OT: dense mouldable material for a cycle trainer flywheel.

[Neal]

Ok folks, weirdish question - but you are smart people

Can people suggest a dense material, mouldable (e.g. putty), and ideally al so curable (e.g. clay/ epoxy resin etc.)

I want to be able to fill a shallow wide cylinder (~2cm high by 10cm diamet er) with this material. I need a ~1cm diameter hole in the centre for acces s to a nut: Imagine a shallow wide bowl, with an attachment in the bottom. (See further down if you really want to know why).

A solid lump of metal is the ideal, but I am limited by ~3mm available thre ad on the existing spindle (hence the fillable cylinder with the nut access ). A machined piece (with suitable centre holes cut) would suit very well, but is beyond my means.

So I have a 'bowl (old bedside light fitting) which is soldi enough, but em pty. I need to fill it with something heavy.

I have air-dried moulding clay. It has a relative density of ~1.3 when wet, but I suspect less than 1.0 (water) when dry.

Alternatives include Epoxy things (e.g. milliput) Fine, but expensive and n ot that dense.

Other curable crafting clay (anyone ever used/seen 'polymer clay'?) may sui t, but I cannot fine useful indications of density when dry. Anyone any exp erience?

Plasticine may work, if i enclose it fully (so complete curing not needed - as long as it done not 'slosh around' or easily leak, it should be OK)

I considered ball bearings and a cover, but the catastrophic failure scenar io is not pleasant. (see below)

Possibly a combination is in order (e.g. strong modelling clay/epoxy, with embedded ball bearings?). But stability and failure scenarios are an issue.

I have some ideas, but brighter/more practical people may have more.

Any responses gratefully received.

Neal

Afterthought: Potentially a metal disc with a central hole (like a really b ig, thick washer) which could be securely fixed inside my 'bowl' would be g reat. I have considered dumbbell type weights, but none I can find suit the size limits I have. And actually, something TOO heavy is likely to strip o f the short thread/nut I have available on the existing end of the axis in the device.)

++++++++++++++++++++++++++++++++++++++++++++++++++++ WHY?

I have a cycle trainer (magnetically braked spinning wheel attached to back wheel of my bike) for excerise etc. during winter. It has an integral flyw heel around 10cm diameter, listed at 1.25 Kg.

Works ok for my son, les for me.

But. I a a BIG bloke.Broadly if I make any serious effort (e.g. stand on pe dals), there is so little angular inertia that I 'fall' rather that add pow er. Not explaining very well, but more simply: I want to add extra weight/a ngular inertia to my existing flywheel.

This is likely to spin at ~ 100 rpm, so if it suddenly fails consequences a re not good: e.g. ball bearing scattering at high speed (have you seen Swor dfish? (don't bother if you have not...)).

Neal

[Brian Gaff]

Unfortunately you do not mention balancing it and at that speed an unbalanced mass is going to vibrate a bit like an out of which washing machine. Brian

[Murmansk]

How about a mould of a suitable size and shape, filled with fine concrete, possibly with a layer of lead flashing with stainless steel screws through it to bind it to the concrete.

Nice and strong The shape you determine by the mould Heavy Smooth

[T i m]

So, ideally the extra mass wants to be around the inside of the existing 'flywheel', to make the additional mass more effective?

How easy would it be to add suitable weights (lead sea fishing weights?) around the inside? You could match them all on some suitable scales first and bond them to the inside using epoxy or silicone etc?

Cheers, T i m

[Tim+]

Before you go too far down the route of making your own flywheel, are you absolutely sure you?re not just experiencing tyre slip? The sudden ?falling? sounds more like slip to me than lack of flywheel resistance.

Tim

[T i m]

Increasing the mass of an existing flywheel ... ?

I was thinking that the only thing the increasing the mass would offer greater resistance to the acceleration, once it's up to speed it wouldn't make any difference.

And as you say, that can only be realised if there was good traction between wheel / tyre and the flywheel.

Cheers, T i m

[Chris Hogg]

There is a popular design of potters' kick-wheel that uses concrete for a flywheel. You need to make a mould. Some people use an old bicycle wheel, complete with spokes and hub, and with suitable shuttering (usually thin ply, tied round with string), fill the wheel with concrete. Other mould possibilities are a straight-sided bucket, a circular bowl, an oil drum, a sand mould etc. If you use a dense aggregate you can get the weight up. With a bicycle wheel you get the axle and bearings included, but other moulds you either add the axle afterwards, or you incorporate it into the mould before you pour the concrete.

Here's a guy on Youtube making a concrete flywheel from a bicycle wheel

Google for more examples.

[tabbypurr]

also curable (e.g. clay/ epoxy resin etc.)

eter) with this material. I need a ~1cm diameter hole in the centre for acc ess to a nut: Imagine a shallow wide bowl, with an attachment in the botto m. (See further down if you really want to know why).

read on the existing spindle (hence the fillable cylinder with the nut acce ss). A machined piece (with suitable centre holes cut) would suit very well , but is beyond my means.

empty. I need to fill it with something heavy.

t, but I suspect less than 1.0 (water) when dry.

not that dense.

uit, but I cannot fine useful indications of density when dry. Anyone any e xperience?

- as long as it done not 'slosh around' or easily leak, it should be OK)

ario is not pleasant. (see below)

h embedded ball bearings?). But stability and failure scenarios are an issu e.

big, thick washer) which could be securely fixed inside my 'bowl' would be great. I have considered dumbbell type weights, but none I can find suit t he size limits I have. And actually, something TOO heavy is likely to strip of the short thread/nut I have available on the existing end of the axis i n the device.)

ck wheel of my bike) for excerise etc. during winter. It has an integral fl ywheel around 10cm diameter, listed at 1.25 Kg.

pedals), there is so little angular inertia that I 'fall' rather that add p ower. Not explaining very well, but more simply: I want to add extra weight /angular inertia to my existing flywheel.

are not good: e.g. ball bearing scattering at high speed (have you seen Sw ordfish? (don't bother if you have not...)).

sand + epoxy

NT

[T i m]

I'm not sure that sort of thing is what is being looked for in this instance Chris.

I believe it to be a small (10cm diameter) 'top' type flywheel that sits on a small floor standing frame and carries the rear wheel of a std cycle for exercise / training purposes.

The 'load' is presented by an adjustable physical coupling between the flywheel and a stator and to make the action smoother, it has a small flywheel effect.

I think Neal is hoping to increase the inertia of the flywheel as he can 'overcome' the existing flywheel and it makes the action feel like a slipping transmission or clutch.

Because the existing flywheel is quite small and most of it's 1.2k KG mass would be round the outside, to offer any real increase in rotational mass you would need to add the greatest density material at the greatest radius. eg, a band of lead around the inside of the existing 'rim' would offer a greater effect than filling the whole thing with concrete.

As the flywheel is only 2cm wide, maybe a band of ally wheel balance weights positioned equally around the inside if the flange would be a good starting point?

Cheers, T i m

[Thomas Prufer]

You could also increase the gear: the faster the flywheel spins for a given pedal axle rpm, the higher the resistance seen from the pedal side...

Thomas Prufer

[T i m]

Cheers, T i m

[Neal]

Yes, balancing could be an issue. But all the vagaries of bicycle, and tyre against runner means that these will *hopefully* be small. Esp with the 10cm diameter. High frequency, low amplitude...

Neal

[Neal]

ack wheel of my bike) for excerise etc. during winter. It has an integral f lywheel around 10cm diameter, listed at 1.25 Kg.

pedals), there is so little angular inertia that I 'fall' rather that add power. Not explaining very well, but more simply: I want to add extra weigh t/angular inertia to my existing flywheel.

I have not explained well enough.

The existing flywheel is a machined Metal cylinder/disc 10cm diameter, 2cm 'deep', weighing ~1.25 kg. I have ascertained that I can add an additional flywheel (the 'bowl' i desc ribe, with added mass) onto the same axle (hence the restrictions)

But yes, sea fishing weights are exactly the kind of thing which may work, especially on the outer rim of my bowl.

Neal

[Neal]

y also curable (e.g. clay/ epoxy resin etc.)

ameter) with this material. I need a ~1cm diameter hole in the centre for a ccess to a nut: Imagine a shallow wide bowl, with an attachment in the bot tom. (See further down if you really want to know why).

thread on the existing spindle (hence the fillable cylinder with the nut ac cess). A machined piece (with suitable centre holes cut) would suit very we ll, but is beyond my means.

t empty. I need to fill it with something heavy.

wet, but I suspect less than 1.0 (water) when dry.

nd not that dense.

suit, but I cannot fine useful indications of density when dry. Anyone any experience?

ed - as long as it done not 'slosh around' or easily leak, it should be OK)

enario is not pleasant. (see below)

ith embedded ball bearings?). But stability and failure scenarios are an is sue.

ly big, thick washer) which could be securely fixed inside my 'bowl' would be great. I have considered dumbbell type weights, but none I can find suit the size limits I have. And actually, something TOO heavy is likely to str ip of the short thread/nut I have available on the existing end of the axis in the device.)

back wheel of my bike) for excerise etc. during winter. It has an integral flywheel around 10cm diameter, listed at 1.25 Kg.

n pedals), there is so little angular inertia that I 'fall' rather that add power. Not explaining very well, but more simply: I want to add extra weig ht/angular inertia to my existing flywheel.

es are not good: e.g. ball bearing scattering at high speed (have you seen Swordfish? (don't bother if you have not...)).

Sounds good. Any recommendation as to an epoxy mix I can use, and the propo rtion of sand I can add to remain stable at high rotations?

Neal

[T i m]

Unless you are able to rig up a system to balance it after casting, it may end up very much out of balance. ;-)

Cheers, T i m

[Neal]

Yes. All of the above. If anyone wants to look further,

shows the default device, with the shiny metal 'disk' as the flywheel. I need to add to that with a separate 'bowl' with as much weight (esp on the outside') as I can safely include.

I appreciate the reply

Neal

[Neal]

Yes, that works. But I have now set it up in the highest bicycle gear, with a fairly low resistance, and it is still an issue (though less than low gear and high resistance - ughh...)

Neal

[Neal]

Hadn't thought of concrete. Sounds very feasible, especially with added weights. I will look into it

Thanks

Neal

[T i m]

Ok?

Yes, I thought I understood that bit ... ;-)

Ah, so that would stick 'outwards' so the two were back to back for example?

On the inside of the outer rim (so they are held on with centripetal force)? ;-)

Rather than trying to pickup on the existing axle thread, could you bolt a suitable solution though the existing flywheel ... or even bond it?

Assuming you can get the two surfaces to be a good mating fit and the additional flywheel concentric, you may find good quality double-sided / ''servo' tape would do the trick?

What about a small steel pie / baking dish, should be thin enough not to take up too much of your thread and you could bond a penny washer on the inside if it did need 'beefing up' a bit?

Cheers, T i m

[Neal]

el resistance.

Yes. I have looked into this. The trainer can be adjusted so that there is no slipping (setting is similar to Mole grips- screw to tighten before appl ying)

I also have this problem on direct drive 'spinning' bikes at the local gym. They have fixed physical characteristics (flywheel = angular inertia) com bined with variable settings (braking as a 'resistance' torque)

But a 'huge' torque ignores resistance and relies on inertia to damp it ( s uddenly realising it is pretty similar to inductance and resistance in elec tronics...)

Thanks for the reply

Neal