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EMTB 02/2021
Reading time 3.75 minutes

Gear ratio assistance

Long flat stages, steep mountains, it is only with a wide range that the gear ratio fits for both. There is a need for action especially when a small climbing gear is missing.

With the last battery bar flashing on the display, but the last mountain not yet climbed and a charging station out of sight, the stress level increases dramatically during the tour. This makes prevention all the more important in e-mountain biking, i.e. the conscious management of battery reserves. Unfortunately, the gear ratio on bikes often sets limits, especially on older e-MTBs. Because when gravel ramps and uphill trails become so steep that the lightest gear can hardly be pedalled, the only thing that helps is to reach for the turbo mode that eats up a lot of power. If you change, however, to a smaller gear ratio, you can save more than just the battery. For tricky and steep uphill climbs a suitable mountain gear is also a must. A too tight gear ratio also wears down the material: It makes the e-biker change too often onto the largest sprocket, which increases the oblique run and thus wear. What do you need for the conversion? A short gear ratio check, a little hobby mechanic know-how and the appropriate conversion parts.

First of all, however, you must be sure in one point: Are you allowed at all to convert the e-MTB drive unit? Background: Unlike mountain bikes without a motor, e-MTBs are subject to the Machinery Directive of the European Union. The Guidelines for the Parts Replacement of e-bikes and electric-assisted bikes developed by the German Bicycle Association (ZIV) indicate that chainwheels and sprockets must only be replaced with identical number of teeth. Expert Dirk Zedler reassures however: “A private person has as little to fear from the legal point of view as with a conventional mountain bike, especially when converting to a smaller, i.e. lighter gear ratio. But prior to converting, retailers should check with the manufacturer.” We will show here whether it is worthwhile making the conversion and how it works exactly.

1 Is the gear ratio ok?
Check 1: Climbing the mountains
The higher and steeper the mountains, the more important are the small gears. You will notice a missing gear for climbing when a higher assistance mode than desired is necessary even though you have already shifted into the lightest gear. If you only just move with the Bosch e-MTB mode or the Shimano trail mode, a conversion could be worthwhile. If you need the turbo mode to climb a mountain, there is definitely a need for action. Second indicator: You fail with nasty uphill challenges, because even in the lightest gear and turbo mode, a round pedalling is no longer possible.

Check 2: On flat terrain
Rarely, the gear ratio is too small. This becomes evident especially on flat terrain, when the chain is already running on the smallest sprocket at normal cadence at the limit of assistance. The problem: The chain wears down due to the oblique run. There should still be one or two gears left at 25 km/h. If you have problems in the mountains as well as on the flat terrain, you should change to a modern cassette with more range.


Dirk Zedler is graduate-engineer and publicly appointed and sworn in expert for bicycles and electric bicycles.

EMTB: Converting the gear ratio on an e-MTB, is that allowed at all?
DIRK ZEDLER: This is in general not forbidden for private persons, as long as the status as electric-assisted bike is not violated. But this is rather unlikely, especially when converting to a smaller gear ratio. However, you should bear in mind: Anyone who converts something is also liable for it. That’s not a big problem with the gear ratio, but if you install a different brake and cause an accident as a result thereof, you cannot expect the bike manufacturer to take responsibility for it. With a normal mountain bike it is not different.

And the retailer? Is he allowed to change the gear ratio?
The retailer is not a private person. With a conversion he can endanger conformity including the CE mark. He may no longer be allowed to put the e-MTB on the market. He can only avoid this risk, if he has the conversion approved by the bike manufacturer.

Can the manufacturer refuse the warranty, if an electronic fault occurs after the conversion?
There is the risk in principle. In my opinion, motor defects as a result of a changed gear ratio are unlikely, but we lack reliable data to be able to judge this conclusively. The same applies here: If you want to be on the safe side, you should request written approval from the bike manufacturer.

Author: Adrian Kaether

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