Rotor BSA30 and Hollowgram

[skinnyrider]

Okay, I hear what you're saying.
And thanks.

But what if you were to use a 128 axel with the with the shoulder machined off. AS the bb with bsa cups would measure 90.4
The current spindle is 104mm on a 68 shell so that's 36 needed.
90.4 + 36 = 125.4. So you've only got to space 1 mm give .5mm.

[dwaharvey]

There's more to this than just getting the cranks on the bike (symmetrically) though - there's the Q-factor and more importantly the chainline to worry about. With the 128 axle you could definitely get the crank on, and have the arms symmetrically spaced, but your chainline is going to be about 12mm off which is a pretty substantial error.
On my new mtb I'm trying that adaptor BB to put a SiSL2 crank on a BSA 73mm frame, and I think it's going to work well (I have it installed, but not yet ridden it). In this case the 132mm axle (standard MTB length) with the external cups and 73mm frame leads to me running only a 0.5mm spacer on the drive side and about 9mm worth of spacers on the non-drive. My crank arms are symmetric and the chainline with Leonardi XX1 spider is good (~50mm).

[skinnyrider]

Where did you get the number of 12mm for the chainline being off from?

I'm a bit simple, so please explain.

[dwaharvey]

So the normal road axle is 104mm and the crank/spider/chainring is designed to give the correct chainline with that axle. If you make the axle longer by 24mm (out to 128mm) the whole thing shifts outwards by half of that distance... ie 12mm

[skinnyrider]

But then it'd be running on a BSA frame that is designed to have a chainline where the shell is 68mm and then the bb cups outside so 91 wide.

[dwaharvey]

I'm not sure how to begin addressing that I'm afraid, but I'll do my best. A BB30 or a BSA 68 frame both have a 130mm rear wheel spacing, and the rear wheel is the same in either case. The spacing of the ideal chainline - the distance from the center of the chainrings to the center-line of the frame is a fixed quantity that the groupset manufacturer dictates (pretty much the same for all road groups). Its supposed to be the same regardless of the frame its on so that the groupset shifts correctly (ie crank in the same location relative to the rear cassette). Now Cannondale design their cranks and BB spindles in order to match this number. Assuming they got this right with their 104mm road spindle, making the spindle 128mm instead is going to throw it off by 12mm. Whether the BB is compatible with external cups or not has nothing to do with this.

[skinnyrider]

Got you. Thanks.

[ms6073]

Now Cannondale design their cranks and BB spindles in order to match this number. Assuming they got this right with their 104mm road spindle, making the spindle 128mm instead is going to throw it off by 12mm. Whether the BB is compatible with external cups or not has nothing to do with this.

To put this into context, if you compare the backside of the spiders of a Hollowgram and BSA24 cranks , you should notice that where the spindle joins the spider for a BSA 24 crank (example FSA K-Force Light MegaExo), this area is 'recessed' into the spider where as the Hollowgram is not.

[topflightpro]

ms6073 said it best. A BB30 crank has a nearly flat spider as the chain needs to line up almost in a plane with the edge the frame at the bb. On a non-BB30 crank, the spider is angled back toward the frame.

For example, on a Shimano crankset with outboard bearings, it is difficult to see drive-side bearings when looking down because they are obstructed by the spider and chainrings.

On the other side of the crank, BB30 non-drive-side arms are angled out, away from the frame to create symmetry from the BB. Non-BB30 NDS crank arms tend to be more straight, as the symmetry is created by the outboard bearings.

[skinnyrider]

Yup. What your saying makes sense.
Not that I particularly enjoy hearing it.

Bugger.


But can someone explain why it would work on a 68 BSA bb MTB then?
As cannondale make the package for it.

[dwaharvey]

Because the Q factor for MTB cranks is wider than for road (to fit the wide tire), the rear cassette 2.5mm further outboard (135mm spacing rather than 130) and because the spiders are designed to give the correct chainline in this setup. Because Cannondale use the same arms on all their SISL cranks, this leads to a longer axle (rather than more flared arms), and that leaves enough room for an external BB on a 68 or even 73mm BSA frame (instead of the spacers that normally occupy this space).
Point being that with a special spider with some curvature, one could correct the chainline to work with an SISL crank on a road fame with an MTB spindle. But that still wouldn't help you with the Q factor, where your Q factor would be 24mm wider than it really needs to be

[eric01]

If you have to make all these crazy modifications to get Hollowgram to work with BSA --- aren't you negating the benefits of hollowgram?

Hollowgram is pretty nice. If I had a BB30 bike I would get one. But is a hacked up hollowgram on BSA better than say, Shimano new Dura Ace?

[ms6073]

But is a hacked up hollowgram on BSA better than say, Shimano new Dura Ace?

Ordinarily yes, but the original poster was asking about an SRM Hollowgram on a BSA frameset.