[email protected] (Zaf) wrote:
>Mark Hickey <
[email protected]> wrote in message
>news:<
[email protected]>...
>> Hey, you can test your theory that you can compress those curved fork blades easily. Just tape a
>> piece of dental floss or other non-stretchy string tightly from the dropout to the crown and
>> push down for all you're worth. If you're compressing the fork the string will go slack. It
>> won't, though... the compressive force you'd need to get just a couple mm of bend would be
>> measured in tons.
>>
>I take it you have done this? If so I am surprised the spring constant (that is a spring constant
>is'nt it?) is this high.
I don't have to do it, I know how the test will come out. The curve isn't there as a "spring" but as
a way to provide proper trail while using a crown that's "square".
>> >I have never seen an curved aluminum fork, if there ever was such an animal this would lend
>> >support to the theory that there is no deflection of the fork blades, deflection of aluminum
>> >would be a bad thing.
>>
>> I'll remember that when I'm riding my fixie today with its Kinesis aluminum (curved) fork.
>
>OK, so there are curved Al forks then pls help straighten out my misconceptions of materials. At
>this point you have forced me to pull my yellow paged mtl sci book from the shelf. Ah, yes there it
>is fatigue (at this point we're OT and back onto frame mtls methinks). I guess I had always assumed
>that forks would deflect more then frames and Al frame loading was within the endurance limit of Al
>whereas forks could not be. So what is a typical net deflection of a AL frame or fork?
I don't know how to define "typical", but would say that an aluminum fork is much more
overengineered than the typical aluminum frame. The impromptu experiment I did shows that
deflections of plus/minus 10mm are certainly possible - though infrequent (I doubt anyone would ride
with constant brake shudder of that magnitude). The greatest flex in a bike frame occurs in the BB
area, where you can get enough deflection between the BB shell and front derailleur cage to cause
contact with the chain (yes, the BB spindle also plays a part, but much less than the frame in most
cases where there's a problem).
Mark Hickey Habanero Cycles
http://www.habcycles.com Home of the $695 ti frame