Rack-mount, belt-drive, new build

Discussion in 'Rack Mounted Engines' started by knobb490, Aug 16, 2010.

  1. knobb490

    knobb490 Member

    Hi all,

    I've started a new rack-mount build. I'm documenting it on my blog which is http://enginepoweredbikes.blogspot.com

    This is a belt drive using a 20" rim for a driven pulley, attached to a 26" mountain bike wheel. Take a look at the blog to see what I've come up with. I'd love some ideas on better ways to attach the pulley to the rim if anyone has any.


    So many freaks, so few circuses
    Last edited: Aug 16, 2010

  2. loquin

    loquin Active Member

    My first question is "why do you want to go 14:1 ???"

    The HF engine is rated at 3600 RPM. But, 400 RPM on a 26 inch wheel equates to 31 MPH.

    So, a 3600/400, or 9:1 reduction gets you 31 mph. Your 14:1 would generate a 50 mph top end, assuming the motor can push you that fast, but, that kind of speed on a bike could be deadly.
  3. Esteban

    Esteban Active Member

    I agree 100%. Speeding on a $99 bike ????????? PLUS, it draws too much interest from the law !!
  4. Wheres my dog

    Wheres my dog New Member

    I have to give credit though....

    VERY ingenious with splitting the screws and using them to attach to the spokes!

    Please keep us updated as to the progress on the setup here, I know I am interested in seeing it come to fruition!
  5. knobb490

    knobb490 Member

    Where's my dog - Thanks for the complement. I'm not sure that I won't be breaking spokes with the set-up but we'll see.

    Loquin and Esteban -

    What? My math is wrong? Dang, why didn't I pay more attention in high school? Okay, somewhere I must have gotten off track. If it isn't too much trouble, will someone please point out the errors of my ways? I have a lot of them according my family, but if you'll just focus on the math errors, I'm working on all the others.

    Goal: Speed of around 20 mph @ 3600 engine rpm. There are a number of conversions that need to take place so that I'm dealing apples to apples.

    Step 1 - Determine distance traveled by 26" wheel in one revolution. I think this is wheel diameter x Pi or 26 x 3.14 = 81.64 inches.

    Step 2 - Convert previous answer from inches to feet. 81.64 inches/12 = 6.8 feet.

    So one revolution of the tire will move the bike 6.8 feet. Now I need to determine how many feet I need to move in one minute to go 20 mph. Since I'm dealing with feet in terms of the tire rotation, I need to covert miles/hour into feet.

    Step 3 - Convert miles/hour into feet/hour. 20 miles/hour x 5280 feet/mile = 105,600 feet/hour.

    Step 4 - Convert feet/hour into feet/minute. 105,600 feet/hour divided by 60 minutes/hour = 1760 feet/minute.

    So the bike needs to travel 1760 feet/minute to make 20 mph. Now I need to determine how many tire revolutions I need to travel 1760 feet.

    Step 5 - I determined from step 2 that one tire revolution moves the bike 6.8 feet, so I divide the distance of 1760 feet by 6.8 feet/tire revolution. This gives me 258 tire revolutions per minute to go 20 mph.

    Step 6 - The engine is rated at 3600 revoutions per minute. I need to reduce engine rpms to 258 tire rpms. How much reduction do I need? Divide 3600 engine rpms by 258 tire rpms to get a reduction of 13.95:1, rounded to 14:1.

    Okay, please straighten me out.
  6. loquin

    loquin Active Member

    No - your calcs are 100% correct. It was me, winning the "D'Oh :goofy: award" for the day!
  7. knobb490

    knobb490 Member

    No problem, please question anything I'm doing any time. I sometimes wonder about what I haven't taken into account and it helps to have someone looking over my shoulder!
  8. ibdennyak

    ibdennyak Guest

    Had me scratching my head there for a bit Lou. :detective:
  9. knobb490

    knobb490 Member

    New way to mount driven pulley to bike wheel

    I've updated my blog with a new way to mount the rim-pulley to the host wheel. You can see it at http://enginepoweredbikes.blogspot.com/. It includes more photos and detailed description. Any thoughts/ideas are welcome.

    If you need a tool and don't buy it, you'll eventually find you've paid for it, but don't have it. -Unkown
  10. Wheres my dog

    Wheres my dog New Member

    Nice work with the P clamps you mounted...

    Just do a google search for the coupon and you will find it online!

    If not, PM me with your email address and I will gladly send you a jpg of one for you to use!

    Keep up the good work....
  11. ibdennyak

    ibdennyak Guest

    Now that's ingenious. Should be easier on the spokes too. :idea:
  12. Esteban

    Esteban Active Member

    Sorry, I had the wrong figures, too.
  13. knobb490

    knobb490 Member

    ibdennyak - Thank you for the kind words. I'm hopeful that it'll work. It sure was faster than cutting slits in machine screws!

    Esteban - I'm glad to have a few folks with experience like you and loquin jumping in to save me from myself. Doesn't matter to me if your figures were wrong, I'd rather have you say something than me risk a problem down the road!

    If you need a tool and don't buy it, you'll eventually find you've paid for it, but don't have it. -Unkown
  14. knobb490

    knobb490 Member

    Wheres my dog - Great suggestions, I found the Harbor Freight coupon for 20% off and printed it out thank you! I stopped into HF and found that the engine is normally $129. It goes on sale for $89 about three times a year. The clerk I talked to said he didn't know when it would be on sale, but based on the previous sales pattern was thinking September. That's not too far away. I'll be keeping an eye open.

    If you need a tool and don't buy it, you'll eventually find you've paid for it, but don't have it. -Unkown
  15. knobb490

    knobb490 Member

    I know that Harbor Freight always has 5-10 different sales going on at the same time, but not knowing when this 97964 2.5 hp engine would next be on sale was killing me. I decided to make some phone calls and see what I could learn. So I called Harbor Freight's 800# and they said that although the 97964 2.5 HP engine isn't currently on sale, it has sold in the past for $89 and they would honor that price for me if I wanted to order it today. Add $11 for shipping and I'll have it in 5-7 days.

    Um, no thanks, I'd rather just pick it up from the store so I can have it right now, but do it for the $89 sale price. Can't they just arrange that? No, they can't. They don't have anything to do with the retail stores and don't know when it will next go on sale.

    Great. I got nothin'. I called one of the two Harbor Freight stores on State St. in SLC, UT. This one is on my way home, but isn't that great of a store. Junky appearance, the help isn't helpful, and their inventory is mostly depleted - they don't seem to have the items in stock that I'm looking for. I told them that the rep at the 800# agreed to sell me a 2.5 hp engine for $89, and would they do the same? Dead air on the phone while the clerk mentally processed what must have been a very complicated question. When the clerk finally figures out what I'm asking, she responds that the 6.5 hp engine is currently on sale for $99 and she can do that.

    Hmmm, this isn't going to work, not with this clerk. I thank her for her time and hang up. I call the other Harbor Freight store on Redwood Rd. in SLC, UT. This was much better. This clerk knew my request would require approval from another person higher up. Her name is CJ, the manager, whom she promptly gets on the phone with me. I gave CJ the background of my phone call with the rep at the 800# and she said she'd match that deal and to see her when I come in. I'm impressed! This HF store knows how to get and keep customers!

    I went in and bought the engine, and had the 20% off coupon which they honored as well (thanks to "Wheres my dog"!), so I was out the door for $76.92 incl. tax. I even got a free flashlight with another coupon. Nice! I'll be back to this store!

    If you need a tool and don't buy it, you'll eventually find you've paid for it, but don't have it. -Unkown
    Last edited: Aug 26, 2010
  16. professor

    professor Active Member

    I also have a HF 79 mounted in a legnthened frame (3"). The motor has been fine- really a nice little engine. But it is wide, so mine is moved up and forward. It just clears my knees and I am not tall.
    I suggest centering the engine so the spark plug is just on the left side of the frame rail (about a 1/2 inch over farther than mine). I cut a little off the engne output shaft too.

    Since you are going to need a jack shaft of some sort, you can buy pillow block brgs and shaft from McMaster Carr, if you use a charge card.
    They also sell V belts and pulleys.
  17. knobb490

    knobb490 Member

    Professor Mike Thank you for the suggestion on placing my engine and the referral to McMaster Carr. What a great site they have! On the jackshaft, I'm trying to build this without one. This means the engine will be offset a ways, but hopefully not too far. This also means I'm abandoning the frame mount option for now and going with a rack mount. That's probably good because I'd like to just keep this thread going and not have to jump over the frame mount section of the forum (grin).

    I've started work on the rack but I'm sure I have several hours of time remaining. I've updated my blog with photos and descriptions for anyone who cares to see. http://enginepoweredbikes.blogspot.com/

    If you need a tool and don't buy it, you'll eventually find you've paid for it, but don't have it. -Unkown
  18. knobb490

    knobb490 Member

    Building the Rack, Phase 2

    I've done a bit more on this, completed an offset engine frame and clamped it on the bike. There's more detail and photos on my blog at:


    Suggestions sure are welcome!

    If you need a tool and don't buy, you'll eventually find you've paid for it, but don't have it. -Unknown

    Attached Files:

    Last edited: Sep 14, 2010
  19. knobb490

    knobb490 Member

    It works!

    Success! I took it for a spin Saturday night. I've documented it on my blog at http://enginepoweredbikes.blogspot.com/, including photos.

    Check it out!

    If you need a tool and don't buy, you'll eventually find you've paid for it, but don't have it. -Unknown

    Attached Files:

    Last edited: Sep 28, 2010
  20. knobb490

    knobb490 Member

    Further refinements

    While it works, it was pretty rough in some spots. I made some adjustments to the clutch lever, making it more narrow and repositioning it so it holds itself in the engaged position until the rider releases it. I added a throttle cable and some no-slip tape to the driven pulley. New top end is 33 mph. I've got more detailed descriptions and pictures at my blog.


    I'd love your feedback on other improvements. Thanks!

    Attached Files: