Difference Between Aluminum/Steel Geometry

[ONLY4STROKES]

During the model years of 2003-2005, the Yamaha YZF's had a reputation of feeling top-heavy...and wanting to stand up in the corners. They lacked in the handling department (corners) when compared to the red bikes. Test riders reported to the factory that the frame geometry needed to be changed in order to fix the handling issues. Thus the factory introduced a four-stroke version of Yamaha's '05 YZ aluminum frame.

My question is...what, exactly, is it about aluminum that makes it so much superior than steel in a motocross frame? Is it rigidity? And is aluminum always a better material to use than steel, no matter the bike? This questions has pondered me for a while because looking at the spec sheet, there is no difference in weight between the '05 steel and '06 aluminum frames.

[kx_rider53]

The difference between is not as how much the weight is but how you distribute it around the bike.It also depends on how high the bike is off the ground. The lower the bike the lower center of gravity is has to handle better.

[Thumprding]

Good question
To start the answer the question you have to compare the physical properties of the two.

The biggest difference is density verses stiffness. If I remember aluminium is 1/3 the density of steel with half the stifness. So if the frame has a weight limit then you can increase the material stifness by swapping to aluminium because for the same weight you can use thicker walled material.

Other differences between the two metals are
Ultimate strength
elasticity
Yeild strength

Other differences.
Heat treatment can alter steel significantly
Heat treatment to aluminium does not do a whole lot. The heat treatment of aluminium is during the fabrication phase not after the frame is made. This is one reason the areas of weld are larger and the base metal is thicker at the weld joints than in the rest of the part.

Is one a better choice than the other
Depends on what you are looking for out of frame. Either material can produce a good frame or a bad frame all depnends on the design of the frame. The one thing Aluminium provides is you can build a similar frame that weighs less and then increase the weight some where else on the bike to increase the durability and life span of that part. Gain this but put it over there.

[Garasaki]

I think both these guys touched on something that makes a big difference.

Because AL is lighter, you can use big gigantic spar frames with very wide members high up on the frame, without having a big impact on the center of gravity. If you tried the same trick with steel, it would signifigantly raise the center of gravity.

So, you are able to use thick members wherever you want. The stiffness of an aluminum frame comes not only from aluminum's property's, but also because the spars can be wider (stiffness is proportional to the cube of width, so a small increase in width can translate into a LARGE increase in stiffness).

This question deals with A LOT of very complicated issues though, things that the Average Joe like you and me don't understand. You'd have to be a professional frame designer to truely understand the impact and the advantages of aluminum use in a frame. We can give a few reasons here, but I think they really only scratch the surface of the answer to your question.

Know what I mean????

[ONLY4STROKES]

Thanks a lot guys!

And Garasaki, yes...I know exactly what you mean! Thanks for clearing it up some.

[*cly.sp*]

Although the new aluminium framed bikes are better, you should not forget they could metal framed bikes just as good if they wanted, its all about where they put the center of gravity on the bike.
I think aluminum frames have took over now because they are "THE IN" thing, I'd gurantee Yamaha wouldnt sell near as many 450/250's this year with the steel frame, even if they had fixed its top heavy-handling issue






2005 YZ250
2004 YZ450F- sold!!

[Team Scream]

Originally Posted by *cly.sp*

Although the new aluminium framed bikes are better, you should not forget they could metal framed bikes just as good if they wanted, its all about where they put the center of gravity on the bike.
I think aluminum frames have took over now because they are "THE IN" thing, I'd gurantee Yamaha wouldnt sell near as many 450/250's this year with the steel frame, even if they had fixed its top heavy-handling issue
2005 YZ250
2004 YZ450F- sold!!

I agree with that.
Theres a reason Ducati still uses trellis designed frames in WSBK.
I know that is comparing apples to oranges here but consider the Aprillia 450 supermoto/mx machine of last year kind of a hybrid design that seems to work really well.
I think Yami needed to jump on the aluminum frame band wagon to keep up with the joneses and they have aparently done a phenominal job of it.
I want one but I always wait a year or 2 after the new stuff comes out to buy into it.
Let em work out the bugs.
They are SWEEET looking bikes for sure.