The Riding Gravel Forum
Register Calendar Latest Topics Chat
 
 
 


Reply
  Author   Comment  
cmcalpin

Member
Registered:
Posts: 48
Reply with quote  #1 
I didn’t know which forum this would be best in but since it pertains to bikes I thought I would post it here. Always lots of discussion about bottom bracket drop. I have even heard people say they don’t like “said” bike for gravel because it does not have enough drop yadadadada. I have been on the fence about it at the mercy of advice from people more experienced than I but still haven’t fully subscribed to the hoopla. On a bike the weight of the cyclist gives it a high cg regardless of bb drop. I talked to a well known frame builder recently and he said it was hype. I won’t get into everything he said cause that’s too much typing but he definitely got me to re thinking. I stumbled across this article tonight and really found it good reading. Maybe you guys have read this and maybe not but for those who can’t decide on what their next frame or bike may be, maybe this might be of interest to you. http://davesbikeblog.squarespace.com/blog/2007/2/21/bottom-bracket-height.html

I certainly don't know everything but I am always on the quest for truth and knowledge.

__________________
 
0
nalax

Member
Registered:
Posts: 45
Reply with quote  #2 
Thanks. Dave Moulton probably knew a thing or two about frame geometry.
0
dangle

Senior Member
Registered:
Posts: 128
Reply with quote  #3 
He's right, but there are two things to add to the conversation.

1) Bottom bracket height will change with whatever tire is on there. How does one measure bottom bracket height unless the exact wheel and tire is used on every bike out there? BB drop is more universal.

2) Yes, the rider determines the center of gravity. Assuming the same crankset and pedal is used with two frames, one with a 70 mm drop and one with a 60 mm drop, it seems the one with less drop would result in a center of gravity 10 mm higher.
0
chas

Avatar / Picture

Senior Member
Registered:
Posts: 160
Reply with quote  #4 

Interesting read.  Thanks for posting.

In my quiver, I have an extreme range of BB drop from 50mm to 80mm. 

For me the biggest and most obvious difference is pedal strike.  The 50mm drop will never have pedal strike (unless I hit something), I can ride on a 45 degree incline with no strike.  The 80mm drop threw me off the road at 30mph when the pedal hit the road turning left.  When your pedal hits the ground, your rear wheel is lifted off the ground and then dropped back down.  Not a pretty site. 

 

I don’t know if I can really attribute much stability to drop.  The wheels are huge gyroscopes and I can easily ride with “no hands” on either bike.  A mountain bike with “long, low, slack” geometry is easier to ride fast down hill, but I’m thinking that is more of a head tube angle thing (and rider position on the bike).  Going down hill standing tall, sitting down, or using a dropper post makes a difference.  But does moving the BB 10mm?  Kinda, a little maybe….?  Clearly a couple of inches (i.e. dropper post) makes a difference for hard mountain biking. 

 

Interesting comments about its effect on the frame.  Higher bottom bracket allows for shorter chain stays (and presumable a more responsive bike).   One can of course make longer chain stays without dropping the bottom bracket.

Bottom bracket is just a part of the picture though. Typically a low bottom bracket (these days) is included with longer wheelbase and slacker head tube angle.   To me, these make the bike feel sluggish and less responsive.  Of course this is a good thing if you like bombing down hill, or take trips that are so long that you are totally wiped out at the end of the ride.

I agree with the author, that on most bikes, a 70mm drop is a good target for a responsive bike.    That is my target.

0
cmcalpin

Member
Registered:
Posts: 48
Reply with quote  #5 
I always struggle with things I read. That goes both ways. Let me put it this way. A year ago I was texting a friend of mine who races occasionally. I was asking him about frame geometries etc in regards to a bike I was riding. He replied, and I quote, "Man I don't know. I just ride the damm things" That stuck with me. If your racing competitively then every little MM helps somewhat over 200 miles. But for the rest of us "Just ride!!"  [biggrin]
__________________
 
0
Previous Topic | Next Topic
Print
Reply

Quick Navigation:

Easily create a Forum Website with Website Toolbox.