Metal Strut Strength: Aluminum vs. Cromoly

[DocToxyn]

This is sortof a mix of physics, engineering and metallurgy so I'm not sure if this is the best place for this, but I figured it will get sorted out.

I am looking into adding an http://www.mtbr.com/interbike/booths/bikecontrol/ device to my recumbent bike which attaches at the rear dropout and what would be the seat stays on a normal diamond frame bike with cromoly stays. This mod has been done on other 'bents but the seat support stays on those are also cromoly. My particular bent has aluminum stays that are adjustable by a tube-within-tube design held in place by a pin. The forces created by the suspension device should be linear to the support stays, thus putting most of the force on the pin as well as where the stays attach to the frame and seat. Is aluminum going to be significantly weaker than cromoly for this application or does it have comparable tolerances for such linear forces? If this is too complicated to visualize without some pics I can post some if desired.

 

[FredGarvin]

Cromoly sounds like a 300 series stainless alloy. Is this the case? If so, you can assume that the aluminum will be about a third the strength. Some pics would be nice though!

 

[Astronuc]

"Specifically, 4130 steel - an alloy steel - which is commonly known in the bike industry as chrome-moly, contains the following alloying agents: 0.28- to 0.33-percent carbon, 0.4- to" 0.6-percent manganese, 0.8- to 1.1-percent cromium, 0.15- to 0.25-percent molybdenum, 0.04-percent phosphorous, 0.04-percent sulfur, and 0.2- to 0.35-percent silicon. The other 95-plus percent is made up of good old-fashioned iron. Now, there are hundreds of kinds of steel, but 4130 finds its way into bike frames because, among other attributes, of its weldability, formability, strength, ductility and toughness. (Many low-buck frames are made with 1020 steel, which is called plain carbon steel, and has significantly lower strength than the chromium-molybdenum steels.)"

from http://www.astro.washington.edu/cowan/cycling/cromoly.html

Geez it's hard to find good data when one needs it.

Basically the steels have similar properties. One could use Al with lower strength and just increase the wall thickness proportionally, so as to keep the stress about the same. If Al-alloy strength is 1/3, then one does not gain much by tripling the wall, because Al density (2.7 gm/cm³) is about 1/3 of steel (~7.8-8.2 gm/cm³ depending on alloying elements).

 

[brewnog]

Pictures please! I'm a bit of a bike nut too.

 

[DocToxyn]

http://www.lightningbikes.com/Shockster_copy.jpg
Here's the Shockster on what I think is a Lightning Thunderbolt.

and here is one on a Lightning P-38.

and finally you can more or less see the struts on my Rocket in https://www.physicsforums.com/showthread.php?t=73657&highlight=speedster (later posts), but I'll get a close-up tonight.

 

[brewnog]

Ahhhhhhhhhh!

I see your problem now. Where on a normal bike (or even the second 'bent above), the Shockster bolts onto the actual frame, on yours you just want it to bolt onto those weedier stays, which aren't part of the frame, but purely seat stays.

Getcha.

At first glance, I don't see any reason why you should be able to do that, although the first picture above seems to have used pretty much the same configuration.

On your current setup, (and I've not thoroughly dissected this yet), all the load from the axle is borne purely onto what I'll call the frame (the red bits, as opposed to the silver stays). If you install a Shockster how I think you want to (bolting its upper onto your silver seat stays), most of the load will be transmitted through the dropouts to the frame as usual, but you might get a considerable load acting to bend the silver seat stays, particularly if you hit a bump. These stays are purely designed to keep your seat up, and aren't supposed to take any bending forces, but I suppose it's just a matter of the magnitude of any bending force which might occur.

In the very worst case, you'll bend your seat stays, but at least you shouldn't cause any damage to your frame.


The difference between Al and CrMo shouldn't itself cause a problem; - although the steel is stronger, your aluminium stays will have been designed with this in mind, and should be comparably strong. Differences in tube geometry will have an effect, - your Al tubes are almost certainly fatter (both diameter and thickness) than CrMo ones on a similarly specced bike. Without knowing numbers, it's impossible to tell whether you'll be alright, but I'd try and get in touch with the guy who mounted his Shockster how you want to mount yours. Alternatively, try it, go steadily, and keep an eye on that deflection!

 

[FredGarvin]

At the first pass, the one thing I would worry about is not the compressive stresses induced. I think the short length at the attachment on the aluminum tube would provide you with some pretty good buckling resistance. If that were a worry, you could always double up on that top section of tube.

Personally, I would worry about the bending moments you're going to get. Even though the shock absorber load is along that tube's longitudinal axis, you will get a bending tendency at the attachment point which will want to bow that seat tube at the middle.

Thanks for the link Astronuc. 4130 is pretty run of the mill (pardon the pun). The big question now is, is it usually in the annealed or a heat treated condition?

 

[DocToxyn]

Ahhhhhhhhhh!

I see your problem now. Where on a normal bike (or even the second 'bent above), the Shockster bolts onto the actual frame, on yours you just want it to bolt onto those weedier stays, which aren't part of the frame, but purely seat stays.

Here's where it gets interesting - Lightning states that you can also put this unit on their Phantom (pictured below - sorry about the size, but man that's pretty!). Those seat stays are just as spindly as mine, although they might be cromoly and not Al like mine. I have yet to find a picture that demonstrates this, I may just have to e-mail the company and ask for their guidance and/or perhaps I'll have to fabricate some cromoly seat stays.
http://www.lightningbikes.com/2002_Phantom_copy.jpg

 

[brewnog]

Here's where it gets interesting - Lightning states that you can also put this unit on their Phantom (pictured below - sorry about the size, but man that's pretty!). Those seat stays are just as spindly as mine, although they might be cromoly and not Al like mine.

In which case, I say go for it.

Like-for-like, Lightning will probably have designed their CrMo and Al seat stays to the same specification in terms of strength. However, I'm finding it hard to believe that they are different materials; any weight saving from using Al over CrMo for the stays would be marginal, and I doubt it would be sufficient to give them cause to use two different materials for very similar items. The frame perhaps, but not the seat stays.

Giving them a bell might be a good idea, but if it were me I'd probably just give it a go, and take it easy for a while until I was confident that it was up to the job. Any excessive bending should show up during light use, and prompt you to do something about it.

Keep us posted anyway!

 

[DocToxyn]

OK, here's the last little tidbit for this project. This is the strut arrangement on my Rocket. I tested the relative strength of the struts by pushing on them and could only deflect then a few millimeters with a considerable amount of force so I think I can rig it if I do it right. There are a couple of other guys on a bent riders forum that I go to that are trying to do the same thing some maybe I'll let them work out the bugs. Also I'd rather not add the weight to this bent until I can truly dedicate it to offroad, right now it's my commuter/road trainer. So this will probably end up being a long-term project, but I'll update as things proceed. Thanks to all for the advice.

http://img226.imageshack.us/img226/7565/rocketstruts3mt.th.jpg