Worried about the long, steep hill on my commute

Discussion in 'General Questions' started by JinbaIttai, Jun 25, 2008.

  1. JinbaIttai

    JinbaIttai Member

    Which engine kit should I buy?

    I weigh about 150 lbs and am in my early 30s. I intend to buy an aluminum framed mountain bike or hybrid.

    The commute will be to and from work, 13 mile round trip. The posted speed limit is 35 mph and one flat stretch is at 30 mph. Traffic speed is usually 40-50 mph.

    My biggest concern is that hill. It goes from sea level to 800 feet over a span of about 3 miles. It is the reason why I want a bike with an engine. I assume that this hill won't be so good for gas mileage or engine life.

    My goals (in order of importance):
    #1 best mpg
    #2 reliable
    #3 quiet (non-high pitched)

    Will a sub-50cc work too hard up the hill and ruin my mpg? Or would it be better to take the mpg hit of a larger displacement engine because the overall mileage will be better? I've never ridden an MB before.
    Can I have my cake and eat it too?

    I am considering a dax 70 cc or maybe a 4-stroke Japanese sub-50 cc. Other than reading through a few forum posts, I am relatively unfamiliar with both. What do you think?
    Last edited: Jun 25, 2008

  2. GEARING! For the DAX 70 go with at least a 48 tooth if not a 50 tooth. You'll be able to get up that hill with pedaling that feels like you're going down a straight road.
    ALWAYS consider helping your engine along when pedaling.
  3. augidog

    augidog Banned

    there is no such thing as a long steep hill with a tanaka 2-stroke...you could enjoy a decent pull with a robin-subaru 4-stroke 35, but you'd have to gear down a bit.

    recommendations are good to get, but be sure to (independantly) explore all your options. also, do yourself a favor & start learning about what happens to bicycles when you motorvate 'em, safety and strength become real issues :cool:
    Last edited: Jun 26, 2008
  4. eljefino

    eljefino Member

    I have a 70cc HT with a biggish ~48t sprocket.

    It pulls hills fantastically! I'm about 195 lbs and the bike maybe another 45.

    I have rolling hills here, and crest them at 10 mph, kill the motor, and coast down the other side.

    on the flats, the setup sounds happy at 24 mph and maybe 1/4 throttle, and would pull away from that if I wanted... but would run out of gear and overrev the motor.

    I get the much-quoted 150 MPG. Might be the coasting. :) But seriously going near full load is pretty efficient for a gas motor. I don't think the simple carb in these things has any "power enrichment" so going WOT isn't that inefficient.

    Incidentally my steel-framed kulana moon dog is (was, I sold it :) ) much much better in the comfort and vibration aspect than my converted alloy MTB. Soft steel and long ape handlebars deaden such things. Arriving at work comfortable and with hands that aren't tingling is worth something.
  5. Hey,

    I use my Moondog w/ a 80cc - motor - so they say: on a 1.4 mile ride to work each day.
    0.7 miles of that is about a 23% uphill grade. I use a factory 44 tooth sprocket, and with pedal assist - no real hard peddling - I can go up my hill at about 20-23 mph w/ very little effort.
    So far, I haven't noticed any problems or strain on the motor at all.
    I weigh about 155 lbs. - it takes me where I want to go w/ no problem.
    If you go w/ a bigger tooth sprocket, you will lose top speed in the exchange.
    IMO - a hill doesn't get much steeper than what I go up every day, so I think you would be fine w/ the stock set up if it's just a gradual incline.


    Troy Fitz.

    Attached Files:

  6. JinbaIttai

    JinbaIttai Member

    Although I didn't include price in my list of priorities, the HT 70cc is definitely priced better, and therefore easier to get the woman's ok to buy.;)

    I'm leaning towards a steel-framed hybrid with front suspension and linear brakes, with a HT 70cc frame-mounted 2-stroke, 44 tooth, with all hardware swapped for high quality stuff, and custom motor mounts.

    I keep reading about how aluminum frames aren't desired since they transmit engine vibrations more noticeably. I wonder, with good motor mounts, would it become a non-issue?
  7. lennyharp

    lennyharp Member

    Gearing is the answer as everyone says. I can ride up long steep grades with no motor if I have a low gear to allow me to pedal easy. That is what Alpine gearing is all about for bicycles. I also pedal up a steep grade with my Staton/ Subaru 35 as it can bog down with my weight (230) and gearing. Easy pedaling really helps the motor out.

    As to aluminum versus steel my main concern is the lack of resiliency. It is stiffer. The vibrations that are being transmitted can do you, the bike frame and as a whole all parts damage. Steel can be bent and unbent numerous more times than a similar strength piece of aluminum, which is the materials fatigue strength. This is a critical difference as steel tends to stretch and break slowly, aluminum goes quickly.

    Lastly aluminum frames also are heat treaded after welding to add strength. To make any welding repairs or changes may ruin a frame. While some steel frame repairs are delicate proceedures, it is more sure of getting a lasting and strong repair. Frame repairs on steel are easy but not on aluminum.
  8. HI,

    I would recommend a large engine (I personally run a Dax 70) and 36T or smaller rear sprocket.....I weigh about the same as you (me: 155 lbs) and I can climb some very steep hills here in VA without pedaling (I actually plugged you info into an angle calculator and if your figures are accurate (800 feet rise over 3 miles)....yields an average 2.9 degrees over that distance (of course that is just average and there may be some steep areas and some flat areas) so the overall angle is not that steep but the distance is fairly long uphill.....I feel that the large engine is good for power (maybe not technically legal without a license but you can always argue "effective displacement" VS swept displacement but that's a whole different argument) and the smaller sprocket would give you a good cruise speed (to keep up with traffic better and lower engine revs for better mileage / wear and tear)....Granted a stock or larger sprocket would be easier on the engine up hills BUT you will severely limit your top speed and or make for a real buzzy cruise....I think you would be happier overall with a smaller than stock sprocket....how small you decide to go is of course up to you....Of course you can simply get a kit....run it stock and if the engine is too buzzy or the bike not fast enough you can always regear from there but the advantage is that you would not have spent any additional $$ on a sprocket until you see what you need....
    Hope this helps you.....Good luck and ride safe!
  9. Mountainman

    Mountainman Active Member

    I guess it depends on how fast we wish to get there - as far as engine size.. I am new to these small bicycle engines - but - after a few days of riding with my Subaru 35cc friction drive - I have hit some pretty good hills already - no problem with a little pedal assist. I live up a 5 mile mountain going from sea level to aprox. 1,500 ft.. I have no doubt that my motor is up for it -- it's just one dangerous road with not much of a bike lane -- many killed on that road since I have lived here -- and they were not on motorized bikes -- in time I am sure that I will go for it -- remembering to PRAY FIRST !!! Happy riding from - Mountainman
    Last edited: Jun 27, 2008
  10. JinbaIttai

    JinbaIttai Member

    You guys are awesome. Thanks for all this help!

    I didn't think about fatigue strength at all. Ok it is looking like a steel frame, with a Dax70 and a 36 tooth gear. I double checked the maps again, and it goes from about 100' to about 900' over 2.9 miles, so the slope is still accurate enough I think.

    I anticipate 1/4ish throttle on the flat portions, and whatever speed that gives me, I'll cruise at. So do you guys think I'll pull off 150 mpg with this setup, even going up this particular hill once a day? I'm thinking of 1.75" all weather-type road tires for a few extra mpg over typical knobby mountain bike tires.
    Last edited: Jun 28, 2008
  11. eljefino

    eljefino Member

    A lot of people like 1.95-2.25" semi-slicks. I took a pinch flat from gravel and the wider tires are more reliable b/c they absorb shocks better.
  12. duivendyk

    duivendyk Guest

    As an engineer I attempt to solve problems based on science,I run the numbers.I hope you can follow me: we'll deal with the hill first.I work in the MKS system (universal except for the US, thanks to that Reagan nitwit);your weight+ bike I will guess generously at 115 kg (250 lbs),the incline is 800ft over 3 miles or 5280 ft,that means a 5.1% grade. The force required to get you+bike up that hill is: 0.051*115= 5.9 kg (13 lbs ,* means multiply).Now we will figure out how much power it takes to get you up that hill at a certain speed,say 20 mph.(32 km/h),32km/h is 32000/3600=8.9 m/sec (29 ft/sec).
    The power required is: Force times speed or:
    5.9*8.9=53 kgm/sec .But that does not take the wind & rolling resistance into account. Guess at around 0.5 HP,one HP=75 kgm/sec., 0.4 HP =30 kgm/sec .Total power reqd is 30+53=83 kgm/sec or 1.1 HP.Assuming 85% drivetrain efficiency we need 1.3HP to get up that hill at 20 mph,and correspondingly less at other speeds,for instance at 15mph it would take about 3/4*1.3=1HP and it would take you 12min to get up that hill,instead of 9 min at 20mph
    You have the choice of a fixed gear ratio (much cheaper) or the variable speed NuVinci hub + Staton gearbox ,(expensive but flexible, also heavy). In the fixed ratio we have the Staton gearbox and the GeBe kevlar belt drive or the DAX Titan countershaft setup. As far as engines go, forget the Chinese (HT) engines,too unreliable.Reasonable choices are the 49cc Honda or DAX Titan.Both engines have a power capability of around 2.5HP but should,in my opinion not be run at over 1.5HP continuously.
    With fixed drive you have a basic problem,to get up the long hill you need to gear down sufficiently to have enough torque, where the rubber meets the road,but you would prefer not to overrev the engine either when cruising on the flats.Peak torque for these engines occurs at around 5k rpm,at around 80% of max power or 2HP,derating this by 30 % to have something to spare we can count on having 1.4 HP on hand at 5krpm.That's enough.
    There is a basic relationship between power (P) torque (T) and speed N (rpm),P= 0.0002*N*T , at 5000 rpm with 1.4 ft lbs torque the power delivered is :0.0002*5000* 1.4 =1.4HP which is more than the 1.3 we needed at 20 mph.With a 26" wheel, 20 mph corresponds to 270 rpm,so we need reduction of 5000/270=18.5
    This is all well and good, but how will we be doing on the flats?.If we limit the rpm to 6.5 k we get a speed of 26mph and you could at 7.5k rpm you can get up to 30mph (bad idea)
    How to get the 18.5 reduction?.The Staton gearbox has a 18.75 reduction so, a 1:1 from the gearbox to the rearwheel would do nicely.They have a hub with a freewheel input from the engine.The DAX Titan has a built in 5.0 reduction and a 1:1 to the jack shaft so you need around a 3.7 reduction.10t to 36t rear sprocket would be about right.There is no freewheel in this drive train, only the centrifugal clutch.One thing to bear in mind is that such a clutch does not disengage if you throttle back unless you brake to a sufficiently low speed.So you could use engine braking going down hill.With a freewheel you have to rely totally on your brakes,watch out for overheating.I guess you are going to find the DAX setup a lot cheaper than the Staton.I am a bit leery of the DAX engines,will the Titan last ???.Fuel consumption is not much different between engines,it takes a certain amount of energy to get up that hill and your fuel has to supply it. I hope you were able to follow my design process,good luck
  13. augidog

    augidog Banned

  14. eljefino

    eljefino Member


    This is probably the slowest (speed) bench racing anyone's ever done... ;)
  15. duivendyk

    duivendyk Guest

    Tortoise & the hare,while you all,HT speed freaks are racing along,**** the torpedos to your next breakdown (you should pull along a trailer with tools+spare engine),we plodding engineers pass you by,could lend a hand,or first aid as required.
    I understand you now have two of these Chinese jewels,smart move,you might keep at least one running at the time.Makes things exciting if you're a glutton for random punishment.Have fun and be sure to carry plenty of life insurance.Someone might appreciate that.