Using motorcycle drive belts from BEHIND an engine

Discussion in 'General Auto Conversion Discussion' started by Doggzilla, Sep 14, 2019.

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  1. Sep 14, 2019 #1

    Doggzilla

    Doggzilla

    Doggzilla

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    It occured to me today while I was thinking about drive belts that a set of belts behind an engine could be used as a PSRU driving a shaft that then runs over the top of the engine between the valley on top of a V engine and to the front. It would use crankshaft bearings in custom mounts that have integrated tensioner bolts for the belts.

    There are several massive advantages to this setup.

    A) Allows gearing without PSRU

    B) Allows auto engines to be used in standard configuration with accessories on front and drive on rear. Greatly reducing the level of modification required.

    C) Allows use of off the shelf parts that can be inspected and replaced easily

    D) Allows keeping radiator on front with fan, greatly simplifying the cooling system and reducing overall length and complexity.

    E) Allows a higher prop mounting for better clearance/larger prop, much like a Spitfire or Mustang

    F) AND MOST IMPORTANTLY: Allows gearing that can be inspected and modified easily without a casing around it.

    Im sure there are more, but this is just what I can think of off the top of my head.
     
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  2. Sep 14, 2019 #2

    henryk

    henryk

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    http://www.reaa.ru/yabbfiles/Attachments/funkfk11_drivetrain.jpg

    https://www.bydanjohnson.com/pilotreport/fk-11/
     
  3. Sep 14, 2019 #3

    AdrianS

    AdrianS

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    It would be interesting if you could design the drive shaft to be torsionally "soft", driving the resonant frequency below operating revs.
     
  4. Sep 14, 2019 #4

    pictsidhe

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    I posted a picture of a rear-redrive briggs mockup a month ago. I can't find it now the post now, but here's the pic again.

    20190804_163100.jpg The grey PVC is the propshaft. White PVC support struts for the front bearing. Powerplant dynafocal mounts to go close to the join of struts and the plywood plate.

    The advantages for me: Gets all the big lumpy bits on the back of the engine. So the powerplant will fit in a scale cowl. Structurally, I ease a lot of problems. The propshaft, which will need to take sizeable gyro loads, is long and very well supported across a good span giving much lower bearing loads. Moments are much harder to deal with with a redrive on the front. The engine mounts are towards the back, where I want them to hang it on the aircraft. It should end up lighter, sleeker and stronger than a PTO mounted redrive. I need a torsionally soft system to prevent spontaneous dissaembly, I will be using sprung idlers to achieve that. I'm also using a much larger diameter but lighter flywheel, on the PTO end. My flywheel is an automotive flexplate. So that is a low dollar and easily sourced part. 47% of the MOI of the Briggs flywheel, but only 24% of the weight. That's a 12lb weigth saving right there.
    A car conversion would better use front and rear propshaft support structures, using the block as a stressed member.
     
    Last edited: Sep 14, 2019
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  5. Sep 14, 2019 #5

    wsimpso1

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    Look up the original BD5 powertrain, it is a layshaft design similar to what has been described. Then search search for an article on the BD5 and torsional vibration. When I get home I will post a pick to the article.
     
  6. Sep 14, 2019 #6

    pictsidhe

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    TV wasn't considered during the design of the BD5 powerplant. That was a mistake.
     
  7. Sep 14, 2019 #7

    BBerson

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    If the overhead shaft was belt driven with belts on both ends of the crankshaft there could be significant TV advantages, I think.
     
  8. Sep 14, 2019 #8

    pictsidhe

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    How?
     
  9. Sep 14, 2019 #9

    BBerson

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    If you drive the prop shaft with both ends of the crankshaft that reduces the crank end torque impulse by half and reduces to total crank flex.
     
  10. Sep 14, 2019 #10

    pictsidhe

    pictsidhe

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    That'll be fun to analyse.
     
  11. Sep 14, 2019 #11

    Jay Kempf

    Jay Kempf

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    There is one big advantage to this suggestion and that is almost infinite possibilities for prop extension. At the cost of some weight. Certainly in a V8 the entire intake could either straddle the shaft or you could have two intake runners on each side. Done all the time. Could still be hollow shaft for stock CS props. The block of the engine could be a stressed member in the thing giving huge stiffness.
     
  12. Sep 14, 2019 #12

    wsimpso1

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    Here is the article that described several projects with severe torsional vibration from an insider's point of view including the BD5:
    http://ibis.experimentals.de/downloads/torsionalvibration.pdf

    Torsional vibration was definitely an issue in the BD5 with multiple failures and a bunch of work to solve it. The system had a layshaft that ended up being part of the solution.

    Billski
     
  13. Sep 14, 2019 #13

    wsimpso1

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    Something to remember about crankshafts - one of the big design requirements is that crankshaft torsional mode resonance usually must be above 2 times max firing frequency to avoid resonance inside the operating range. Same is also true for things like camshafts and their drives and accessories like distributors and oil pumps. Now this is not to say that they are immune to issues from flexing, but but they do tend to have high stiffness to mass/inertia ratios.

    Car engines at the crank flange end are designed for full engine torque and torsional accelerations due to firing pulses, which means that they are robust to torsional loads. Neither the bending on the crank end nor lateral loads were intended, so you usually need a separate bearing set on the outside of the sheave to be robust enough to belt loads. Many belt PSRU for car engines do this.

    The front end of automotive crankshafts are not even that sturdy, with the design intent being to drive comparatively small accessories - alternator, water pump, power steering pump, and AC compressor. Yes with belts, but with small torque compared to the other end.

    If you put drive pulleys at both ends, you would have to do things similar to existing belt PSRU at the crank flange and come up with some sort of crank add-on at the front end to carry bigger torque there. Vibrationally, you would have firing pulses seeing both a system with only the sheaves and belt and a system with sheaves, belt, and springy shaft. Yep, two different vibrational systems that both have to be designed to keep resonance frequencies out of the range of firing frequencies. I do not see how this added complexity would improve the vibrational situation...

    Billski
     
    Last edited: Sep 15, 2019
  14. Sep 14, 2019 #14

    wsimpso1

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    This sort of thing has been done on airplanes and in other vehicle types. It is called a layshaft design and I first saw them in mini-bikes and go-carts in the 1960's. Of note, such a system was used in the BD5, and they had much fuss and numerous failures in development. See the attached file on another post I made on this thread.

    While it does get rid of the gearbox and its parts, the described system IS a PSRU, with all of the issues of carrying prop loads, rotating machinery, and managing vibration.
    While true, where is the advantage? Accessories can be bulky. We have more space around the rear of a conventionally arranged engine. Putting the accessories around the front can drive a larger front end to our cowlings, which usually means lower prop efficiencies and higher drag.
    True and can simplify our arrangement unless we decide we want a more streamlined cowl, in which case, we rearrange things to get that better cowl shape and end up with custom accessory drives anyway.
    There is ZERO need for a fan on a conventional aero application. See Ross' posts on belly mount radiator ducts etc.

    Putting the radiator in front of the engine will drive significantly higher weight and drag than can be obtained otherwise in water cooled engines.
    True, but is no better or worse than other available PSRU for engines with vertical offsets.
    Picking a gear ratio and prop diameter that keeps the prop tips below sonic speeds is pretty straightforward. Damage detection of geared units is rather simply achieved with a COTS chip detector in the oil. Now if you wanted to conduct experiments to play with different gear ratios, hey, knock yourself out, but I see no gain from this "advantage".

    I do not see these points as big pluses and they might indeed be big minuses. If anyone thinks this is the way to run a V-8, go for it. I see it as the long way to turning a prop from a V-8 given the successful drives already on the market for reasonable money.

    Billski
     
    Last edited: Sep 15, 2019
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  15. Sep 17, 2019 #15

    pfarber

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    There was never a suitable power plant for the BD-5.

    And look at the P-39 Aircobra... an almost 8 foot driveshaft AND reduction gears. Heck every major WWII V12 has reduction gears... so I'm not sure why PSRUs get a bad rap.
     
  16. Sep 17, 2019 #16

    AdrianS

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    Read up on how many PSRU (and crank) failures they had on the dyno before the design was signed off for production.

    Is a reliable PSRU possible? Definitely.
    Is designing a reliable PSRU a simple task? Only if the power rating is low and/or the weight budget is generous.
     
  17. Sep 17, 2019 #17

    BJC

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    PSRUs in general don’t get a bad rap; there are many flying today. Two of the highest production GA aircraft engines, the Rotax 912 and the M14, use reduction gears.

    The problem has been with PSRUs that were flown without adequate design and or development. That is understandable, since very few homebuilders have the education, experience, time and money to develop one. There are some successful ones in service, and some new commercial offerings that may prove to be acceptable, but the list of failures in E-AB aircraft is long enough to have resulted in the bad rap.

    A successful complete auto conversion FWF package that weighs about the same as a Lycoming of the same HP, occupies about the same volume, runs reliably for 2,000 hours, and costs significantly less than the Lycoming will be widely accepted by E-AB aircraft builders.


    BJC
     
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  18. Sep 18, 2019 #18

    Rik-

    Rik-

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    You can use a "Gilmer Belt" which is a cog belt like is used on Blower Motors. Gates and GoodYear both makes these as power transmission belts.

    They come in various pitch (distance between the cog's, various lengths and the power ratings vary accordingly.

    I've used them, 14MM Gates Belt, with Kevlar reinforcement (same as you'd see on a Top Fuel Dragster) in a 4" width coupled to a Lycoming T53-L13B turbine engine in a boat. 300 hrs and no failures.

    Beauty of them is that they require no cooling nor lubrication.
     
  19. Sep 19, 2019 #19

    RSD

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    It would be interesting to see how well they handled a Mazda Rotary to prop or ducted fan application
     
  20. Sep 19, 2019 #20

    Rik-

    Rik-

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    I can’t see why it would be a problem. There are companies that make the belt sprockets/pullies for the filler belts. Just research blower motor belts and sprockets and you can get them made in various sizes.

    Then you will need a structure that attaches and holds your shafts and bearings. You can use a sealed plate mount bearing like they use in industrial fans. Low maintenance low cost
     

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