There are several objects around us that are ubiquitous and we take their working for granted. The humble bicycle is one such object. If you’re wondering what could possibly be interesting about bicycle tyres, lets begin by asking a simple question and trying to answer it:
Does the hub of a bicycle wheel hang from the top spokes? or does it stand on the bottom spokes?
(Pardon the poor labelling, I didn’t really find anything better)
Does the central part of the tyre (called the hub) ‘stand’ on the bottom spokes? or does it ‘hang’ on the top spokes. Seems like a simple problem to represent physically, but the answer can be ‘it hangs’ , ‘it stands’ or even both. How? lets find out.
Lets take a simpler wheel
Okay, nothing to see here. The entire wheel is solid so there is no question of pushing/pulling. These were our earliest wheels until somebody realised they could be made a whole lot lighter by cutting a bunch of holes into them.
This is a fairly typical wagon wheel. It is still a simpler design than bicycle wheels because rather than being all criss-crossed the wooden ‘spokes’ are all radially connected to the centre. We have studied such wheels, and there is no doubt that in such a wheel, the hub stands on the lower spokes. How do we know? the most literal way possible. Keep the wheel with a load on it (presumably as a part of a loaded cart?) and cut the spokes one by one (While standing still, of course ).
What we see is that cutting even 5-6 spokes from the top doesn’t do anything, while removing even one or two spokes from the bottom causes the wheel to collapse.
So, only the bottom spokes are actually supporting the wagon. The ground pushes up into the rim, which pushes up into the spoke, which pushes up into the hub. The hub pushes back down on the spoke. The spoke is said to be in compression, because both ends are being pushed in towards each other (compressed).
Now, moving on to the modern bicycle wheel
Superficially, it appears similar to our wagon wheel. however there is a very big difference. Steel wires are not good at taking compressive loads . A Wagon wheel is simple to explain, all the wooden spokes are rigid solids, so the spokes directly in contact with the ground bear the load of the wheel and of course, keep the hub in place.
However, Bicycle spokes are made up of stainless steel tubes, which bend rather easily (You can bend them with your hand) however they are much more resilient to tension (stretching) . Bicycle wheels are built to take advantage of that stretching.
When a bicycle wheel is built, all the spokes start out as lose wires which are gradually tightened. This is a process referred to as pre-tensioning since you are applying tension even before you put any load on the wheel. The tension must be balanced. If you screw it up, the rim of the bicycle folds to one side, and this may even ruin the rim.
So your hub is in the centre, while it is being pulled towards the rim in all directions equally. The spokes start out with having nearly equal tension……….until someone sits on the bike (a load is applied), If you measure tension when someone sits on the bike, The lower spokes are the only ones that lose tension (become looser) and hence you can say the lower spokes get compressed, just like the wagon wheel.
How can something like a wire hold that much compression?
The spokes can take compression because they are in tension and remain in tension even after the load is applied. This means that the ‘compression’ is not actually there, it is much like a fictitious force. It is true that only the bottom spokes register a distinct change in tension, but that is because they are only in compression relative to the other spokes. Thus, it can also be said that the hub hangs from the upper spokes. The bottom spokes are pulling downward (towards the rim, like all other spokes) and cannot be said to be holding up the wheel.
If tension is all that matters, can a bicycle wheel be made up of thread?
The answer is yes, although it would have to be very strong thread to maintain its shape after loads are applied. In fact materials such as Kevlar fibres have been used as bicycle spokes, though they are not common because Kevlar is more expensive than steel and there are no special advantages to using it.
So what is the conclusion?
The question of how to best describe the hub and spokes forces is still an open question (Check the Wikipedia article). It is a great example of something that is easy to represent mathematically, but hard to put into words. In a bicycle tyre all the spokes are pre-tensioned, so all of them play some role in keeping the hub in place, not just the ones at the top and the bottom.
As for why the spokes are crossed? It is perfectly possible to make a bike with radial steel spokes but the torque on the hub is more, which can cause damage. So we cross the spokes so that this load is applied tangentially.
Thank you for reading 🙂
References:
1.) https://en.wikipedia.org/wiki/Spoke#Reaction_to_load
2.) http://hea-www.harvard.edu/~fine/opinions/bikewheel.html
3.) https://www.quora.com/How-do-the-spokes-of-a-bicycle-wheel-work-How-is-a-wheel-able-to-be-light-weight-and-take-such-high-loads
4.) http://poehali.net/attach/Bicycle_Wheel_-_Jobst_Brandt.pdf
Sapience 