Gebla Rohbox: adjustment, reset and care

The Gebla Rohbox is a mechanical adapter that lets Shimano GRX brake and shift levers operate a Rohloff gear hub. If the bike shifts poorly, the cable tension usually needs adjusting.

A cable that is too loose can cause missed shifts or no shift at all. A cable that is too tight can make the hub shift two gears at once. Open the Rohbox cover, check the position of the internal mechanism, and make small adjustments with the two cable tension screws until the mechanism sits in its correct middle position.

Caution

Do not ride the bike if the shifting is unreliable or stuck, skips gears, shifts two gears at once, or the hub does not engage the selected gear properly. If the adjustment is unclear, or the problem remains after resetting, contact us.

Why a Rohbox?

Rohloff designs the SPEEDHUB for its own twist shifter. Under pedal load, shifting the hub takes more hand force than a derailleur does, which a whole hand on a twist grip can apply easily but a thumb or a single finger on a lever cannot. That is why Rohloff makes no lever shifter for the SPEEDHUB.

On our drop-bar bikes, the Gebla Rohbox solves this. It takes the place of the Rohloff cable box on the hub and converts the cable movement of the GRX levers into the movement the hub needs. Because it works with precise cable tension, small changes in tension affect the shifting.

The advice on pedal pressure from Riding with your Rohloff applies here too: ease off the pedals for a moment when you shift.

Applies to

Booda bikes fitted with a Gebla Rohbox, that is, bikes with Shimano GRX levers shifting a Rohloff hub.

Care

• Check from time to time that the shifting still feels precise.
• Check whether the cable tension has changed.
• Check that the cover screws and the large centre screw are tight.
• Keep the outside of the unit clean and free of dirt and moisture.
• Inspect the cables and housings for wear, fraying or drag.
• Readjust the system if shifting becomes slow, imprecise or inconsistent.

If the bike starts missing shifts or shifting twice, check the cable tension before suspecting a problem in the hub.

Common problems

• The cable tension loosens over time.
• The adjustment screws sit in a tight, hard-to-reach position.
• If the cable is too loose, the bike may not shift properly.
• If the cable is too tight, the bike may shift two gears at once.
• The lever needs too much travel before the shift happens.
• The mechanism rubs internally if the tension is too high.
• A loose screw creates play in the mechanism.

Before you start

The Gebla Rohbox is sensitive to cable tension. Make only small adjustments, and test the shifting after each one before you change anything else.

Resetting the Gebla Rohbox

1. Remove the cover

Remove the two screws that hold the cover plate with a T25 Torx key, then remove the large centre screw.

2. Check the position of the mechanism

The middle gear wheel drives the shifting mechanism: shifting down turns it one way, shifting up turns it the other way. The aim is to bring the mechanism back to its correct centred position.

3. Find the two cable tension screws

Two cables enter the Rohbox: one shifts down, the other shifts up. Each cable has a tension screw, which you use to loosen or tighten that cable.

4. Adjust the cable tension

Make small adjustments only, and use the drawing above as a reference:

Setting	What it means
Too loose	The lever has to travel too far, and the bike may not shift properly.
Correct	The mechanism sits in its correct middle position and shifts cleanly.
Too tight	The mechanism can rub against the gear wheel, and the bike may shift two gears at once.

5. Test the shifting

Before you close the Rohbox:

1. Shift up and down through the gears.
2. Check that the mechanism moves freely.
3. Check that the lever does not need excessive travel.
4. Check that the mechanism does not rub or bind.
5. Make sure that one lever movement gives one clean gear change at the hub.

6. Refit the cover

When the shifting feels right, refit the cover plate and tighten the screws.

Tightening the screws

If the system feels loose or the cover has play:

1. Remove the cover.
2. Check the large centre screw.
3. Check the cover screws.
4. Tighten the screws securely.
5. Check the shifting again after reassembly.

Note

The screws sit in a tight space, so access can be awkward. Work slowly and do not force the tool in at an angle.

When the cables need replacing

The Rohbox uses two shift cables, and their condition directly affects how precisely the bike shifts. The cables or housings need replacing if:

• a cable is frayed;
• shifting becomes heavy or sticky;
• the cable tension no longer stays stable;
• a housing is damaged or contaminated;
• the system can no longer be adjusted reliably.

Replacing the Rohbox cables is a workshop job: please contact us. How the cable end is fitted in the GRX lever is shown in Fitting the cable end cap in a GRX lever.

Final check

Before riding, make sure that:

• the Rohbox cover is secured;
• the large centre screw is tight;
• both cables are seated correctly;
• the bike shifts up and down reliably;
• the hub engages properly in the selected gear;
• one lever movement gives one clean gear change.

Caution

If the bike skips gears, does not engage a gear, shifts two gears at once, or the lever feels blocked, do not ride the bike. Contact us.

My bike with GRX levers shifts poorly. Is the Rohloff hub faulty?

Usually not. In almost every case the cable tension at the Gebla Rohbox needs adjusting: a cable that is too loose causes missed shifts, one that is too tight can shift two gears at once. This guide shows how to reset it; if adjusting does not help, contact us.

Read next

• Fitting the cable end cap in a GRX lever
• Riding your Rohloff: shifting, noises, transport