angle , to deal with friction at the boundary layer ... as free air becomes ducted .
A simple radius does the job , every thing else is for show .


Re: Bellmouth design article
kevin #753873 10/24/18 6:51 pm
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Another side to this.
Speedway bikes are limited by the sports regulators to a 34mm choke diameter.
In order to extract 80-90 BHP @ 12,000-13,000 RPM very short inlet tracts and carburettors with a thin guillotine like flat slide are used. The favoured Blixt carb is about half the length of an AMAL
The short length flows more air. (less drag from the port walls??)


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Re: Bellmouth design article
henryanthony #753902 10/25/18 12:05 am
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Originally Posted by henryanthony
I was flipping through my archives looking for something else and found a link to an article about bellmouth design that may be of interest to any engineers here. Way above my head though.

www.profblairandassociates.com/pdfs/RET_Bellmouth_Sept.pdf


lots of information there, henry. thank you.


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Re: 3D printers
kevin #754101 10/26/18 4:23 am
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Just printed off a couple of bellmouths, one for the stock triple head, the other for the four valve. Not a super fine finish (0.2mm layers) but this is just to compare the 2 and 4 valve heads. 2:1 ellipse on a 1.062" throat and 1.40" throat. The black streaks are from a quick swipe with emery paper.
[Linked Image]

Re: 3D printers
Rohan #754119 10/26/18 11:03 am
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Originally Posted by Rohan
Some models of Ducs have electrically adjustable-length air trumpets,
you could dial in a length on the fly.

Assuming they are in your ballpark for length to begin with.
Not exactly a low tech solution though....


The new Ducati 4 cylinder engine has complicated full electronic engine management and is light years away from anything on this forum . For street use it's 214 hp at 13,000 rpm grin It's used in lieu of variable cam timing that is even more complicated.. But a clever guy could make use a small servo motors to work the variable stacks..But who knows if it would be worth the effort on much lower output engines with carburetors..




Last edited by Hillbilly bike; 10/26/18 11:06 am.

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Re: 3D printers
kevin #754153 10/26/18 5:39 pm
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Dave, wouldn’t a rougher finish be better for air flow dynamics over a smooth finish?


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Re: 3D printers
DMadigan #754188 10/26/18 10:46 pm
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Originally Posted by DMadigan

[Linked Image]



those are beautiful, dave. what is the plastic? have you checked for solubility in gasoline?


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