Uber-light turbo killerbocks?

[PaulH]

OK, so I can now walk on my ProJumps, but that's as far as I've got.  Forgive me the enthusiasm of the novice!

Anyway, I was wondering how far they could be improved, in terms of lightness and comfort.  I can imagine making something pretty curvy and light out of carbon fibre, custom-fitted.  The adjustability of the knee-bar seems to add a lot of weight/complexity, without delivering a very secure fit or great comfort.  Also, as far as I can tell, the main upright (I mean the big square-section aluminium tube with holes in it) is only ever in tension, not compression, so it should be possible to treat it as a tension member and lighten it a lot (or does it take a lot of torsion, in use?).

But what do I know?  I'm sure people here have tried all sorts of things, and I wondered what the limit is.  I was thinking a 1kg bock wouldn't be that difficult with the right design and materials - do such things exist?

[Kazbahn]

I know barely anything about bocks, but i'd say you're very overly ambitious. I'm pretty sure the spring weighs more than 1kg for a start.

Also, the upright, box-section support, that goes from footplate-spring, is in a LOT of compression when in use. It depends on how strong your springs are, and how much they're being pushed, but it's under sudden jolts of compression multiple times every few seconds. I think you'd have to be very careful about doing anything other than basic mods to the bocks, but if you do manage to do something absolutely amazing, get it mass-produced and sell them why dont you

[darfgarf]

the springs weigh quite a lot, people have had ideas about CF frames etc, but they would jsut cost wayt too much to be worth making

the new flying locust stilts are supposed ot be lighter than most, but not by much, powerskips are also lighter, but still about 3.5kg i think, so good luck making anything lighter

[PaulH]

I think commercially it wouldn't make sense - you're right.  But I'm not interested in commercial possibilities - just interested in whether I could make myself something amazing.

Springs are fibreglass, yes?  I'm pretty sure carbon fibre composite would give you the same stiffness and energy storage for about half the weight.  If you've never played with CF materials, they are unbelievable in terms of stiffnes-for-weight!

Kaz - are you *sure* about that upright (the one that runs up the back of your calf) being in compression?  As far as I can see without modelling it, it's in tension:  in effect, it's working like a stirrup to suspend the foot platform from the top of the spring; the spring itself is the main compression member. 

I'm imagining a thin carbon-fibre-composite shell like a rigid riding boot, but opening like a clamshell to get your foot and calf in, and padded inside.  Top of the boot reinforced with an integrated aluminium cuff to provide the top anchor point for the spring; sole likewise reinforced to take the pivot-point for the hinged supporting strut.  The only thing I'm not sure of is whether you need any freedom in your joints below the knee - I'm assuming not, since the straps on the existing model are meant to keep your ankle joint fixed?

[toddy]

ok
me and jason is playing with bock designs etc
and it is made in this way to minimuse the amount of material that is used, so of corse you could loook at different materials but then the cost will be way over priced and it will be well fradgile.
and PJ's springs are now carbon, they are not that much lighter than a glass one.

and as for using carbon for the main frame i would no way surgest to do that as if you get a little inpact in the wrong place the carbon will be buggered and you wont know it untill it shatters on you

and this is comming from a good mtb dh side of me, carbon is not allway the best material, just because it is light and strong in certian directions does not make it a go for all parts, as i have had carbon frames and yes they are light but they larst about 1/10 of the time compared to an alu one, and i have destroyed an alloy frame in about 4 months of generial riding.

[PaulH]

Yep, point taken about damage resistance - hadn't really taken it on board.  On the other hand, some components seem to be protected from damage by their position (I keep looking at that horrible square upright behind the calf).  Also, a question:  is that upright in tension or compression?  And if compression, how?

[toddy]

that is in tension untill you bottom out

[PaulH]

...hmmmm.  And when you do bottom out, it's not really doing anything.  So it could be replaced by a tension member, provided the tension member could flex a little to absorb compression when you bottom, no?

[toddy]

if it flex's you then could really hurt your self, it is designed to be ridged so that the foot plate does not start to rotate etc.

my advice to you if you want to get a lighter bock is look at exstroded sections, not cast(as they are cast).
as well look into other materials but they have to have be strong in mupile directions and not fragile. so i would say look at exstroded TI sections in sted of the case alu

[PaulH]

Hmmm.  Point taken about the torsion - I guess it has to stop the heel swinging out sideways.  I still think CF may be an option as long as the part isn't prone to impacts (and that upright is pretty well shielded); I've found a place that does CF tube and box-section, and may try a mod; I also plan to extend that post by a few inches to allow for an improved and lighter knee brace (but will have to look into the spring mounting).  I'll probably just wind up breaking something, but it'll be fun trying!

Re. titanium - another good idea, thanks! 

[toddy]

as well there is one major problem i can see about cf, except for knocks etc

is the holes at the top and bottom, they will most probilly spilt the tubing when in use

[PaulH]

Yes, I agree the fixing points are the weak point.  My plan (I have a plan?  Well, OK, my vague idea) was to buy over- and under-sized tubing, and epoxy short sections at the fixing points (so, locally, the tube wall will be 3x thickness).  Then use an internal bush (like they use on the current versions) to prevent compressing the tube.  Probably it'll all go pear-shaped, but who knows.  I ordered some carbon fibre to play with so I can get a feel for it.

I spoke to one of our engineers about titanium.  He warned me about fatigue limits, which I guess are an issue; and also about the difficulty in machining it (eg, surface hardening if you machine too slow).  Some of the good aluminium alloys are easier, but I assume that the ProJumps already use the best alumimium alloys for the job?

One major weight-contribution seems to come from the knee-brace and the hardware that fixes and adjusts it.  I'm thinking about better integration of the upright and the knee-brace, which would make it non-adjustable but lighter (and I don't think my legs are still growing!).

[toddy]

well with TI is are you going to be machineing it? no
and the alu point, no way is it the best allows for the job, and it is a casting, so if you used an exstroded tube, it can be thinner and lighter but still having the same strength

[Spud]

I don't understand much about all this stuff but I doubt projumps are made using the best alu alloy. Considering powerskips are also made of alu, weigh 1.5kg less per bock and are stronger. You should probably look at poweskips a bit. They have the knee bar bracket pretty much to a minimum. Might also look at the new model projumps aka upwings. They actually weigh the same at the moment but I think its because of the materials used. They are less bulky looking.

CF bocks has been discussed on the other forum. I think they established that it would be possible, just would cost a lot to do.

 

[XS]

but I assume that the ProJumps already use the best alumimium alloys for the job?

HAHAHA that's funny.  The aluminum is just about the cheapest and crummiest alu you can get.  If you actually made a pair of bocks out of aircraft grade stuff then they'd easily lose 2kg, but the cost and labor would drive up the price, which is why powerskips cost so much and they're 2kg lighter

[Kazbahn]

Kaz - are you *sure* about that upright (the one that runs up the back of your calf) being in compression?  As far as I can see without modelling it, it's in tension:  in effect, it's working like a stirrup to suspend the foot platform from the top of the spring; the spring itself is the main compression member. 

You've got to consider the direction the spring is working in. It's stretching the upright, as it bends down. It's also pressing it forwards, so it needs to resist snapping. And after all that, it then springs back up into place, meaning the upright needs to take a lot of sudden jolting forces, which are worse for most materials than continual pressure (fracture points, weak spots... Think dropping a cup/pressing it base first onto the table with a wood block[to prevent death of hands].... which is easier to break it with?)

[Jason]

Right where do I start

Springs as Toddy said come in Glass and Carbon fibre composite and have slightly different actions due to this

A carbon frame would be possible but I agree length of service could be a problem but the main fault I could see would be the speed of failure Ally will show cracks and bend before breaking whereas Carbon would fail instantly (as my fishing rods do) and I for one wouldn't like to be standing on a set when something went 

The alloy used by Pro-jump is a reasonable quality (not the best but you get what you pay for) skips use a better quality
I am machining up a few bits for mine at the moment and I am using HE30 a good machining quality alloy (probably what skips use or HE15 machines easier but not as strong)

The rear upright post is mainly in tension but any presure on the front of the footplate (as in when jumping) puts it into some compresion due to the support rods also when the spring straightens as you jump the top bracket slams down and causes a temporary compression

The hammering effect that wears the bearings/bushings out is enought to show how carbon would survive and it seems to be the top bushes that go first

The skip parts are very light and the main weight is in the spring as Toddy said so although it may be poss to lighten them more it would be at the cost of strength so I would say 1kg stilts are at the moment nearly unattainable with todays materials

The moving foot is worth a look PaulH as I have found this to be amasingly comfortable due to you being alowed movement during jumping so it feels so much more natural and ballanced

As for lightening the existing design once you have fixed the kneebar position cut the lower section off ant you will save a few grams better still change to cuffs and cut the extra bit off
Moving foot Skips don't have a footplate at all you bolt your shoes to the alloy and so save weight (this would work on PJ's and save a few more grams

Have I missed anything

Jason 

[PaulH]

Wow!  First, many thanks to everyone for the comments.  Second, I'm impressed at the level of thought and engineering here on the forum.  I can see that I just jumped in with the naive enthusiasm of the newbie, and that people have though about this a lot already.

If powerSkips are 1.5kg lighter per bock (really??), then they've already achieved what I wanted to, so I guess the smart move is for me to wait until I've broken my leg on the ProJumps and then save for some PowerSkips while I'm mending :-)    After all, no point starting with a Volvo and trying to lighten it, when I could buy a Porsche and start from there.....

One question:  Jason, you mentioned a moving foot - this is on PowerSkips?  I like the sound of that.  On the PowerJumps, I find the top of my foot, and my ankle, get fatigued even though they aren't moving - I think it's because they are trying to move against the fixed foot-plate. 

Are PowerSkips generally regarded as the better bocks, or are there pros and cons (apart from cost)?

Thanks again to all for your patience with a noobie!

[Spud]

Powerskips are 1.5kg lighter. But it comes at a cost. The pro's (with the moving footplate) are like £750 I think...They are about as good as they get. I don't think are any cons as such. Rancher doesn't like the pixie boot thing on his standard ones so put a normal footplate on them but I think thats about it. Oh, and the springs tend not to last as long, but they are the best springs you can get and are really responsive.

The only thing better than powerskips are velocity stilts, which i think are just modded powerskips. Think they go for about 1k

btw, poweskips are the originals...from the designer. Everything else is a copy, for the people who can't afford £700. Things like PJs are decent, i love mine but...to make them at the price they do something had to give, and thats why they weigh more I guess. Cant use the best alu and stuff like that.

 

[sprog]

Powerskips are 1.5kg lighter. But it comes at a cost. The pro's (with the moving footplate) are like £750 I think...They are about as good as they get. I don't think are any cons as such.

The only con is the economy, the price to import them has rocketed to almost £1000.

The only thing better than powerskips are velocity stilts, which i think are just modded powerskips. Think they go for about 1k

Indeed, usually around $2000 ~ £1300, and they are stripped skips. The only major difference is that they have cuffs and normal footplates opposed to the pixie boots. Henry Holloway was on Radio Bock in 2007 talking about them (as he seems to be the only person on the planet who owns a pair and bothers to interact with the community) deemed them "a complete ripoff"

[Spud]

Are PowerSkips generally regarded as the better bocks, or are there pros and cons (apart from cost)?

But seriously, 1K for powerskips! damn economy! I think the velocities are probably good, But I just cant see where the extra money goes

 

[sprog]

Are PowerSkips generally regarded as the better bocks, or are there pros and cons (apart from cost)?

Pfft shhh Spud.

• Pros with skips are they're aerospace aluminium, making them a hell of a lot lighter.
• The springs are incredibly responsive and do last a while if you're not a spring slayer on 720's, which are unlisted on the skip site as they're not... as perfect as the others... one guy in my local group went through 1 pair a week or so, until powerskip refused to give him any more, to give you an idea. The other weight springs seem to last well though and maintain character really well.
• Hooves are like a shoe sole rather than rubber, when you wear them out, you buy new soles and take them to a shoe shop to be fitted. The hooves wear down fairly fast, so I guess that would be a con lol.
• Rather than snowboard bindings, they have what we've dubbed the "pixie boots", which is a soft, resin-like structure that does up like a shoe with laces, and holds your foot in. Some people love it as it distributes the pressure, others hate it

That is about it really.

I think the velocities are probably good, But I just cant see where the extra money goes

Into Jeff Jay's pocket.

[PaulH]

Yes, 1.5kg less weight would make a BIG difference.  Do you know if a lot of that weight saving is below the foot?  (That's where I guess it would count most.)

Also, the "pixie boots" - I'm dubious about any sort of "shoe" structure, since I have big feet and I choose my own shoes carefully.  Any comments on these? Can you (as I think someone mentioned) bolt your own shoes to the platforms for comfort?  (I did this once with a pair of home-made stilts and it seemed OK).

[sprog]

They only come on standard powerskips, the powerskip pro model you have to bolt your shoes to as it uses a tilting footplate. The footplates are quite different so I'm not sure it would be possible to bolt shoes onto the standard model.

[Spud]

I would imagine the weight lost is actually spread evenly over the bock since the design is pretty much the same as projumps ect. Altho..I would expect the hoofs to be a bit lighter as they are quite a bit smaller. Not sure tho. Possibly the footplate too. I imagine the yellow stuff is lighter than the plastic and also the knee bar bracket as there is not much to that really.

As for bolting shoes on. These are original powerskips. No moving footplate and have the "pixie boots"


Here are the pro's.


I cant imagine it being possible to bolt shoes onto the standard model. Can change to a normal footplate with snowboard bindings but the only other way to do it would be to buy a powerskip pro moving footplate. I think Jason stopped his moving somehow. Anyway, if you want to bolt shoes on then just get the pro's. Would be a lot cheaper than modding originals, especially if you buy the pro's moving footplate.