Bicycle Mechanics - Bottom bracket drag

Anyone know of any studies on this? Just got a new Truvativ ISIS BB, and just sitting there, it feels like it has significantly more drag than previous new BBs I've had.

Can't seem to find anything on the web discussing it though, nor can I find that site with all the cycling calculations (analytic cyling?)....

You might have it over tightened.

The stiffness may also be attributable to its oil seals, in which case it will loosen up a bit as it breaks in.

It's not installed yet, so it's not overtightened. ;)

Once I get some riding time in on it, I'll have to subject it to the drag tests.

It is probably the oil seals. Most BBs these days use sealed cartridge bearings and they tend to be a bit stiff until they are used for a bit.

Most makers mention seal drag in the instructions.

It's not installed yet, so it's not overtightened. ;)

Once I get some riding time in on it, I'll have to subject it to the drag tests.

Havent installed it yet.....and you are thinking it has too much drag by turning it in your hand.http://burns.thefinaldimension.org/cwm/3dlil/idea.gif http://www.gamers-forums.com/smilies/contrib/corky/smilies3.gif

Can't seem to find anything on the web discussing it though, nor can I find that site with all the cycling calculations (analytic cyling?)....

anybody?

Other posts have mentioned that oil seals etc. will be responsible for drag. I presume you are just turning the spindle by hand, without a crank mounted and that the drag is smooth?
If so, then the drag is really only a relatively small torque. When you consider the effort required to produce this torque by a pedalling force acting at the length of the crank it will not be significant. The energy you dissipate into this torque is related to the torque multiplied by rotation speed and is similarly small considering that the extreme speed of a BB (cadence) is probably 120 RPM for most riders i.e. a small torque at rather low rotation speed.

This is not the whole story, however, because the resistance presented by the bearing will also change depending upon the load forces exerted upon it whilst pedaling, in addition to the drag due to rubber seals. This must be a major design consideration for the BB because it sees the highest loadings of any bearing in a bicycle.

Cheers,

Ed

"This is not the whole story, however, because the resistance presented by the bearing will also change depending upon the load forces exerted upon it whilst pedaling, in addition to the drag due to rubber seals. This must be a major design consideration for the BB because it sees the highest loadings of any bearing in a bicycle."

Right. Obviously the equipment required to test this during a pedaling cycle would be rather complicated and expensive, so I was hoping someone had seen such a study before.

Studies have shown that sealed bearings have more drag/friction than conventional ball bearing/cup designs. There are two reasons for this. First, the seals as mentioned above contribute to drag and many sealed bearings have the bearings held in place by cages. The cages themselves will increase the friction.

The only studies done have compared new sealed bearings to conventional cone/cup bearings. In all tests cup/cone bearings are best. The authors did state that sealed bearings loosen up with use and their friction losses decrease after broken in.

If you want to test your BB be my guest. As fellow posters have noted above, give it some time. Check it a week after installing it and you'll see a marked decrease in the amount of effort it takes to rotate it by hand.

I have always said a break in period is necesary for any cartridge bearings. I believe that any manufacturer of items with cartridge bearings will agree. However they do get smoother and more free flowing with use.

Anyone know of any studies on this? Just got a new Truvativ ISIS BB, and just sitting there, it feels like it has significantly more drag than previous new BBs I've had.

Can't seem to find anything on the web discussing it though, nor can I find that site with all the cycling calculations (analytic cyling?)....

I experienced the same thing with a brand new Dura Ace BB that had been ordered for my first bike build. Holding it in my hand and turning the axle manually, there was substantially more drag than I had expected. Since the BB threads had to be chased (the frame had been refinished), I chose to have a trusted mechanic install the BB after the thread clean up. Once home,I double checked his work. I went ahead and did a partial uninstall and then a reinstall to make sure that there wasn't any lateral tension being created by the BB locking rings. The drag was still there. But, after the crankset installation (including peddles) the BB felt buttery smooth. It was easily as smooth (or more so) than two or three other high end bikes in my garage. It might just be a normal drag sensation caused by such a low mechanical advantage (when turning the axle with your fingures).