chrome molly aint allways best ?

[zardoz]

i was watching a TV program on demolition derby's the other day, when an interesting fact about chrome molly was put forward. the guy reckons that chrome molly has become the de facto standard choice for everything that needs to be tough these days. With some guys simply using it for everything under the sun, as they reckon tougher is better full stop. Well he went on to point out that strength and hardness are different things, and needed for different occasions. for instance making a roll cage from chrome molly tubing might seem a good idea. However with a cage made from chrome molly it will take more punishment than ordinary mild steel, however it will not deform, but suddenly snap in two. Which one would not want with a roll cage, where deformation is all part of the safety feature, as it takes the impact and deforms instead of breaking in half.

I would be interested in comments from other forum members who know more about these properties and uses of different steels, than i do.

 

[Garreth]

From Custom cages website -

Custom Cages uses two materials for its roll cages: T45 and CDS; both conform to FIA specifications and have excellent bending and welding qualities.

T45 is the strongest and lightest of the two tube steels. High Tensile Carbon Manganese Steel (T45) is approximately twice as strong as CDS so the same strength can be achieved with thinner wall tube. For example, a T45 International roll cage for a Fiesta Mk 6 (40kg) is 10kg lighter than the equivalent CDS design.

However, lower weight comes at a higher cost so T45 is more widely specified for the higher end of motorsport and by professional teams where the additional cost is judged to be small in the overall value of the car. For some models, where the weight of the two types of roll cages is similar, the T45 cage will be significantly stronger.

Cold Drawn Seamless Carbon Steel (CDS) is the alternative to T45 and is widely used for roll cage manufacture where cost is the priority.

All our roll cages are supplied as a kit of bent tubes with profiled ends ready to be assembled and welded and with the necessary steel reinforcing plates specified by the FIA. The roll cages are unpainted. Certain bolt-in cages welded and fitted at Custom Cages can be powder coated.


Hope that sheds some light !

Garreth

 

[TechnicAl]

CrMo steel will be stiffer than MS, thats true. Roll cages need to be stiff, bending wouldnt do you any favours. The rest of the car can bend.
They have been making roll cages out of CrMo for as long as I can remember (and thats a long time)

The latest anti-submarine car seats are a similar concept. They are stiff enough to hold together whilst the rest of the car crumples.

 

[stormin]

I'd have to disagree with Technical Al.

If you are involved in a big crash you most definately want the cage to deform "to some extent" and absorb the impact. If the cage does not deform and absorb some of the energy from the crash then it is transferred to your body with disastrous effect.

Obviously in a lighter impact it is desirable for the body structure, if there is much of one, to absorb the impact and leave the cage intact, but for the bigger accidents the cage is going to have to deform.

I've seen several off road buggy crashes were the cage has snapped rather than deforming, quite frightening really.

 

[greyshirtguy]

Hmm these are interesting:

http://chronicle.augusta.com/stories/031501/oth_nascar1.shtml
http://www.4wheeloffroad.com/techarticles/body/131_0602_4x4_buggy_building_tubing/index.html
http://www.offroadfabnet.com/forums/showpost.php?p=50758&postcount=8
http://www.opelgt.com/forums/racers-forum/3165-roll-cage-progress-4.html
http://aerodraft.wordpress.com/author/aerodraft/

On the surface it seems like you would go Cro-Mo where strenth, weight and resale were most important (and I imagine strength leads to the rigidity needed to achieve the level of handling that is required to try and avoid even more crashes)....but if you worked for Volvo and your mission was crash survival you would want something (else) to absorb much more of lethal crash forces. It seems to be a classic case of conflicting constraints that you balance off with each other - an improvement in one degrades the other.

The more I think about it...it kinda makes sense....It's all about the forces exerted on your body during the crash - a frame that is stronger will crumple less.....and therefore absorb the kinetic energy over a shorter period of time resulting in higher impulse forces on the soft and gooshy body strapped inside!

 

[TopherWiles]

Tips for Welding Chrome Molly

We're interested in trying to build a couple of Recumbent Bike Designs.

The one we've done so far was made directly from steel. We're considering redoing it now in Chrome Molly for better look and lighter weight.

Someone recently told me that we couldn't MIG weld Chrome Molly. So does anyone have any tips or suggestions from experience on the best ways to weld it?

Topher

*link removed*

 

[chunkolini]

Hi Topher.
A recumbent builder? lets see some pictures.
I have been going to build mine for about 10 years.
The chrome moly thing? dont know much about it so will follow this thread.
Put up a post in the introductions section so we dont hijack this thread, and maybe some pictures in the projects section.

Chunko'.

 

[Nimbus]

Of course you can mig cromo... almost all decent mountainbikes are mig welded 4130 cromo. As for the strength / stiffness thing, stiffness is usually a product of design - in tubes if you double the diameter you will square the stiffness. Bicycle forks generally taper toward the end to provide springiness otherwise the force is concentrated at the fork crown where the welding happened. 'Exotic' steels tend to be more sensitive to heat and need careful handling or further post production treatment to maintain strength.

 

[chunkolini]

Quality steel bike frames are Tig welded. or occasionally at the real high end brazed.
There are some very advanced steel alloys used these days, a bit out of touch with it all.
You will only see Mig welding on cheap junkers.

Sadly quality steel frames are becoming quite rare these days. ant beat them though. My commuter bike is an ancient Kona Kileauea, one of the best mass produced steel frames ever. A lovely thing.

My commute is a fairly serious one, a 20 mile round trip on tiny country lanes, with three massive hills to get over.
Commuter hell.
I took these shots a couple of weeks back.

No it isn't upside down


I follow the river for a mile and a half then I have to go over the hill in the background.

Chunko'.

 

[Nimbus]

Guess my suppliers were fibbing to me... 'course back then I knew even less about welding than I do now - I only ever brazed frames together except 753 which was so delicate it could only be silver soldered... Not so fond of steel frames myself as they lack the rigidity of oversized Al alloy.

 

[hotrodder]

Welding 4130 Article is about thin wall tubing, thicker sections require pre heat and slow cool/post heat. With MIG going to end up with more brittle untempered martensite than with TIG 'as welded'.

Chromoly tubing has always been used by Reynolds for their low end/entry level/tough rather than light tubesets, 500 and 501 were CrMo IIRC as are 525 and 725. 531 and 753 were a manganese moly (or carbon manganese depending who you speak to), 753 being a lighter wall heat treated version of 531. Silver solder required to avoid ruining the temper and destroying properties hence the test coupon Reynolds required before they'd sell you any. Nowdays there's 631/853 which are some micro alloyed air hardening steel and 953 which is a maraging steel

Ride qualities and the like were always more influenced by tube section than alloy used with tube sections being determined by the properties of different alloy steels and their manufacturers

 

[chunkolini]

For hardtail use I prefer steel every time. I used to own an aluminium Cannondale bike.
Great offroad for the really nasty stuff, high speed runs over boulder fields etc. But not nice cruising or on rough tarmac etc. A quality steel frame has some resilience, and just feels 'nicer'. A cheap steel frame is a vile thing. Planklike and heavy.
Hotrodder some quality information there.

Chunko'.