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Only the rear wheel of a bike is connected to the drivetrain in order to propel the bike forward. I assume the choice of rear wheel instead of the front wheel is for stability (e.g. avoiding 'fishtailing' and other problems).

However, would there be any potential benefits if, somehow, both wheels were connected to the crankset so that the input power could be shared between the wheels? (This hypothetical system need not use a chain – c.f. shaft-driven drivetrains like CeramicSpeed.)

One advantage of a two-wheel drive system is that there could be improved grip and control on uneven, off-road terrain but the disadvantages probably outweigh this benefit.

The obvious disadvantage is you would be powering two wheels instead of one (so the rotational inertia would double?). You would also have to deal with the fact that you have to steer with the front wheel too!

I doubt any sort of bike like this has been made* / will be made in the future, but it's interesting nonetheless to imagine what this could be like...

*EDIT: As the comments have pointed out, they do actually exist.

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    bikeradar.com/news/… says they've been built and even sold in the past – Chris H Aug 8 '20 at 14:45
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    christinibicycles.com/bikes/christini-mountain-bikes/… is another manufacturer, but it's not so clear how the transmission works – Chris H Aug 8 '20 at 14:48
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    There won't be any change in rotational inertia - on a normal bike you're still turning the front wheel, just using the road to do it. – Chris H Aug 8 '20 at 14:49
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    There's actually a pretty good explainer on their website, though it leaves a little to the imagination christinibicycles.com/awd-tech/how-it-works – renesis Aug 8 '20 at 15:13
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    2WD motor bikes also exist and also have interesting transmissions - e.g. the front wheel is driven at a lower gear than the back, with a freewheel, so it only pulls when the back slips. The first link I posted could also do that. Motorbikes share some of the same characteristics, but the bike:rider weight ratio is very different, while a motorbike rider adopting a tricky body position doesn't prevent them putting the power down – Chris H Aug 8 '20 at 16:37
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Many two wheel drive mountain bikes exist. Here's one article advertising a new one:

There’s been no shortage of attempts to build a workable two wheel drive bicycle over the years, but this latest effort from Double of Japan looks like one of the most compelling yet.

https://www.bikeradar.com/news/this-is-a-two-wheel-drive-bike-done-right-sort-of/

They do offer better traction in some situations but I've never seen one with a limited slip differential between front and rear, which means they need a freewheel for each wheel to avoid stressing the drivetrain (you really don't want one wheel pushing backwards!). The extra weight and drivetrain complexity is always there but the extra traction is only useful very occasionally.

The other problem chain driven ones have is limited steering range (see the one in the article above). That's not a problem on FWD speedbikes where the steering range is limited by the fairing (often to +/- 5 degrees or so), but for technical mountain biking it's a huge issue. I commonly steer +/- 90 degrees in technical sections, and even on normal trail climbs half that. But getting a +/-45 degree swing out of a universal or CV joint is hard to impossible.

There are also flexible cable drives. Those trade more steering movement for more losses and generally shorter service life (ie, the only one I tried broke, luckily not when I was riding it). With a flexible shaft some wind-up is actually useful, as it gives a bounce in the drive to help you over large bumps. But it's disconcerting when riding and overall burns effort from the rider to counter the wind-up all the time while helping occasionally.

A rigid shaft drive could in theory have a rotating linkage with 180 degrees of steering freedom, but would be quite complex to build. It would also need a telescoping section if there was front suspension, and likely significant gearing to reduce the shaft size (and the torque on the sliding section as well as on the steering).

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There have been front drive designs, but all variations I've seen are either impractical, structurally inferior, or both. There is no real benefit to having both wheels deliver power, and that would add a gigantic amount of weight and likely be quite problematic. Imagine needing two separate drive trains that involve separate chains, cassettes, derailleurs, etc.., and that al would need to be tuned together somehow? No matter how you designed it,it would add substantial weight to the bike, and no real functional benefit. It might be novel and fun, but just not practical.

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  • Would it not improve grip on uneven, off-road terrain? I would imagine there may be some benefits for mtb/gravel bikes. As you say though, current designs are probably too cumbersome and bulky to be useful for most purposes. – ThymeTravel Aug 8 '20 at 15:24
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    @ThymeTravel One situation where having a driving front wheel would be cool is climbing a technical trail. The rear wheel can loose traction (literally hang in the air) when overcoming a steep bump, leaving only bicycle's inertia as means to overcome it. Come at it too slow, and you won't make it. At this moment, having a driving front wheel would allow one to still overcome the obstacle by pedaling. In conventional bicycle designs, this situation is solved by having rear suspension, which allows the rear wheel to better follow the terrain and less often hang in the air. – Grigory Rechistov Aug 8 '20 at 16:38
  • Some 30ish years ago there had even been several attempts of building two wheel drive cross motorcycles. They had been abandoned because of the power losses in the power transmission to the front wheel. imagine the impact of loss in a pedal-powered vehicle! – Carel Aug 8 '20 at 17:52
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    @Carel I read recently about some current 2WD motorbikes used by the military (special forces) – Chris H Aug 9 '20 at 8:13
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    There's a Russian all-terrain paratroop motorbike that is 2WD and incredibly capable, but also very limited in what it can carry and how fast it can go. It's the classic "if that's what you need that's the one bike that will do it" solution. – Móż Aug 10 '20 at 1:11
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As long as you can transmit the force you want to the terrain through one wheel, there is no advantage to driving the second one. As a road biker, I have no problem here. I cannot generate enough torque to break the rear wheel free, so it wouldn't help me at all to drive both wheels. The reason off-road drivers want four wheel drive is that one or both rear wheels can lose traction in bad terrain and they want the front wheels to pick up the slack. That may apply to mountain bikes, I can't say. Carrying drive force through a steering joint is complicated. You need to find a reason to justify that complication, not just a wish for symmetry.

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    But for mountain bikers loss of traction on the rear wheel is a common problem, leading to problems ranging from fatigue to castration. It's one of the things that puts the "technical" in "technical mountain bike trail". I also find that being used to loss of traction at both ends improves my road cycling by helping me avoid crashes. – Móż Aug 9 '20 at 12:00
  • Even on a road bike, a 20%+ gradient on a wet road with a bit of gravel/mud and you can spin the back wheel. But on the same road dry you can lift the front wheel in a low enough gear (@Móż) – Chris H Aug 10 '20 at 10:03
  • @ChrisH It's easy to lift your front wheel when going uphill: Your center of gravity will be at a steeper angle above the rear wheel due to the slope. Lifting your front wheel while going downhill is hard. (And, please, don't try, I guess it's rather dangerous as well!) – cmaster - reinstate monica Aug 11 '20 at 7:27
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    @ChrisH "long wheelbase and heavy" sounds like my bikes :-) – cmaster - reinstate monica Aug 11 '20 at 9:14
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    @cmaster-reinstatemonica TBH nevr mind the bikes, it describes me pretty well too – Chris H Aug 11 '20 at 9:28
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As anyone can easily conclude, having a driving front wheel on a traditional bicycle would be mechanically more complex than achieving the same result with the rear wheel. The front wheel is primarily used for steering, that is it rotates around a vertical (almost) axle. Relegating the steering to somewhere else (i.e. to the back wheel) is not an option as it makes a very unstable system (riding a bicycle rearwards proves it). The rear wheel is either fixed relative to the cranks, or at least stays in the same plane as the frame.

Side note: children's tree-wheelers and ancient penny-farthings have pedals directly attached to the front wheel and do not suffer from this problem.

So, any design that has a driving front wheel needs extra complexity to overcome this issue: to deliver power from the pedals in the middle of the frame to the wheel. This is doable, but results in more components, more weight, less reliability and less efficiency. If we only want to power one of two wheels, the rear wheel wins.

Now, suppose we want to have a design where both wheels are powered by the rider. It has to be noted that the golden rule of mechanics mandates that no advantage in power will be achieved: the same power generated by the rider will be distributed over two places, instead of only one (plus losses on friction, higher in the case of two drivetrains). If there is any potential benefit in having two-wheeled drive, it has to lie somewhere else, e.g. in increased control over the ride.

Side note: we do not consider additional "pedaling with hands" here. It would theoretically give the rider more power at the cost of less steering control.

Let's consider two categories of bicycles: road (primarily racing) and off-road (primarily MTB but also e.g. fat bikes in snow)

  • road bicycles are ridden over pavement of different quality. Both tires touch the road all the time. The traction between tires and the surface is already good enough to transfer all power generated by the rider into forward acceleration of the bicycle. The second drivetrain won't change or improve over that.

  • off-road bicycles are ridden over uneven or loose terrain. There are often moments when only one wheel touches the ground, while another one is briefly in the air or has otherwise lost traction. In such conditions, having the remaining wheel as driving does become useful. In conventional designs, rear suspension allows to increase traction of the driving wheel by allowing it to better follow the terrain's profile. Simply speaking, when encountering a bump, the wheel, instead of jumping off and flying into the air, stays closer to the ground and regains traction faster.

It would seem to make sense to have a full-suspended and dual-wheel-driven bicycle to keep both types of benefits off-road.

This is becoming really involved at this point. Modern rear wheel suspension designs are extremely complex and finely tuned, partially because they have to deal with the chain that limits their efficiency (see pedal kickback). Repeating the same thing at the front wheel is possible, I would assume (e.g. with a linkage fork instead of a traditional telescoping one), but again it results in additional weight and complexity.

Who knows, maybe one day such designs will become available to athletes and regular consumers, once (and if) other options to improve bicycle capabilities have been exhausted. In the end, the bicycle design is limited by human abilities: there is no point in making a bike with crazy high traction if no human with enough power to climb it exist.

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  • Oh, and ore more interesting point is distribution of human effort between two drivetrains/wheels. Should it be fixed at 50/50? But then it would mean a half of power is lost to useless spinning of a wheel with no traction. Should it be dynamically distributed to favor one, optimal wheel? If so, how to achieve that? – Grigory Rechistov Aug 8 '20 at 18:03
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    A freely spinning wheel would have to be spinning extremely fast to take up the same amount of power as a wheel pushing the bike forward on the ground. Spinning through the air doesn't use much power. The simplest scheme would be to ensure both wheels always go at the same speed. That would mean if only one wheel had traction essentially all power would be used for that wheel. – bdsl Aug 8 '20 at 22:54
  • @bdsl Yes, that is a solution that comes to mind. I guess it calls for a two-wheel-driven fixie bicycle as an "easiest" solution... – Grigory Rechistov Aug 9 '20 at 5:37
  • @bdsl like a locked differential on a rear wheel drive truck, that would get you out of a slippery patch affecting one wheel – Chris H Aug 11 '20 at 9:35
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While I'm aware of the fact the link-only answers are frowned upon in this community, I'd just leave this here. I'm not too sure, but I guess you might as well be able to buy one if you so wish.

https://twicycle.com/

Twicycle equipped bicycle

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    If you have enough reputation, you'll be able to leave comments on all posts. – Mast Aug 8 '20 at 17:16
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    And this picture of a woman on Twicycle kind of defeats itself: twicycle.com/wp-content/uploads/2017/03/… Getting hair caught in the front drivetrain sounds no fun. – Grigory Rechistov Aug 8 '20 at 18:17
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    @Grigory looking at the specs for gears, I don't think their target market is people likely to aero-tuck. – Chris H Aug 9 '20 at 8:17
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    @ChrisH indeed, the word "workout" is mentioned many times in their materials – Grigory Rechistov Aug 9 '20 at 9:08
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    arm-assist systems are somewhat notorious for their popularity with non-researching inventors and their rapid disappearance from the market. Racers are typically cardiovascular limited not muscle limited, so arm power doesn't help after the first ~5 seconds – Móż Aug 10 '20 at 1:21
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However, would there be any potential benefits if, somehow, both wheels were connected to the crankset so that the input power could be shared between the wheels?

There would be some benefits but more drawbacks.

For example, currently on bikes rear tire is the one to wear fastest. This suggests a tire replacement schedule where the new tire is installed to the front and the old front tire is installed to the rear, as otherwise the front tire would die of old age and cause a life-threatening accident due to its sudden failure. Both-wheel drive would not necessitate this tire replacement schedule, but most bicyclists do not see observing the correct tire replacement schedule a problem.

However, the drawbacks outweigh benefits. As the front tire is the one that is steered, it would require some special power transmission mechanism. The automotive solution (CV joints) does not work given the form factor of a bike. I suspect the only reasonable power transmission is electric, i.e. put a motor in the front wheel hub. Then you would need a generator in the rear wheel hub. Those would unnecessarily increase bicycle weight and work at such poor efficiency that the energy loss would be huge, far greater than the energy loss in chain drive.

Some electric bikes actually have a hub motor in the front. Those are not favored over mid-drive motors because hub motors have poor torque at high speeds (the marketers may quote quite high torque figures but those are at standstill) and weigh a lot because reduction gearing is not easy in the front hub unlike it is in mid-drive motors. Furthermore, the limited torque in front hub motors does not go through the drivetrain, thus meaning you lack the inherent advantage of the dynamic speed/torque range given by the bicycle derailleur gear system. Also a front hub motor requires a torque sensor on the bottom bracket so it anyway requires a non-standard bottom bracket. Mid-drives make the bottom bracket the only non-standard component. One advantage of front hub motors is that they even out the wear between front and rear tires, but this advantage does not outweigh the disadvantages of front hub motors.

Bicycles have an uneven weight distribution so on flat terrain 67% of the weight is on the rear wheel and 33% is on the front wheel. Thus both-wheel drive would give only 50% more traction and not 100% more traction. Not only that but bicycles don't require much traction except when going up a steep hill, and if the hill is steep, nearly 100% of the weight is on the rear wheel anyway with the front wheel practically unloaded. This front-wheel-unloaded phenomenon also shows why front hub motors in electric bikes are a bad idea: if you climb a steep hill sitting, and if your height is long, the front wheel being unloaded causes the front drive motor to be dangerous if it has enough torque to actually propel the bicycle up that hill. The front wheel will slip, causing you to fall!

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  • I'm not convinced by your description of electric bicycles, sorry. I think you're correct that low system efficiency is what stops motor-generator setups being useful, but your reasoning is poor. The BB torque sensor is a legal requirement rather than a technical one, just as one example. – Móż Aug 10 '20 at 1:14
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    @Móż the efficiency is an issue, but in hilly terrain this could be more than made up for by the ability of regenerative braking. The inability to do that is IMO the stupidest thing about mid-drive e-bikes. – leftaroundabout Aug 10 '20 at 8:52
  • IMO the front-e-drive would have a lot of promise for MTB, if somebody actually implemented it correctly. The biggest problem is that a hub motor adds unsprung mass. Your concerns about slip are however unwarranted, in fact the opposite is true: first, a front motor adds some mass that helps keeping the wheel on the ground. And if it does slip, that's actually a good thing because it means you'll be able to steer/balance even at zero speed! (It's the opposite of a locked front wheel, which is dangerous because it doesn't steer despite going at speed.) – leftaroundabout Aug 10 '20 at 8:52
  • Bikes don't require much traction when going uphill, but they require a lot of traction when doing corners. Unfortunately, drive train design can't help with that. – cmaster - reinstate monica Aug 11 '20 at 7:35
  • Mid-drive may be preferred, but hub motors (often rear unless retrofit) are much more common IME. For pedal assist the BB doesn't have to be special with a hub motor - the sensors can be built in to the crankset. I've even dismantled one that sensed the rear wheel speed! (But that had some very odd design decisions). A motorised front wheel intended for low-speed traction probably has a sensible torque curve anyway - actually a lot of the benefit of (commuter-type) e-bikes is in acceleration rather than top speed so low-speed torque is often useful. – Chris H Aug 11 '20 at 9:42
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That would effectively give you an all-wheel-drive bicycle with torque force halved on each of them. While both wheels have road contact, you do not loose anything except for the extra parts' weight. But the problem is bicycles don't have a differential device. Meaning your wheels will likely get different torque and angular speed, meaning you will waste muscle power on struggling with tyre resistance, plus leading to uneven tyre wearing out, making the effect even more noticeable. Also mind different path radii on stiff turns - all these require differential, and all automobile have one, especially AWD, 4x4 and heavy trucks.

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Looks like the strong point of the dual wheel drive motorcycle is very high climbing angle, as high as 60 degrees. Hence dual drive mountain bike should have a good potential for a very steep climbing. Ok you can also pull a bear with this bike but unlikely to happen very often.

But I think human cannot sustain the necessary power for more than few seconds. Only some purpose-built trail full of very steep sections few meters long would maybe justify such a bicycle.

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