Announcement

Collapse
No announcement yet.

Mongoose Dolomite Spec'd for BBSHD

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

    Mongoose Dolomite Spec'd for BBSHD

    I picked up a Mongoose Dolomite the other day and thought I'd power it with a BBSHD. The first task was to measure the bike clearances to see if the BBHD would fit and, if so, whether to use a 100 or 120mm.



    Mongoose Dolomite with 120mm BBSHD
    Click image for larger version  Name:	Mongo Dolo2.jpg Views:	1 Size:	1.12 MB ID:	91081


    The bike has a 100mm BB shell width with an ID of 33.6mm, a 190mm pedal axle, 190mm rear dropouts, 4 inch tires, and a 14 to 28 7 speed freewheel . The chain line is 82mm. I used a section of 1 inch sched 40 pvc pipe with a slot cut for a rule to measure from the bottom bracket (see pics below). I used the measurements from the Bafang installation manual and those from the forum thread, Chainring Offset Guide, for the drawing. Again, see below.

    I determined from the measurements that either the 100 or 120 BBSHD would work, but the 120 results in more symmetrical peddle offsets. Essentially, the drive side of the motor remains fixed in relation to the bike with either, but the non drive side would be shortened by 20mm when using the 100mm model. This would result in 103-87 right and left axle lengths as opposed to the 103-107 lengths with the 120mm model (See the drawing). Both installations would require non drive side spacers.

    An unknown is the crank arm clearance. If someone can provide the Bafang crank arm offset, I would appreciate it. As can be seen from the drawing, there's not much clearance for crank arms at 170mm. I doubt that the standard Bafang crank arm would clear on the non drive side using 100mm installation and I'm not sure about the clearance using the 120mm install. One way to determine the offset would be to put the arms back to back at the top and then measure the distance between them at the pedal attachment point. Or, place the top of the arm back side down on a flat surface and measure the rise of the inside of the arm at the pedal end.

    The drawing suggests the 46t chain ring would sit considerably inboard of the bike's design chain line of 82mm. A smaller chain ring is probably a better choice for gearing and would move the chain line outboard. For example, using the Luna 40t would move the chain line out by 10 mm to 75mm which is closer to the original and would probably allow usage of all the freewheel cogs.

    I believe the Bafang measurements are applied correctly, but I would appreciate people double checking them. The bike measurements, I think, are within a couple mm, but please check before doing a similar install. There seems to be some inconsistencies between frames as built.

    I welcome your comments and questions and I am anxiously waiting for someone to give me the Bafang crank arm offset measurement. I will continue this thread as the build progresses.




    Click image for larger version  Name:	bafangsnip.JPG Views:	1 Size:	31.0 KB ID:	91082
    Attached Files
    Last edited by Jay Aitchsee; 08-07-2019, 07:24 AM. Reason: formatting

    #2
    Originally posted by Jay Aitchsee View Post
    I picked up a Mongoose Dolomite the other day and thought I'd power it with a BBSHD. The first task was to measure the bike clearances to see if the BBHD would fit and, if so, whether to use a 100 or 120mm.



    Mongoose Dolomite with 120mm BBSHD
    Click image for larger version Name:	Mongo Dolo2.jpg Views:	1 Size:	1.12 MB ID:	91081


    The bike has a 100mm BB shell width with an ID of 33.6mm, a 190mm pedal axle, 190mm rear dropouts, 4 inch tires, and a 14 to 28 7 speed freewheel . The chain line is 82mm. I used a section of 1 inch sched 40 pvc pipe with a slot cut for a rule to measure from the bottom bracket (see pics below). I used the measurements from the Bafang installation manual and those from the forum thread, Chainring Offset Guide, for the drawing. Again, see below.

    I determined from the measurements that either the 100 or 120 BBSHD would work, but the 120 results in more symmetrical peddle offsets. Essentially, the drive side of the motor remains fixed in relation to the bike with either, but the non drive side would be shortened by 20mm when using the 100mm model. This would result in 103-87 right and left axle lengths as opposed to the 103-107 lengths with the 120mm model (See the drawing). Both installations would require non drive side spacers.

    An unknown is the crank arm clearance. If someone can provide the Bafang crank arm offset, I would appreciate it. As can be seen from the drawing, there's not much clearance for crank arms at 170mm. I doubt that the standard Bafang crank arm would clear on the non drive side using 100mm installation and I'm not sure about the clearance using the 120mm install. One way to determine the offset would be to put the arms back to back at the top and then measure the distance between them at the pedal attachment point. Or, place the top of the arm back side down on a flat surface and measure the rise of the inside of the arm at the pedal end.

    The drawing suggests the 46t chain ring would sit considerably inboard of the bike's design chain line of 82mm. A smaller chain ring is probably a better choice for gearing and would move the chain line outboard. For example, using the Luna 40t would move the chain line out by 10 mm to 75mm which is closer to the original and would probably allow usage of all the freewheel cogs.

    I believe the Bafang measurements are applied correctly, but I would appreciate people double checking them. The bike measurements, I think, are within a couple mm, but please check before doing a similar install. There seems to be some inconsistencies between frames as built.

    I welcome your comments and questions and I am anxiously waiting for someone to give me the Bafang crank arm offset measurement. I will continue this thread as the build progresses.




    Click image for larger version Name:	bafangsnip.JPG Views:	1 Size:	31.0 KB ID:	91082
    What a look.
    tellthebell

    Comment


      #3
      Did you install the BBSHD? I'd like to see

      Comment


        #4
        Hi Jay,

        I'd recommend using the 120mm BBSHD. I used the 100mm unit, and had to trim 1/4" off the right side of the bottom bracket (sprocket side).

        I went with a 42 tooth Eclipse sprocket from Luna. The chain and sprocket alignment required a shim under the sprocket to even out the spacing. I'm pretty happy with the gearing using the 42t sprocket. The Shift sensor is a waste of money; my Dolomite shifts fine under power.

        If I had a do-over, I'd go with a battery mounted on the downtube as opposed to the rear rack, to even out the weight distribution. I do admit I like the extra weight planted over the rear axle; the extra traction helps me stay upright!

        I went with a 52V 20AH battery. I have yet to deplete the battery in a day's ride!

        Comment


          #5
          Refer to the attached pictures for actual clearances. This is with a 100mm unit, and .250" trimmed off the sprocket side bottom bracket.

          Go with the 120mm unit. You'll get more reasonable Left Side Crank Clearance & Full Threads on the fastening nut.

          BTW, the original crank arms will bolt up. That would also provide more clearance, as the stock Mongoose arms have a little flare in them.


          Click image for larger version  Name:	Left_Side_Crank_Clearance[1].jpg Views:	0 Size:	1.25 MB ID:	102139
          Attached Files
          Last edited by BraveMan; 02-23-2020, 11:40 AM.

          Comment


            #6
            Thanks, BraveMan. You've confirmed what I thought. I'll definitely go with the 120mm. Good tip on the crank arms. Hopefully, I can get started soon.

            Comment


              #7
              Sweet mary mother of god I wonder though, if 100mm could work with a smaller chainring?

              Comment


                #8
                Hi everyone. I just bought the 120 mm kit for the mongoose. My question is how long should the chain be or a specific chain to buy for the 42t ring? Thank you in advance

                Comment


                  #9
                  Hi, Lordchilly.
                  Any good quality 8 speed chain should do.
                  A good rule of thumb is that the chain should be two links longer than the length needed to fit around the largest rear and front cogs simultaneously without being threaded thru the derailleur.
                  For everything you ever wanted to know about chains, see:
                  https://www.youtube.com/watch?v=O0YibMDWBAw
                  https://www.sheldonbrown.com/chains.html
                  https://www.sheldonbrown.com/derailer-adjustment.html#chain

                  Comment


                    #10
                    Doing the exact same thing. Post most helpful. What did you decide on the sprocket, 42 or 46? Which one would be most inline?

                    Comment

                    Working...
                    X