How much weight goes through feet?

It's a simple mechanical relationship between the forces applied.
Someone could draw a freebody diagram to work out the maximum gradient you could climb or the max speed you could push on the flat. Will depend a bit on the length of your cranks, the gears you run, and the size of the wheel/tyre.
Generally I'd say flat easy rides ok, but there will be logistical issues if you stop the bike and have to hop around to your crutches... Maybe you are thinking of a static bike or turbo trainer?

Yeah you could come up with an estimate, my brain is a bit fried after a long day but it'll be something like this:

Guess a power, say 200 watts for knocking along at a half decent effort. That's 100 watts per leg (yeah right, but let's go with it)
Watts = N × m/s, therefore force (N) = watts/speed (m/s)

Guess a cadence, say 90rpm, and guess crank length = 175mm, that gives a circle of about 1.1m per crank (who knew!). So that's (90/60)*1.1 = 1.65m/s leg speed.

So, Force = 100/1.65 = 60.6N, or 6.2kg.
That's very idealised and assumes a lot of things, including that you maintain the same force all the way around the stroke. I don't have a power meter so I have no idea how much it does vary but I bet it's about 50% or more, so call that a peak of 10-15kg.
Also thats for spinning along at a decent leg speed, half that cadence and you'll double the force so I imagine that your force will spike dramatically when starting off, and you know how much harder it feels going uphill..

So by no means take this as saying that it's ok but if you're just pottering you can probably keep the force low ish, but it really will be pottering. I'd be concerned about what happens if something goes wrong and you need to crank out of a tight spot, and not knowing if you've spiked over 30kg.

It'll depend entirely on how you ride and how much effort you put in. If you stand up and mash at the pedals, you'll basically be putting all your body weight through each leg in turn. If you stay seated and mash at the pedals, you'll still be putting most of your weight through each leg. If you cruise around gently spinning away, it should be fine.

I love this place. @ico86 excellent fag packet work.

Cheers, but I can't emphasize enough how idealised that is and how variable your actual power will likely be.
Another way of thinking about it is remember what it feels like to do a body weight squat, each leg is basically pushing half your bodyweight. If you weigh more than 60kg, don't push as hard as you would doing a squat. That's probably as accurate as my numbers above.

Can you cable tie some electronic scales to the pedal?

…so the question is…how much weight (i’m 90kg) does seated cycling put through the pedal? Obviously i probably don’t want to be going nuts and pushing enormous gears…but is there any kind of formula for this?

You have 3 main points of contact so it's simply a case of dividing your weight by 3....Maybe. Ok, maybe 5 points including the hands but they just do the steery bit really so let's discount them.

Bum is 30kg, left foot 30kg and right foot 30kg.

So you'll be fine. Just don't stand up to pedal. 😉👍

IANAscientist

Not to sound too much like your mum, but be cautious.

What will you do if you fall? Or wobble? Or have to put a foot down unexpectedly at a red light?

How you would it affect your recovery if you damaged the ankle?

I’m sure in normal riding, sitting and spinning, you’d be well less than 30kg through the ankle, but you don’t just need to consider the best case here.

Totally different if you’re considering a trainer or spinning bike or similar. If that’s the case then it’s a fabulous idea!

When you pit you'll need a crew member ready with your crutches...

E bike sounds just the ticket. Road/gravel to reduce the chances of an awkward dismount

in these calculations, don't forget there is a joint at the ankle and it is this that can't see more than 30KG of weight.

Depending on what is actually wrong and what is at stake should it be exceeded, I think I'd be sticking to indoor cycling. Introducing any unknowns, such as trail conditions, the need for a sudden foot down could spike the loading.

When I snapped my Achiles Tendon, the first few footsteps and rides were very nervous and let me know just how much force goes through the various body bits. For this reason alone, I'd be suggesting keeping it easy and calculated for as long as it takes

Yes thinking about going on the turbo, and sticking to a set cadence and low wattage. Thanks very much anyway, I've made some vague calculations to stick well under the 30kg limit.

It's over 30kg as soon as you get out of the saddle.

Sounds like a good opportunity for some endurance training.

Sounds like a good opportunity for some endurance training.

Exactly this. Zone 1-2, glass cranks.

Assuming you are on flat pedals and therefore unable to pull up
all your force is going into the downstroke pedal

ignoring any sort of yanking on the bars, an uninjured person stood up pedalling hard would put 90 kg though the leading foot. So you can do a third of that, max.

with mental discipline and low gears (and a tough taint) its theoretically possible, Ico gives some good numbers but based on average power generated not the power stroke so, as he says, its only going to be an estimation.

The key point is, as speeder says, the instant you stand up - and I mean stand up coasting just to
get over a rough bit of tarmac, not an out of saddle sprint - you have put 45kg through each foot. There is no way around this, the cranks are free to rotate, if you have them at 3 and 9oclock the load must be balanced.

This to my mind eliminates the ebike option.

@joe when you think you're ready try it (sensibly obvs ), you'll know very quickly whether it's to much or not.

I broke three metatarsals and dislocated the ball of my right foot in March and was told 6 weeks minimum before trying any cycling. Tried it at six weeks and knew within a couple of yards it was too much even seated. Left it another couple of weeks and tried again and was ok seated but couldn't stand up at all unless all the weight was on my good foot. Fast forward to now and I can cycle normally, but I won't be landing any jumps for a while as the ball and big toe won't be happy about it.

There's no way you could do it outdoors, you'd be well over 30kg just going over a bump in the road unless you take your foot off.

Yes thinking about going on the turbo, and sticking to a set cadence and low wattage. Thanks very much anyway, I’ve made some vague calculations to stick well under the 30kg limit.

I would normally say an automatic no but this sounds pretty sensible tbh

Guess a power, say 200 watts for knocking along at a half decent effort. That’s 100 watts per leg (yeah right, but let’s go with it)
Watts = N × m/s, therefore force (N) = watts/speed (m/s)

Guess a cadence, say 90rpm, and guess crank length = 175mm, that gives a circle of about 1.1m per crank (who knew!). So that’s (90/60)*1.1 = 1.65m/s leg speed.

So, Force = 100/1.65 = 60.6N, or 6.2kg.
That’s very idealised and assumes a lot of things, including that you maintain the same force all the way around the stroke. I don’t have a power meter so I have no idea how much it does vary but I bet it’s about 50% or more, so call that a peak of 10-15kg.

Very much what I was thinking. But half a pedal stroke is no power as that’s the up stroke. So my guess is that it’s more like a quarter of the stroke so more like 24kg

I think you need to start on a static bike or trainer taking it really easy. Static is more controlled and less ok an issues with stopping. If that seems ok then tentatively outside. I suspect based on my experience that less good leg will just do very little work and the pedalling will be fine

I did very similar calculations a couple of months ago! I broke my hip in February and was allowed only 'touch weight-bearing' on the bad leg for 12 weeks. That was quantified as about 15 kg. I did some sums and convinced myself that at steady state 200 W on the turbo was probably just about OK. Surgeon (a cyclist) agreed that turbo was OK after 6 weeks provided I stayed seated and stuck to fairly low force work.

I started very low (like 100 W or so) to get the range of motion smooth, then built up from there. I did relax my bounds a bit in the last week or so and push beyond 200 W but no really big surges. I agree with others that seated work on the turbo is a whole different ball game from outside where, in various scenarios, the peak force will be much higher.

I found that getting my leg moving and the muscles doing some work really helped my recovery and stalled that atrophy.

Good luck!