[Closed] Pivot point for front wheel lifts ?

you rotate about the axel. If you tried to rotate about the bottom of your wheel part of your wheel would have to sink into the ground!

I think you'll find it's a complex rotation about both points.

See, I'd say the other: pivot around the contact patch.
That's certainly what happens if you're motionless and balancing and bring it up into the back wheel.

No idea, but during a wheelie a different part of the wheel, relative to the rest of the bike, is in contact with the ground, so I suspect it's what chiefgrooveguru said.

The clue is in the name: pivot.

Good, that's that sorted out then 🙂

Actually, for the sake of this argument I think we can ignore the fact that the bike is moving and just see whether you can move a bike from horizontal to vertical while keeping one point stationary. That will be the pivot point. By my reckoning it can't be any higher than the rear axle, but could be lower I think.

Which still leaves the mystery of why I find it easier to lift the front wheel on the turbo than on the ground. The pivot point is lower relative to the centre of gravity in the latter case, so that should make it easier, not harder.

it definitely the axle. when riding there is also a translation (linear movement) of the rear wheel forwards when manualling.this is why you have to press forward on the pedals to 'kick' the bike forwards underneath you. the reason it is easier on the trainer is that the clamp is providing a horizontal reaction force at the pivot which effectively replaces this kick (in crude terms).

Neither, as the front of the bike comes up and back it slows, as you push with your legs the back accelerates so the pivot point moves around the middle of the bike. Easier on the turbo because you have removed all the complexity.

I've been practicing manuals for a bit and couldn't lift it far too start but I realised by accident that after the first foot of lift the rest of the lift is done with your legs pushing. Still can't do them properly yet though

Thanks Folks,

That makes sense. I was wondering whether the turbo was preventing the bike from being pushed backwards (or at least decelerating) with the rear weight shift and thinking that I'd need to do something with my legs to counteract that on the ground.

I have noticed that it's when I really focus on my legs that I tend to end up dumped on my arse. On the plus side, the fact that I'm looping out must mean that I can do this. I just need to work out how to control it. Give me a few years and maybe I'll crack it yet (if I don't crack myself first):-)

Depends on how you lift the front wheel.

If you lock the back wheel and pull up on the bars, the back wheel has to rotate back to allow the bike to come up. Thus you're actualy rotating arround a point under the ground, but as the grounds there you dont actualy move arround it, but arround it and out a bit from the 'pivot'.

Or you could push on the pedal (top/foreward pedal) and pull on the bars, pushing the rear wheel forewards under you (the classic wheelie or manaual, both achieve the same effect in the same way, one just used the pedals to drive the chain and wheel forewards, the other pushes them both to push the back of the bike forewards). Then the center or rotation is arround your COG(ish), which also has to rise a bit, which is why it's easier to manual with your arse hovering over the back tyre, less distance to have to lift your mass.

In reality you do something between the two. You wont lock the back wheel, but it wont move entirley freely either.

On a turbo the bike obviously pivots round the axle, but your bodyweight is unlikley to do exaclty the same, as you'll shift about to make it wheelie.

And that's before the added complication of suspension. Some designs (Giant Maestro IME) wheelie/manual with no effort, others seem to counter all the forces (spesh FSR) and keep the front wheel down, Orange 5's seem fairly neutral in between.

What about when Chuck Norris manuals?

It's also worth noting that when attached to the turbo, you can't fall sideways, so this gives your brain one less thing to think about, and hence you are more likely to be able to do it better when this constraint exists

sorry tracker1972 and thisisnotaspoon but you're both confusing translation with rotation. the bike ALWAYS pivots only about the rear axle. when rolling, the relative forward push of the bike acts on the rear wheel but the effect on the front wheel is offset by the bikes rotation, thus reducing the amount by which the front wheel is accelerated horizontally relative to the rear.

in the case where the rear brake is locked, the bike rotates about the rear axle and is translated backwards: ie it rolls over as a whole. you wouldn't say that a single rolling wheel has a centre of rotation that is subterra simply because it is moving horizontally as well as rotating.

any suspension action simply provides deformation of the mass being rotated. this will affect the rotational momentum and impact the ease with which the wheel may be lofted, but does not affect the centre of rotation

^^ This ^^

If it pivoted anywhere other than the axle (e.g. the contact point) then the axle would have to get closer/further away from the ground. How would that work?

If it pivoted anywhere other than the axle (e.g. the contact point) then the axle would have to get closer/further away from the ground. How would that work?

Ah, yes of course. It's obvious when you look at it that way. The only way to keep the axle at a fixed height is to pivot about the axle.

Thanks,

Andy

It's not really pivoting arround the axle then it is, it's pivoting arround your COG (the couple being the reaction force from the ground and the change in angular momentum of the rider giving a force at the bars and pedals). All with a translation verticaly upwards and backwards (you'll allways lose some foreward kinetic energy and transfer it to GPE).

If you look at the pivot as having to be fixed it wouldn't work as the energy to lift he bike has to come from somewhere. In a manual it's from the kinetic energy of the bike+rider, so the pivot is translated backwards.

I see what you're saying this is not a spoon: the cog moves and the angle of the mass changes. the same argument could be applied to say that the bike pivots around the front wheel, or the saddle or the brake pad....

the problem is that the movement of the cog cannot be described by a single linear direction hence it is not a translation, but many translations, or a constantly changing translation.

the motion of the axle, however is purely straight line.

another way to look at it is that the centre of the axle is the only point whose direction of motion matches their acceleration. therefore it has mo rotational element.

I'd agree if you stated that the rider therefore has to pedal enough that the energy required to lift them up is equaled out, i.e. the rear wheel moves under them as they rotate, rather than in a manual where I think the COG moves back and up and velocity decreaces in proportion the the height gain.

Here's another thought, watch a trials rider lift the frot wheel up so the bike's almost vertical. Their body doesn't rotate noticably so there's very little rotation at all (assume bike weight it negligable). They pedal really hard and kick the rear wheel forewards whilst jumping up with their bodyweight to unweight the bike. Their COG goes verticaly upwards and the bike stands up under them. The rider/bike isn't a fixed shape pivoting on a single point, it's several objects interacting with each other.

Wibble

Excellent thread and I will have to think about this

I was going to start one asking the same question about going over the bars 29 vs 26 which boils down to the same thing where is the centre of rotation

that is something slightly different to what we were talking about but the principal remains. in this case, the bulk of the energy is turned into gpe as you have talked about. this is achieved through a deformation (or rearrangement might be a better word) of the mass.

it is a similar move to a wheelchair user performing a standing wheelie. yes, the energy comes from the propulsion force; yes the cog rises; yes, the gain in gravitational potential energy must be equal to the energy imparted to the wheel - assuming a skillful enough performer to not require a braking force compensation. but if the wheelchair wheelieer, er wheelyer.....wheelier (?) were to sit inert like a sack of spuds, then they would be tipped on their arse. to perform the trick, they need to shift forward in their seat, thus raising their mass. the rotational energy of the chair is counterbalanced by a relatively small rotation in the much heavier human being. thus momentum is conserved while the thrust energy is being converted to gpe.

the bike and rider rotate about the axle. in opposite directions. the bike moreso because it weighs less. the mass of bike and rider rearrange, causing a vertical shift in combined cog, absorbing the thrust.

picture the rider performing the manoeuvre with the wheels set in concrete and you can see that his own centre of mass does rotate about the axle.

my brain's getting a sweat on! 🙂

sorry, my last post was in response to thisisnotaspoon but it took me too long