Schlumpf Mountain Drive

Experience with the Schlumpf Mountain Drive

The Schlumpf Mountain Drive ist a planetary gear within the bottom bracket. Normally it is used in combination with one chain ring only. The direct gear (1:1) corresponds to the physical chain ring. The mountain drive (2.5:1) reduces the physical chain ring into a virtual one. Arithmetically the number of teeth of the real chain ring is divided by 2.5, or in other words, the gears are reduced by the factor 2.5. Thus a 70-teeth chain ring turns into 28-teeth.

Schlumpf with torque support, bottom bracket housing from and 70-t. chain ring

Short version – my experience after 8,000 km

Rider: born 1962, 77 kg weight, annual riding 12,000 k and 100,000 meters up. For the time being I have used 2 different chain rings in my velomobil DF XL in combination with the MD.

1. At home (3,000 k, 30,000 m up)

  • 70 t /(MD 28 t) chain ring, see above
  • back wheel, 26″ with 11-40 Sunrace cassette
  • Velomobil DF XL without luggage 26 kg
  • all climbs up to 12% managed without problems
  • Development 1.4 – 12.7 m

Fritz + Brigitte, Isle of Skye, Scottland, 2018

2. Scotland tour (5,000 k, 45,000 m up)

  • 60 t (MD 24t)chain ring
  • back wheel 26″ with 11-40 Sunrace cassette
  • Velomobil DF XL with luggage (48 kg
  • managed all climbs up to 20% without problems
  • Development 1.2 – 10.9 m.

    Schlumpf Mountain Drive with 60 t chain ring

3. Problems?

Minor adjustments such as reducing the gear backlash with a hook wrench, tightening some screws or securing the shift button with Loctite  needed to be done. No severe problems.

Important: Always carry a hook wrench with you.

4. Sound

The MD is virtually silent. The high noise level in the direct gear faded away after some time.  The chain noise can be reduced by a piece of gymnastic mat under the seat.

5. Chain guard

A chain guard to be mounted right from the beginning avoides chain drops.

6. High comfort of shifting 

One-hand- steering without problems as you only have one side shifting. Compared to a double or triple combination the shifting is easier, no readjustments necessary.

7. Development (in metres)

All climbs up to a maximum of 20% on bad roads could be managed, even with full luggage. Including  starting on hill.

8. Maintenance

I refilled some grease twice into the slot on the chain ring spider. After the first time the running noise reduced.

9. Switch

You can easily switch from the direct gear into the MD, even while standing or under load at a slope.

10. Load on chain and idle

In the mountain drive mode heavy forces can apply on the chain and the front idle. During my tour the chain broke 4 times, probably due to shifting under too much load. I have a stronger axle (10 mm instead of 8 mm). The the axle of the idle is still ok.

=> Important: Always carry a piece of wire (to pull the chain through the tube) and chain locks!

11. Price

780 € for the Schlumpf MD incl. assembly of the bottom bracket housing by, 155 mm crank arms, 70 t chain ring and maintenance set (as of feb. 2018)


I am more than happy with the Schlumpf MD and would always buy it again.

Detailes report – my experience after 8,000 kilometers

About me

• born 1962, weight 75 – 79 kg

•  annual riding performance for decades between 5,000 – 8,000 km, 50 – 70,000 meters height, single bike or tandem

• since riding a velomobile (first one in 2017, Quest, 37 kg) I have increased the perfomance to appr. 12,000 km with 100,000 meters height, tendency increasing

Why a Schlumpf drive?

I bought my DF XL velomobile second hand, equipped with a double chainring (Rotor 53/39) and a cassette 11 – 36. In this constellation the DF XL runs smooth in flat areas. For strong riders it works also in hilly regions. I needed a broader spread to be able to manage everything. Which means an easier gear ratio going uphil and a higher one downhill to be able to speed up.

If you search the internet regarding the best combination of chain rings and cassettes, you can easily get heavy self-doubts. It might be a similar male specific phenomenon like in the car tuning scene. Some riders use more extreme combinations than professionals would do. Although I am not the stronges rider, I like climbing hills with 10% or more and I would like to get up without problems. With a cadence around 80 and without pain in my knees or achilles tendon. With the built in combination I could not do this.

Isle of Sky, Scotland

Secondly our this years trip was to lead us to Scotland. The area is known for very steep grades (17% + x) and very rough roads. Including the luggage of 22 kg the velomobile DF xl (26 kg) had a total weight of 48 kg. Uphill quite a challenge.

Fritz Horsthemke with DF XL

Therfore I had different options for the DF xl:

  1. new cassette 11 – 40 plus broader spread in the front 28 – 60 or 32 – 70
  2. new cassette 11 – 40 plue 3 chain rings, 28/48/70
  3. new cassette 11 – 40 plus new drive (Pinion or Schlumpf)

First I changed the cassette into 11 – 40. Then I changed the 53 t chain ring into an oval one with 63 teeth, as well as the 39 t ring into a 34 t chain ring. On routes with little slopes I could continue pedalling even with a speed of 60 km/h or more.

Uphill I was not strong enough to pedal with a cadence of 80 over a distance of 1 to 3 kilometers with this combinations. I thought about installing three chain rings. In my Quest I rode with triple chain ring 30/42/53 and cassette 11 – 34. It was fairly good to handle, but I wanted a broader spread. But with a wider spread you face more problems with the front derailleur when shifting gear. And I might have faced problems in the DF XL due to the Q-factor.

The idea of having only one chain ring and another smaller virtual one was tempting. Which would also be ideal for the chain line. As the Schlumpf Mountain Drive is not really a bargain, the purchase should be well considered.
So I asked others about their experiences in the velomobile-forum in Germany ( Most of the answers were positive. I installed a Schlumpf MD with a 70 t chain ring, which gave me a 28 t chain ring in the mountain drive mode. I rode about 3,000 km in our area (low mountain range,Teutoburg Forest, Sauerland).

My experiences here at home were throughout positive. I managed all grades without problems. But I asked myself, whether I would be able to ride steep grades (12 – 20%) with full luggage.

On our way to Scotland we stopped for a check-up in Dronten, Netherlands, at Intercitybikes (manufacturer of the DF). We changed the 70 t chain ring (28) against a 60 t one (24). It turned out to be the right decision.

My experience with the Schlumpf Mountain Drive (MD)


In the mountain mode (1:2,5) the Schlumpf is absolutely noiseless.  In the direct gear the first 5 gears are quite loud in the beginning. With the number of kilometers rising the number of gears affected decreased. And the noise got less. You often can read that the noise is related to the hight speed of the chain. I never understood this. If I pedal with a constant cadence regardless the gears, the chain should always run at the same speed in the direct drive. My explanation is different: The chain resp. the rear derailleur has more tension in the lower gears as the chain runs on the bigger sprockets. You can feel under the seat that the chain tube is lifted a bit while pedaling. The tube is pressed against the seat. The chain inside produces vibrations and the seat with its fixed connection to the velomobile body increases the noise.

I placed a soft plastic (piece of gymnastic mat) around the tube. The noise war almost gone.

Gear backlash

The chain ring of the Schlumpf Mountain Drive should not move sideways while being slightly movable in forward direction. The backlash can be easily adjusted with the hook wrench (maintenance set). Within the first 2,000 km I did that 3 – 4 times. Since then no more adjustments were needed.

The torx-srews on the spider were not tightened fast enough. I thought first, that the gear backlash was not adjusted correctly until I checked the torx-srews and tightened them.

Shift button

At the beginning the shift button sometimes fell out on the left hand side. Thanks to the velomobile being closed underneath nothing can get lost. I put it back in again and clamped it. But it kept falling out, now even more frequently. Maybe the drive moves when shifting from left to right while the button is held still when pressed with the shoe. After several times of shifting the button is loose and falls out. I added a drop of loctite to the button. Since then I rode several thousands of kilometers without any signs of loosening.

Chain drop

In the beginning I used the 70 t chain ring without a chain guard. The chain fell off once in a while. It occured when riding fast on bumpy roads and curves. My explanation: On bumpy ground the back suspension is beeing squeezd, the chain hanging loose. In the curve a sideways movement leads to the chain falling down. After installing a chain gurad the problem is solved.

Maintenance and lubrication

I refilled twice some of the special grease from Haberstock. You need to loosen a screw on the spider of the chain ring, add some grease and close it again. Very simple. After the first time the noise was clearly reduced. The second time I repeated the treat after 5,000 km preventive.


The step between the direct gear (1:1) and the Mountain Drive (1:2.5) feels strange at the beginning. Going uphill I need to shift down 3-5 gears on the sprockets when shifting into the Mountain Drive. That is if you want to continue pedaling with the same cadence and similar pressure on the pedals. You get used to it fairly quick and with the gripshift gearing it is simple and comfortable. The MD is excellent when you want to start again after a sudden break. Just shift into the mountain drive and you are some 5 gears lower.

The top 4 gears of the mountain drive overlap the 4 lowest gears of the direct drive. When I ride the highest gear in the MD (11 t sprocket = 4.3 m development), then change into the direct drive, I need to shift down 5 gears (24 t sprocket = 4.9 m development). Downhill no problem. Going uphill in the lowest gear (40 t sprocket = 3 m development) in the direct drive I need to shift up 6 gears when changing into the MD (18 t sprocket = 2,65 m development) in order to be able to pedal on a bit easier. This can also be usefull when it gets very steep as the velomobile rapidly looses speed.

Harris, Scotland


The Schlumpf adds ca. 300 gr. compared to the Rotor, incl. switch.

High forces

High forces impact the chain and the axle of the front idle in the mountain mode. Therefore it is important to shift without pressure (truism). Probably I did not follow this rule during our tour. Four times I caused bent up chain links followed by a chain break. Presumably some of the chain links did not lie exactly on the sprockets. The teeth must have expanded the chain links. As soon as the chain breaks it disappears in the tube. Therefore you should carry a piece of long stiff wire which should reach from the bottom bracket to the cassette. If you push the wire through the tube you can fix the loose chain onto it and pull it out to the front. This procedure is also possible with a spare brake cable but not as easy as with the wire. Just take out the broken chain link and put in a chain lock. A job of minutes only.

After I heard about bent axles of the idle I installed a stronger axle for the front idle ( 10 instead of 8 mm). The idle as well as its axle are still ok.


For a short stop at a store you can use the Schlumpf as an immobilizer. Just shift into the highest gear and into the direct drive mode. Then draw the brake. Who ever tries to go off with your velomobile has to find the break and loosen it. Then he has to ride off with a development of 13 meters. This is hard if you do not know about the shift button.


With the 70 t (28 t) chain ring I ride comfortably in my home area (low mountain range, Teutoburg forest, Sauerland). Grades are up to 12%, seldom more. In average 1% (1,000 meters up within 100 km).

In Scotland I rode the combination 60 T (24 T) and 11 – 40 cassette, which worked out perfectly for me. I consider the mix of road surface and and profile as intermediate to challenging, compared to the rest of Europe. Sometimes about 3,ooo meters up within 100 km.

  • The asphalt is up to 85% rougher than in Germany. This leads to less speed and you need to ride lower gears.
  • The roads often have pot holes. Therefore never go with a maximum speed or let go downhill without attention.
    • Scotland is characterized by constant ground waves, lower or higher hills and several mountains. Having smooth asphalt you could run over them using your momentum, but as you need to ride with less speed the momentum does not last over the top.
    • You will find steep grades up to 20%. Often after a steep downhill ride with blind curves, sometimes in between high hedges riddled with potholes. That meant for me with a system weight of 130 kg to break downhill and then ride up constantly again. It always worked. For test reason I sometimes stopped at a 20% slope and tried to start again. I never got into trouble. We rode the pass of the cattle to Applecross, which is said to be one of the hardest in Great Britain.

Trossach, Schottland

Pass of the cattle

Pass of the cattle

Pass of the cattle

My conclusion about the Schlumpf Mountain Drive

For sure the Mountain Drive is no bargain, but it is worth the money. After more than 8,000 kilometers and 80,000 meters up I am more than happy with it. Now, I would install the Schlumpf from the beginning.


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