Weights of things

Randy Grein rgrein at q.com
Sat Jul 14 23:25:08 PDT 2012


Hmm, gotta chime back in here....

Al has generally 1/3 the strength of steel, and 1/3 the density. It gets horribly muddled because there are different alloys of both, but that's the consensus. Problem is that strength is not stiffness, although they are related. The easiest to look at is a solid beam. The strength is related to the square of the depth times the width. The stiffness is proportional to the cube of the depth times the width (and we're ignoring the length, too complicated). The same calculations can be done with an ibeam, a hollow box section or any other  regular shape. Now, if the material is substantially less dense a beam with the same thickness will be bigger around, and we can choose where to adjust so if we need stiffness we make the beam deeper. So the Al framed bike tends to be stiffer because the tubes are bigger. Actually, there's another factor - they hit the practical limits of steel tubing some time ago. Bikes need to be light so for stiffness you want thin wall tube - but it crumples if the tubes are TOO thin. With the much less dense aluminum a tube of the same diameter will have walls 3 times thicker, so you can increase the diameter more with the same strength material, have it weigh the same and not crumple. The same effect is used with magnesium parts which is even less dense, but proportionally weaker - you can cast a nice, stiff part that can be either lighter and weaker but just as stiff, or the same weight and strength and stiffer, or any combination in between.

I wasn't onboard when Bridgestone was making bikes, but hanging around with the CB 160 crowd you can't help hearing about some of the other vintage bikes. Yeah, I raced one occasionally here in Washington. A lot of fun at a slower speed; it's funny watching a guy with 15 hp and skinny tires dust a modern sportbike in a corner, but then braking is almost optional. (grin)

Randy Grein
rgrein at q.com



On Jul 14, 2012, at 3:22 PM, Erik Weber wrote:

> I'm way out of my area of expertise here but I always thought aluminum
> was generally stiffer than steel anyway? I do know that a steel-framed
> mountain bike flexes more than the stiffer aluminum framed bikes (not
> counting suspension). Or at least that was the case back in the stone
> age when I bought my Bridgestone (yep they made bikes).
> 
> On 7/14/12, KC <kc at brgracing.com> wrote:
>> An unsolicited testimony :-)
>> 
>> 
>>                         K.C.
>> BRG Racing Products and Dyno Services
>> Where we have a Sickness for Quickness
>>          Email kc at brgracing.com
>>  Address 110 2nd Ave South Unit D12
>>              Pacheco CA 94553
>>          Shop phone 925-680-2560
>> 
>> -----Original Message-----
>> From: sv650-bounces at micapeak.com [mailto:sv650-bounces at micapeak.com] On
>> Behalf Of Randy Grein
>> Sent: Saturday, July 14, 2012 1:23 PM
>> To: Suzuki SV650 Mailing List
>> Subject: Re: Weights of things
>> 
>> Yeah, it worried me putting my fat ass on it - but it didn't bend or break.
>> 
>> Randy Grein
>> rgrein at q.com
>> 
>> 
>> 



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