Schlumpf hubs: general discussion

[Eric_aus_Chemnitz]

@Eric aus Chemnitz

I still do not understand whats wrong when the crank touches the spacer at 18 Nm. What stops you from tightening it the remaining 22 Nm? At which tightening torque would you expect it to touch the crank (40 Nm would be too late!). The spacer is not supposed to just be gently touched by the crank but to be clamped by the crank properly.

@UniDreamerFR

I also wonder.
On my Quax non geared 36er there is about no torque until the crank reaches the spacer (by memory, I didn’t measure the torque) , and then I put much torque so that the bolt stays at its place.
I don’t understand the issue with the 18Nm stuff.
Maybe the ramp of the spindle (isis like) is too “flat” to keep the crank from slipping , or the spacer is too weak to support the missing torque from 18 to 40 Nm, or it is too large.

The 40 Nm are part of the ISIS specification (see here) and not a special Schlumpf phenomenon. ISIS was originally designed with a crank stop, we use spacers as crank stops. It is not specified, at which torque the crank shall touch the cranks stop (spacer) but there is an amount of preload/ press-fit defined in mm.

The crank interface maintains a constant minor diameter bore of 17.90-18.10mm and a constant major diameter of 22.0-22.4mm. The tapered profile of the crank interface exactly matches that of the spindle (1° inclined
angle relative to the spindle axis) with 2 exceptions: the constant major and minor diameters found in the crank interface as just described, and the offset
of the gage lines to produce the required amount of press-fit between the
parts. […] As mentioned above, the interface requires a certain amount of press-fit between the crankarm and spindle. The magnitude of press-fit is defined as the distance between the female shoulder on the crankarm and the male shoulder on the spindle when the crankarm is pressed onto the spindle by hand. […] Each manufacturer must determine the allowable magnitude of the press-fit for their specific crankarm. However, to avoid problems with the crankarm loosening and ensure accurate crankset location, it is recommended that the crankarm “bottom out” against the male shoulder of the spindle prior to reaching the full installation torque specified by the manufacturer. When aluminum alloys are used for the crankarm, a press-fit between 3.0 – 6.0mm has been found to produce the best results.

(see here)

Also:

If a crankarm has too much pre-load it will not bottom out against the crank stop on the spindle when initially installed. This will lead to the crank arm “walking” up the taper during some initial rides and a consequent loosening of the crank bolt. If the bolt is not constantly tightened until the crank finally contacts the stop, the result could be the crank falling off while riding or damage to the interface.

means: too little or no spacer can damage the interface

and:

If the crankarm has too little pre-load it will bottom out on the crank stop prior to attaining a press-fit with the spindle. Depending on the magnitude of the press-fit, this may result in anything from a slight reduction in the load carrying capability of the interface to a “sloppy” fit between the crank and bottom bracket. This may also lead to creaking of the interface due to the slightly “sloppy” fit between crank and spindle.

(see here)

So, if Silva needed 18 Nm to bottom out the crank, it did definitely not bottom out on the crank stop prior to attaining a press-fit with the spindle and everything shall be just fine!