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Help Gearing 8-speed MB with BBSHD (First Post! First eBike!)

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    Help Gearing 8-speed MB with BBSHD (First Post! First eBike!)

    Hi all. Just wanted to say thanks in advance for the wealth of knowledge here. I'm getting ready to pull the trigger on a BBSHD kit for my old but trusty 2003 Ibex Alpine 550. This will be my commuting bike for a new job I'm starting in North Carolina. Before I get into it, I've read a TON of posts about gearing, but I was hoping for some advice on my particular situation.

    Me: 6'1" 190 lb, with about 20-25 lb of cargo. I bike for transportation and exercise. Days of trail riding and single track are gone until my kids get interested in it.

    The bike: Not many IBEX's around, but this bike has been good to me. 26" tires, medium frame hardtail, 3x8 speed with Deore components and 11-30t cassette.

    Where I'll ride it: Primarily commuting and neighborhood bike rides with the family. FLAT roads where I'm moving (couldn't find a hill if I tried). 7 mile round trip for commute. Hope to cruise between 20-25 mph, with a 1/4 mi max speed sprint.

    My conundrum: I want to get some exercise on my commute, so I'm trying to gear the bike so that I can maintain a natural cadence at a comfortable cruising speed for a majority of my commute. I suspect a low PAS setting would accomplish this for me (as long as I'm not running late). There is a short (0.25 mile) of dangerous road I'll need to ride on where I would want MAX speed to keep up with traffic (throttle only ok). I wanted to get an idea of what kind of speeds I could obtain with the chain rings available to me, so I made a spreadsheet to look at some gearing options.

    A couple scenarios:

    1) Estimating a comfortable cruising cadence of about 60-80 rpm, a 52t Lekkie Bling Ring, and on the 11t sprocket, I calculated 23.6mph, 27.6mph, and 31mph for 60, 70, and 80rpm cadence, respectively. I'd need a cadence of about 90rpm to reach 35 mph, so I'm hoping I can use throttle only to reach 35mph for the stretch of 'busy' road with no bike lane. Because the 52t presents the opportunity to lug the motor in stop and go situations, I also thought I'd swap out the 11-30t cassette for a wider range 11-34t cassette. That gives me a little more versatility (barely) and I'm pretty sure my RD can handle it.

    2) I go with a 48t Luna Eclipse and shed about 2-3mph for the respective 60,70, and 80 rpm cadence.

    3) I drop down to the 42t Luna Eclipse. For that chain ring I calculated 19mph, 22mph, and 25mph for 60, 70, and 80rpm cadence, respectively. To hit 30mph, the chain ring is doing about 95 rpm. I think I'm throttle only at that point.

    So, I'm looking for some advice on what path to take. I want to be able to use all 8 gears, which I don't think will be a problem with any of the chain rings listed, but it's impossible for me to know what offset I need until I get the motor installed. I also want to be able to pedal for exercise and to increase my range. I'd like to get 2 trips (14 miles) out of a trip if I can.

    I also realize that cruising with high loads on that little 11t sprocket is going to mean increase wear rates. If I cruise with a 52t-13t combo, my gear ratio is still higher than the 42t-11t combo and maybe I extend the life of my rear sprocket by 10%....

    Any thoughts? THANK YOU SO MUCH!

    -Ryan

    #2
    Increase your spin rate. I have the BBS02 mounted on a tandem with an 11/36 cassette and a 44 tooth chain ring. We spinout at about 35 on the 44/11 combo. 60/70/80 RPM, to me, is slow as I am a spinner and prefer 90+ rpm's. I use a good old car manual 4 speed drive tranny as my reasoning. Grind along in 4th at 25 and punch it, what happens? Not much of anything until you get the motor wound up. do the same 25 mph in 2nd and punch it, you get 'er going right quick. In bike racing, its easier to respond to attacks and such if you maintain a higher rpm. Now we are not talking racing and such, but its all the same, higher cadence = more exercise and better motor performance. Like I said, get comfortable spinning and keep you cassette and go with a 44tooth ring. You don't have hills and you needn't go faster than that on a frigging bicycle anyway, there is a safety issue here.

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      #3
      Totally hear you. I'm really just guessing at 60-80 being my norm. It's hard for me to estimate my cadence. I found a tone generator app for my phone, so I'm going to set it at 1.5 Hz and see if 90 rpm can be a comfortable cadence for me. Of course, unassisted, it will be at a much slower speed.

      I'm really interested to compare energy usage verse rpm on the BBSHD. I suspect there's an optimal motor speed for lowest W/h at constant velocity. Might be an interesting experiment (but cost a small fortune in chain rings) :-)

      Thanks again

      Comment


        #4
        Sheldon Brown IGH gear calculator. For "gear units" select MPH @ 90 RPM if you don't have any other preferance. Put in your chain ring, rear cog and wheel size, select your hub. Hit calculate.
        You can put in several cogs and it will solve for all of them at once.
        https://www.sheldonbrown.com/gear-calc.html

        Comment


          #5
          Sorry about the IGH post. that was another thread. You might take a look at my 2004 Gary Fisher build. flat terrain, BBSHD, 8 speed. I ended up with 50Tx11-40 casette. I'm not much of a spinner so 175mm cranks for me and long gears. But the Sheldon Brown gear calculator works for casettes also. I can lay down a 1/2 mile at 30mph any time I want to.
          https://electricbike.com/forum/forum...-bbshd-project

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