enginners help, comparing strengths of hollow vs solid bars

**Supra967** · 27-04-2010, 07:02 PM

I put 33kg on each wheel and 90Nm of torque. I'm getting really low stress with the section size you gave, less than 10% of what 4130 is rated to. Should be right so long as the thin wall can act as a section and not fail locally.

**brett_celicacoupe** · 27-04-2010, 07:28 PM

in your design thing (not just a comparison) there are another couple things to consider

when you introduce torsional loads then you need to take additional steps to calculate stresses.

stresses are superimposed accordingly

you will need to examine an element on the top and side. then calculate principle stresses from mohrs circle or the equivalent equations.

alternative is to do a bench test haha

**Sam_Q** · 27-04-2010, 07:29 PM

33kg per wheel is an usually accurate guess!

anyway I was going to use 25 x 1.27 for the fronts, I can see it's so far over what I need, neat. I will have to machine the outside and then drill/bore the inside or else I would get too much warping when I try and machine it.

**roadsailing** · 27-04-2010, 07:57 PM

taht looks like a very steep angle on that uni joint sam, if i were you i'd lookign at using a flexdrive type arrangement of you can get away with it.

Based on my uncanny guessing ability i would just aim to use some thickwall tube for the axles, like 3-5mm wall because I like over-design, in this case as is often the case in engineering, you will be limited more by what tube you can get and the manufacturing processes you can use.

wouldn't a recumberant be really hard to ride off road compared to a 2 wheeler?

**Sam_Q** · 28-04-2010, 07:15 AM

yes its a steep angle however that's only a theorietic drawing. I usually aim for the most theoreticly and then make my compromises from there. My total angle will be in the region of 35 degrees, I will be able to run more angle on the wheel than the uni-because of an unusual trick I am going which I use the acherman compensation to reduce the angles equally between sides whist steering.

Now as for riding it's a mixed bag, I like riding in places where it's just not practical to ride an upright bike. However in many places it is harder but I find it less entertaining.

**Supra967** · 28-04-2010, 07:56 AM

How did you come by your torque Sam? I thought the low gears were quite reducted on push bikes?

**Sam_Q** · 28-04-2010, 05:39 PM

length of crank arm vs force pushed against it in kg vs the twin under-drive gearing all divided by 3, I will try and remember who and where it was calculated

**Sam_Q** · 28-04-2010, 06:52 PM

so anyway whats a flexdrive? I am lucky enough to have a cro-mo supplier about 20 to 30 min away. I have a very good choice of sizes and at this stage I will use the 2mm tube I have and bore out part of it on the inside where I think it's less stressed to 1.5mm or so wall.

**SillyCarS** · 29-04-2010, 09:29 AM

Originally Posted by Sam_Q

yeah I have to disagree with you there, any time I have seen when engineers want to make something stronger and have had the option they have gone for a larger diameter that's hollow over a solid bit of round. Just imagine how easy it is to flex a piece of 10mm rod compared to a 20 x 1.2mm which has less metal in it.

brett_celicacoupe: thats some really full on information. But it feels like I am jumping in the middle and not having much of an idea, do you perhaps suggest maybe a book on the subject or something like that?

i havent read the whole thread, apologies if thas been covered

larger hollow diameter is better due to moment of intertia about the axis. the biggest prob with rotating shafts is the fatigue factor in the middle as it oscillates. it may not be much but at a couple of hundred RPM you'll know about it. anyone driven a car with an unblanace tailshaft?

**Sam_Q** · 29-04-2010, 07:13 PM

so not covered, so what you mean is that because the weight is pushed further out away from the turning point it has a greater inertia and hence any im-balances are amplified?

Here is another one for you guys, I had a look at bearing ratings and they give it in newtons (or some stupid imperial measurement). Now this confuses me as I would of thought of load testing in a single direction and showing the resulting max load in KG, as in one plane only. How does a torque relate to the load of a bearing?

**AdrianS** · 29-04-2010, 08:59 PM

9.8 Newtons = 1 Kg force.
You are thinking of Nm (Newton metres) for torque.

**brett_celicacoupe** · 29-04-2010, 09:26 PM

Originally Posted by Sam_Q

so not covered, so what you mean is that because the weight is pushed further out away from the turning point it has a greater inertia and hence any im-balances are amplified?

moment of inertia is what we are referring to here, not inertia.
it is a property of the object geometry. in this case, its a property of the cross section. we can choose a specific cross section to reduce materials (and thus weight) and increase strength in a specific direction if required.

ie a ruler is easy to bend on the big flat side but not on the narrow side.

Originally Posted by AdrianS

9.8 Newtons = 1 Kg force.
You are thinking of Nm (Newton metres) for torque.

on earth

**Supra967** · 29-04-2010, 09:55 PM

Your principle is right Sam. The further out you go, the higher the force, but mass is equally responsible as well. I don't think you can go too wrong either way unless your welding on those spoke hubs is no good/ really uneven.

With bearing load ratings it depends on what you're looking at, but the basic ratings are usually the direct weight loads, eg if you have 33kg per wheel and 2 bearings etc. Weight/100 for kN.

The way you generate torque effects the bearings, not the torque itself. If you consider the load conditions on a driven wheel, you'd have a weight force from the rider/frame acting straight down, plus the force from the chain pulling the sprocket around.

**AdrianS** · 30-04-2010, 12:22 AM

Originally Posted by brett_celicacoupe

on earth

Unless deep under a mountain... but I assume Sam is earthbound like the rest of us. Should he decide to ride on the moon, static weight loadings will be less, but he's still got to factor in bump loading.

Seriously, impact loading from bumps will multiply the static load by a factor of at least two? five?, more if it hasn't got suspension. Also consider weight transfer when cornering : if you can lift the inside wheel, the outside wheel is taking twice the weight. Something to keep in mind.

Autospeed (http://autospeed.com/) have done a whole series of articles on recumbent suspension design; they're worth a read.

**Sam_Q** · 30-04-2010, 07:03 AM

ok sounds straigfht-forward enough, checking the ratings of the bearings I am well and truly safe, well unless I ride on a dwarf star.

Adrian your right the tires would take a bit of the load but still I am allowing for a 10x loading seeing that's all I have in the suspension department. I know about Edgers articles, I think their pretty good even if he is the self proclaimed master.