Physics thoughts....

richardjiles

So we all know about locking your fork and locking your shock, and how it saves energy and is more efficient, but is it really? if you think about it, lets just say you push down the shock and fork with 50J of energy, that means it still has a potential of 50J, and in some way or another it will return this energy to you, it might push up against your weight at some point, returning the energy.
Your thoughts?

gezebo

The way the suspension works is by converting the energy into another form ie Kinetic into heat and sound and ? In this case. The shocks don't act as an energy store that you can tap into.

However maybe you've stumbled onto an idea...

This is just a quick explanation, I'm sure someone may explain the finer points.

richardjiles

I understand some is dissipated, but this would have to be minimal, compared to the energy going in.

Slow Loris

The forces are acting vertically, perpendicular to the direction of travel therefore are no assistance or hindrance in moving you forward, and therefore are just a waste of energy.

mattv

The entire point of a damped suspension system is NOT to return all of the energy put into it as kinetic energy. If it did, it would react like a coil shock on an Apollo, ie like a pogo stick. This is dangerous as the bike tries to buck you off all the time. Just try riding a bike with the shock too hard and the rebound all the way up.

Twelly

I get your thinking (I think)...

As you push down, your energy goes into compressing the fork but the fork has to come back up through the potential energy stored in the spring/compressed air so when it does come back up the fork is pushing back up opposite to the direction you are pushing the pedals in.. In theory the same effect as pulling up on the bars to get a more powerful push on the pedals?

I know this is wrong though and I'm sure a wannabe NASA physicist will be along in a second though to tell me why!

dhobiwallah

Wrong way round ref pulling on the bars i'm sure :?:

Newton found out you need an equal and opposite reaction etc etc. If you stamp on your pedal you can only exert a force equal to your mass (weight x acceleration due to gravity).

If you can fix your body by holding on to something solid you can apply a force greater than this (up to a muscular strength limit). Since your bike is rigid (for this purpose) holding on to the bars and pulling down while mashing the pedals effectively meets this criterion.

If you are compressing a suspension fork while doing this your weight is shared between 2 'springs' so some of your mass goes to compressing the fork and some (not all) goes to pressing down on the pedal. As the fork extends it doesn't help you either - some of the return energy offsets your mass (making you 'lighter' ) as well as ensuring that your fixed point in space isn't fixed so you can't get as much pull to mash the pedals as the gap you are forcing against is increasing in size as you are applying force.

As regards general assistance - as has been said the travel is perpendicular to the direction of travel and so adds nothing. And the damping is entirely there to dissipate the stored energy. (oil is quite good at cooling - look at the temps in car engines; but eventually the 'wasted energy' will boil it.

The one place where it helps is in keeping the bike in contact with the ground so the forces transmitted through the tyres (steering or propulsion) help you. and in many cases this benefit far outweigh the negatives previously discussed!

Twelly

TwellySmat wrote:

I know this is wrong though and I'm sure a wannabe NASA physicist will be along in a second though to tell me why!

dhobiwallah wrote:

Newton found out you need an equal and opposite reaction etc etc. If you stamp on your pedal you can only exert a force equal to your mass (weight x acceleration due to gravity).

If you can fix your body by holding on to something solid you can apply a force greater than this (up to a muscular strength limit). Since your bike is rigid (for this purpose) holding on to the bars and pulling down while mashing the pedals effectively meets this criterion.

If you are compressing a suspension fork while doing this your weight is shared between 2 'springs' so some of your mass goes to compressing the fork and some (not all) goes to pressing down on the pedal. As the fork extends it doesn't help you either - some of the return energy offsets your mass (making you 'lighter' ) as well as ensuring that your fixed point in space isn't fixed so you can't get as much pull to mash the pedals as the gap you are forcing against is increasing in size as you are applying force.

Aaaand there he is... 8)

Re the pushing pulling on the bars... you would pull up not push down when pedalling.. pulling up locks you against the bike so you can use your arm strength to add to the force you are putting on the pedals through the pedals surely? I certainly pull up on the bars when I'm going for it up a steep hill out of the saddle..

YeehaaMcgee

richardjiles wrote:

I understand some is dissipated, but this would have to be minimal, compared to the energy going in.

Actually, there's a substantial amount of energy dissipated into the shock, it is designed solely to do so.

However, almost all modern full suss bikes have anti-squat built in, so that driving torque on the cranks tries to extend the suspension, to counteract the bobbing.

King_Pin_Rich

Not entirly sure what the OP is getting at, but as far as I can see any suspension movment as a result of pedaling force is not being translated into forward motion at the wheels and therefore wasted effort.
In an ideal world mashing on the pedals would have no effect on either shock or fork, there a lots of variations in suspension design that attempt to isolate pedal forces from the suspension.

dhobiwallah

TwellySmat wrote:

Re the pushing pulling on the bars... you would pull up not push down when pedalling.. pulling up locks you against the bike so you can use your arm strength to add to the force you are putting on the pedals through the pedals surely? I certainly pull up on the bars when I'm going for it up a steep hill out of the saddle..

You are absolutely right- if the forks are locked out - but that is the point of the OP. If the forks are compressing/extending both those actions work against your climbing efficiency as explained in the previous post :?
If you wished to maintain a constant pull on the bars while climbing in order to prevent fork compression you could still maintain a greater force on the pedals than your mass alone - but you are wasting part of that energy keeping the forks at full extension which could be used more usefully in pedalling :P

My point about you getting it the wrong way round is that I read your post to mean that your understanding was such that if the bars are rising on their own then that is helping me as I would be pulling them 'up' anyway. Which, I hopefully explained previously in a way as clear as all the mud on my current loop isn't the case....

YeehaaMcgee

Erm, you can't stop the forks from diving just by pulling on them. Your momentum is travelling downwards anyway. If you manage to stop them diving, then what you're actually doing is reducing the downward force of your legs.

dhobiwallah

I don't see why if you keep pulling up on the bars and pushing down on the pedals/bottom bracket you couldn't prevent compression. As long as the force you are exerting in keeping the two parts of the bike 'apart' is greater than that the force compressing it due to the downward momentum of your mass.

If you have a vertical stack of weights and springs in series so at the top you have 1 weight than one spring then one weight then one spring then both springs will compress if you drop then to the ground and the weights will move closer together as the middle spring compresses as well as the lower spring.
If you repeat the experiment but put a hugely more powerful spring (or indeed a solid bar)between the weights the bottom spring will still compress (more than previously) but the 2 weights will not move closer together despite their total downward momentum; as if you were pulling up on the bars and pushing down on the pedals (hugely stiffening the middle spring).

Not that anyone would ride like that - as has been said; purely hypothetical...

dhobiwallah

Sorry, just re-read your post. Yes you are reducing the force your legs are producing - but they can still produce more than your mass alone so still = more power. The point is - why would you do that when you could just lock out the forks...

YeehaaMcgee

dhobiwallah wrote:

I don't see why if you keep pulling up on the bars and pushing down on the pedals/bottom bracket you couldn't prevent compression. As long as the force you are exerting in keeping the two parts of the bike 'apart' is greater than that the force compressing it due to the downward momentum of your mass.

But you're not, in any way, exerting any force that keeps the bike "apart". The parts that are compressing are (effectively) the fork crown, and the floor. In order to counteract it you'd have to apply a force between the floor and the crown of the fork.
The force on the pedals, no matter how great, cannot overcome the compression, because it is being applied to the wrong part of the system.

Twelly

YeehaaMcgee wrote:

Erm, you can't stop the forks from diving just by pulling on them. Your momentum is travelling downwards anyway. If you manage to stop them diving, then what you're actually doing is reducing the downward force of your legs.

My point wasn't about reducing the compression on the fork by pulling up as you pedal but by increasing the leverage you have on the pedals by pulling the bars up. The bars and pedals are indirectly linked with no suspension or movement between the two so pulling the bars up while pushing pedals down increases the torque going through the cranks. Nothing to do with the fork movement.

dhobiwallah

YeehaaMcgee wrote:

But you're not, in any way, exerting any force that keeps the bike "apart". The parts that are compressing are (effectively) the fork crown, and the floor. In order to counteract it you'd have to apply a force between the floor and the crown of the fork.

As the man in the orthopedic shoes said "I stand corrected" You are of course completely correct

TwellySmat wrote:

As you push down, your energy goes into compressing the fork but the fork has to come back up through the potential energy stored in the spring/compressed air so when it does come back up the fork is pushing back up opposite to the direction you are pushing the pedals in.. In theory the same effect as pulling up on the bars to get a more powerful push on the pedals?
........
My point wasn't about reducing the compression on the fork by pulling up as you pedal but by increasing the leverage you have on the pedals by pulling the bars up. The bars and pedals are indirectly linked with no suspension or movement between the two so pulling the bars up while pushing pedals down increases the torque going through the cranks. Nothing to do with the fork movement.

So - at a second attempt :oops: We still end up with my earlier post where compressing the fork reduces the riders effectve mass at the pedals (by 'absorbing' downward weight in compressing the spring) therefore requiring more muscular effort to achieve the same force by levering against the bars.

The fact the bars then move back up relative to the ground won't affect the force you can apply between bars and pedals as that distance remains constant 8) The upward force from the spring fork then unweights you, still working against getting weight through the pedals and again requiring extra leg strength and leverage to make up the shortfall.

But none of the energy can be used to prevent fork compression - that was just tosh

But the impact force at the bottom of the stroke is another story... If a force is applied over a short period of time the effect is much different to the same force applied evenly. (Ever tried pushing a nail into a block of wood with a hammer?). So - at the bottom of the stroke the weight is effectively magnified (as when you push through the pedal at the apex of a corner for extra grip).

So - does the 'extra weight' as the momentum changes direction counter the 'loss of weight' during the up/down stroke?

cooldad

I think you guys are overthinking this.

YeehaaMcgee

TwellySmat wrote:

YeehaaMcgee wrote:

Erm, you can't stop the forks from diving just by pulling on them. Your momentum is travelling downwards anyway. If you manage to stop them diving, then what you're actually doing is reducing the downward force of your legs.

My point wasn't about reducing the compression on the fork by pulling up as you pedal but by increasing the leverage you have on the pedals by pulling the bars up. The bars and pedals are indirectly linked with no suspension or movement between the two so pulling the bars up while pushing pedals down increases the torque going through the cranks. Nothing to do with the fork movement.

There would still be a downward force that's not being being counteracted.

However, this is a very simplified view of the system. In reality, when you're stomping on the pedals, your mass is on a downward trajectory, which is trying to compress the suspension, but, the effect that has on the bike is to accelerate it, and when a bike accelerates, the front tries to raise up. Not only that, but if you have a full suss, then the anti-squat will kick in too.
There's a lot of interplay of various mechanics going on, and they all affect each other.

But, yes, compressing the suspension is taking away energy that would otherwise go towards driving the wheels, and the only workaround is to use an efficient pedalling technique (where the energy loss is minimal), and, ideally by having a bike that has it's anti-squat set up for your weight and power output.
(EDIT for clarity: The wasted energy doesn't affect pedalling directly, it merely burns up a few more calories that you could do with using to pedal later on in the ride)

So - at a second attempt We still end up with my earlier post where compressing the fork reduces the riders effectve mass at the pedals (by 'absorbing' downward weight in compressing the spring) therefore requiring more muscular effort to achieve the same force by levering against the bars.

Hmm, not really. The rider/bottom bracket connection remains contant, but the whole lot moves up and down on the bike's suspension.
What you're really doing is using unnecesarry energy to compress the suspension, which will tire you out quicker. It's not going to affect the drive.

dhobiwallah

YeehaaMcgee wrote:

What you're really doing is using unnecesarry energy to compress the suspension, which will tire you out quicker. It's not going to affect the drive.

But as this is a physics question rather than a bike design q isn't that the same thing? If you are having to use extra energy you are affecting the drive, you are having to replace a force due to gravity with one due to leg strength- which is why we have lockout on shocks...

YeehaaMcgee

dhobiwallah wrote:

But the impact force at the bottom of the stroke is another story... If a force is applied over a short period of time the effect is much different to the same force applied evenly. (Ever tried pushing a nail into a block of wood with a hammer?).

Er, no.
Totally different principles.

YeehaaMcgee

dhobiwallah wrote:

YeehaaMcgee wrote:

What you're really doing is using unnecesarry energy to compress the suspension, which will tire you out quicker. It's not going to affect the drive.

But as this is a physics question rather than a bike design q isn't that the same thing? If you are having to use extra energy you are affecting the drive, you are having to replace a force due to gravity with one due to leg strength- which is why we have lockout on shocks...

That's not how I see it.
Imagine if you will that you're using, erm, say 1 Joule, of energy (you're a really eficcient rider ) to pedal.
Due to the suspension compressing, you're still using 1 Joule to pedal, but you're now also using an additional 0.1 Joule to compress the suspension.
If you see what I mean.

Twelly

TwellySmat wrote:

As you push down, your energy goes into compressing the fork but the fork has to come back up through the potential energy stored in the spring/compressed air so when it does come back up the fork is pushing back up opposite to the direction you are pushing the pedals in.. In theory the same effect as pulling up on the bars to get a more powerful push on the pedals?
........
My point wasn't about reducing the compression on the fork by pulling up as you pedal but by increasing the leverage you have on the pedals by pulling the bars up. The bars and pedals are indirectly linked with no suspension or movement between the two so pulling the bars up while pushing pedals down increases the torque going through the cranks. Nothing to do with the fork movement.

So - at a second attempt :oops: We still end up with my earlier post where compressing the fork reduces the riders effectve mass at the pedals (by 'absorbing' downward weight in compressing the spring) therefore requiring more muscular effort to achieve the same force by levering against the bars.

The fact the bars then move back up relative to the ground won't affect the force you can apply between bars and pedals as that distance remains constant 8) The upward force from the spring fork then unweights you, still working against getting weight through the pedals and again requiring extra leg strength and leverage to make up the shortfall.

But none of the energy can be used to prevent fork compression - that was just tosh

But the impact force at the bottom of the stroke is another story... If a force is applied over a short period of time the effect is much different to the same force applied evenly. (Ever tried pushing a nail into a block of wood with a hammer?). So - at the bottom of the stroke the weight is effectively magnified (as when you push through the pedal at the apex of a corner for extra grip).

So - does the 'extra weight' as the momentum changes direction counter the 'loss of weight' during the up/down stroke?

I'm not saying pulling up reduces fork compression at all and my first post was not what I think but my interpretation of the OP's thinking behind the original question. I know that the fork compressing when you pedal reduces your efficiency but I could see how someone might think the fork rebounding up would put energy back into your stroke.

I don't think I'm explaining myself clearly to be honest because all these replies I completely agree with but people seem to be disagreeing with things I'm not actually saying.

IN SUMMARY!

Suspension forks absorb energy that could otherwise have gone through the drivetrain and into your forward momentum.

Pulling the bars towards you whilst pushing the pedals down increases power because you are using your upper body to move the BB in the opposite direction the the pedal stroke, making the pedal travel further relative to the BB than your foot has moved relative to you (in my opinion). This is true whether its a suspension fork, a rigid fork or a soggy rizla attaching your front wheel to the frame... the fork is irrelevant for the point I was making.


EDIT: Yes we are thinking about this too much. Lets stop 8)

nicklouse

We still end up with my earlier post where compressing the fork reduces the riders

nope.

Pulling the bars towards you whilst pushing the pedals down increases power because you are using your upper body to move the BB in the opposite direction the the pedal stroke,

it allows you to use more than body mass to react against the power created by the leg.

this is why recumbant bikes can have riders putting more power to the rear wheel.

Twelly

nicklouse wrote:

Pulling the bars towards you whilst pushing the pedals down increases power because you are using your upper body to move the BB in the opposite direction the the pedal stroke,

it allows you to use more than body mass to react against the power created by the leg.

I meant that

cooldad

nicklouse wrote:

recumbant bikes

This sort of thing should be in CC only - wash your mouth out.

dhobiwallah

Well its over 20 years since I looked at this sort of rubbish so I think a quick blast through the mud has sorted it out a little...

YeehaaMcgee wrote:

Imagine if you will that you're using, erm, say 1 Joule, of energy (you're a really eficcient rider ) to pedal.
Due to the suspension compressing, you're still using 1 Joule to pedal, but you're now also using an additional 0.1 Joule to compress the suspension.
If you see what I mean.

I think on that point we are really talking semantics. I would argue that you are having to expend 1.1J to pedal because you don't have a locked out fork - or you could still put 1J of energy in but only 0.9J would go to pedaling...

YeehaaMcgee

dhobiwallah wrote:

Well its over 20 years since I looked at this sort of rubbish so I think a quick blast through the mud has sorted it out a little...

YeehaaMcgee wrote:

Imagine if you will that you're using, erm, say 1 Joule, of energy (you're a really eficcient rider ) to pedal.
Due to the suspension compressing, you're still using 1 Joule to pedal, but you're now also using an additional 0.1 Joule to compress the suspension.
If you see what I mean.

I think on that point we are really talking semantics. I would argue that you are having to expend 1.1J to pedal because you don't have a locked out fork - or you could still put 1J of energy in but only 0.9J would go to pedaling...

Maybe I didn't explain what I meant clearly enough. It won't affect the efficiency of the actual pedalling, but it will be causing you to use your energy in unrelated ways. If you see what I mean.

dhobiwallah

No - I agree with you; that's why I think we are talking about semantics now.

The OP was talking about energy return through fork compression and alluding to lockout forks.
My take is that if you can't lock your fork out you will have no choice to expend the 0.1J in wasted compression so that is part of your pedaling energy 'cost' regardless of how you have 'chosen' to waste it. (I accept the non-fork related effort is only 1J)

In the same way that,say, 0.1J is wasted raising my body temp - I can't simply remove that from my efficiency equation and say I am spending 0.9J pedaling, 0.1J compressing my fork and 0.1J increasing my core temp.

In the above example with pogo-ing forks it takes 1.1J for you to pedal uphill regardless of how you wish to break it down

dhobiwallah

And I think I have my head round the system by reducing it to a stick man on a triangle bike with springs attaching both wheels (full suss!)

As we are only looking at vertical motion we can exclude the rotation of the cranks and only consider vertical pedal force (which we can then make act through the bb).
Pedaling can be represented by a spring that does not return its energy at full stretch (as it has been used in propulsion - but is moving downwards as if stretching through the power stroke).
Initially consider rider is hands-free and standing (out of saddle)

So downward force (pedaling) through the bb will stretch the bb 'spring', compress both shocks as well as raise the riders mass upwards, further off the saddle (if combined force in bb and both shocks is less than riders weight - if it is equal riders mass will stay where it is vertically). At the bottom of the power stroke the bb 'spring' is removed and replaced by an new untensioned one (the other pedal at the top of the stroke).
In an undamped system the removal of this force means the shocks will now want to return to original length, the riders mass (through the bb) will want to resist this (and stay where it is) so as the frame moves upwards the 'bb spring' will extend (as the bottom of this spring has the riders weight acting through it) giving the free propulsion energy the OP was talking about.

As has been mentioned the reason this doesn't work is that the shocks are not un-damped and the damping does actually take a large amount of energy. So while some energy does come back (contrary to what I said in previous posts) just not as much as you would like.

Now with regard to the pulling on the bars. As has been said it is the only way to get a downward force greater than the riders weight. In the simplified example above, the frame is moving upwards as the fork extends whether you are pulling on the bars or not. As the fork extends you do get some 'free' energy back as the frame moves up, your mass stays where it is and so the pedal moves with less effort. So either you can get the same result by pulling less hard on the bars or a quick extra burst by pulling with the same force.

All of the above is pretty academic as you will probably waste more energy trying to get the power down smoothly as you pogo all over the shop!

YeehaaMcgee

dhobiwallah wrote:

No - I agree with you; that's why I think we are talking about semantics now.

The OP was talking about energy return through fork compression and alluding to lockout forks.
My take is that if you can't lock your fork out you will have no choice to expend the 0.1J in wasted compression so that is part of your pedaling energy 'cost' regardless of how you have 'chosen' to waste it. (I accept the non-fork related effort is only 1J)

In the same way that,say, 0.1J is wasted raising my body temp - I can't simply remove that from my efficiency equation and say I am spending 0.9J pedaling, 0.1J compressing my fork and 0.1J increasing my core temp.

In the above example with pogo-ing forks it takes 1.1J for you to pedal uphill regardless of how you wish to break it down

I'm still not quite seeing it as a matter of semantics. The pedalling does not use any more energy. But the overall system uses a little more energy. So, pedalling efficiency (is that what we're talking about? I've lost track now ) doesn't really change. You're not getting any more or any less propulsion from the energy put into driving the bike forwards.
However, it is entirely possible to pedal in a smooth manner, where the forks don't compress.
And, it's also common for the forks to extend rather than compress whilst mashing the pedal, because the bike will "rear up" (rear/rare? not really a horse guy) due to acceleration.