Carbon Fibre-advantages/disadvantages

[alienator]

If the steerer on your fork is flexing, there's a problem with the headset.

No. You're wrong. You should talk to people who design forks. There are thread elsewhere, as well as tests done, that demonstrate this.

It sounds like you're confusing oscillations of the entire frame with oscillations of the individual tubes.

No, I'm not. That's your assumption

It's not about selectively extinguishing certain mode shapes,(whatever that's supposed to mean) but simply picking out a tube set that's going to transmit energy at the lowest possible frequency. For the record, I am an engineer, and I have designed tube structures with dynamic constraints. It's not brain surgery.

Did I say mode shapes? No, I said modes. In terms of harmonics, modes, as used by me and by others refers to harmonic modes, i.e. the frequency at that harmonic. Engineering certainly isn't brain surgery, but engineers should strive to have a better understanding of physics, rather than absolutiely trusting what Pro/E or some FEA programs returns. For example, your engineering expertise hasn't allowed you to understand exactly how a fork flexes.

 

[artmichalek]

No. You're wrong. You should talk to people who design forks. There are thread elsewhere, as well as tests done, that demonstrate this.

You'll have to provide some citations for that. The only fork deflection test I know of is Damon Rinard's, which showed deflections around 5mm. If the legs were perfectly rigid, the top 30mm of the fork would be rotating about 0.11Rad. That comes out around 5.5% strain at the leading/trailing edges of the crown. Admittedly that's a pretty rough calculation, but 6061-T6 aluminum yields at 0.37%.

Did I say mode shapes? No, I said modes. In terms of harmonics, modes, as used by me and by others refers to harmonic modes, i.e. the frequency at that harmonic. Engineering certainly isn't brain surgery, but engineers should strive to have a better understanding of physics, rather than absolutiely trusting what Pro/E or some FEA programs returns. For example, your engineering expertise hasn't allowed you to understand exactly how a fork flexes.

An understanding of physics doesn't provide one with common sense. You don't need any FEA program to know that changing the tube schedule is going to shift the spectral response of the frame.

 

[Insight Driver]

What are the advantages and disadvantages of carbon fibre frames?
I currently have aluminium Avanti Monza triple chain ring, which weighs 22kg

with 2 bottle cages and clipless pedals. Its a great bike , but I'm thinking of upgrading ( 42 years old , live & ride in hilly area, and knees not getting any
younger! ).

--How would Carbon fibre forks improve an aluminium bike?

There is little improvement. There might be some slight difference in feel. The difficulty is finding a fork in carbon that is the same dimensions as the aluminum fork. The reason is a different fork may have a different rake, which will make a difference in trail, which will have an affect on the handling of the bike that likely would only be subtle. The weight difference would be minor. The carbon fork would be lighter and have greater fatigue strength. That said, you likely would be in the market for a new bike long before an aluminum fork wore out.

--How would carbon fibre seat or chain stays improve an aluminium bike??

The improvement there would also be so minor as to be insignificant.

--How would carbon fibre seatpost improve a bike???

No improvement whatsoever. You would be waisting money for an aluminum seatpost does the job and it is an undemanding one.

--Would I only notice a significant improvement from my aluminium bike, if
I bought a bike with carbon fibre frame & fork????
Yes you would, but not because it's carbon fiber. Just because it's different. Any time you get a new bike it feels much better, but there are so many variables in bike geometry that affect the perceptions we have of a bike that we can't say the material of the frame and fork added anything to it.

--Would a carbon fibre bike be strong enough for steep descents down bumpy
sealed country roads?????

Yes to this question. They make carbon fiber mountain bikes nowadays because they can build them strong enough for those demands. A carbon fiber bike is not fragile. It has different failure modes than metal does. Obviously there are now a lot of carbon fiber frames out there and there is no indication that there is any fragility to those bikes.

--Could we expect carbon fibre to become relatively cheaper as it gets
more common??????

Yes it will. The issue though is carbon fiber demand is currently higher in the world than the supply. This will keep the cost of the raw materials high for the foreseable future. That said, due to increases in the use of carbon fiber in bikes in the industry, prices are dropping. There are some amazingly good carbn fiber bikes coming out of China, Korea and Taiwan. Even some American bike makers that make their own frames have reduced the prices of their carbon fiber bikes.

Thanks for any unbiased information-- I want to be educated! Dave.

 

[Insight Driver]

What are the advantages and disadvantages of carbon fibre frames?
I currently have aluminium Avanti Monza triple chain ring, which weighs 22kg

with 2 bottle cages and clipless pedals. Its a great bike , but I'm thinking of upgrading ( 42 years old , live & ride in hilly area, and knees not getting any
younger! ).

--How would Carbon fibre forks improve an aluminium bike?

There is little improvement. There might be some slight difference in feel. The difficulty is finding a fork in carbon that is the same dimensions as the aluminum fork. The reason is a different fork may have a different rake, which will make a difference in trail, which will have an affect on the handling of the bike that likely would only be subtle. The weight difference would be minor. The carbon fork would be lighter and have greater fatigue strength. That said, you likely would be in the market for a new bike long before an aluminum fork wore out.

--How would carbon fibre seat or chain stays improve an aluminium bike??

The improvement there would also be so minor as to be insignificant.

--How would carbon fibre seatpost improve a bike???

No improvement whatsoever. You would be waisting money for an aluminum seatpost does the job and it is an undemanding one.

--Would I only notice a significant improvement from my aluminium bike, if
I bought a bike with carbon fibre frame & fork????
Yes you would, but not because it's carbon fiber. Just because it's different. Any time you get a new bike it feels much better, but there are so many variables in bike geometry that affect the perceptions we have of a bike that we can't say the material of the frame and fork added anything to it.

--Would a carbon fibre bike be strong enough for steep descents down bumpy
sealed country roads?????

Yes to this question. They make carbon fiber mountain bikes nowadays because they can build them strong enough for those demands. A carbon fiber bike is not fragile. It has different failure modes than metal does. Obviously there are now a lot of carbon fiber frames out there and there is no indication that there is any fragility to those bikes.

--Could we expect carbon fibre to become relatively cheaper as it gets
more common??????

Yes it will. The issue though is carbon fiber demand is currently higher in the world than the supply. This will keep the cost of the raw materials high for the foreseable future. That said, due to increases in the use of carbon fiber in bikes in the industry, prices are dropping. There are some amazingly good carbn fiber bikes coming out of China, Korea and Taiwan. Even some American bike makers that make their own frames have reduced the prices of their carbon fiber bikes.

Thanks for any unbiased information-- I want to be educated! Dave.