Forum home Road cycling forum Workshop

Bike geometry questions...

BhimaBhima Posts: 2,145
edited November 2009 in Workshop
I've almost finished disigning a custom bike frame for myself, but I have a few questions:

1) Integrated seatposts: are they "better" than making a normal frame with a seatpost hole? If so, how?

2) On my current bike, the tubes which start at the rear wheel and slope upwards towards the seatpost end right at the top of the seat tube, but what are the benefits of having them end 3 quarters of the way up the seat tube? Like this: http://www.wiggle.co.uk/images/bmc-slx01-09-zoom.jpg

Also, what's going on with the top tube on that frame? Why is there a hole? Weight saving gimmick?

3) Wavy lines: what's the benefit? See here: http://www.nytro.com/prodimages/1775-RED-L.jpg

4) Are the features mentioned in question 1, 2 and 3 only available in carbon? The frame will be steel/aluminium, but I've only ever seen these features on carbon bikes...

5) It will be a geared road bike, but could I have track dropouts, so I could convert it to fixed if I wanted?

6) Paint: could I make a significant weight saving by not paining it? How many grams are we talking? I kind of like the shiny silver look associated with old-skool-looking steel bikes anyway...

7) Why are some bikes made with downward-sloping top tubes? The frame design looks like it could be a lot stronger because it's bigger. Is this true? Here's an example: http://i193.photobucket.com/albums/z96/ ... G_1546.jpg

These questions can probably be answered by the frame builder, but the guy i'm using is on holliday for a bit and I want to get this design finished today! It's looking really good so far... :D

If anyone's interested, i'll post pics over the weekend. Doing it on paper, so it'll take a while to figure out how to use my scanner again.

Posts

  • InfamousInfamous Posts: 1,158
    Bhima wrote:
    1) Integrated seatposts: are they "better" than making a normal frame with a seatpost hole? If so, how?
    Nah, some look nice though.
    Bhima wrote:
    2) On my current bike, the tubes which start at the rear wheel and slope upwards towards the seatpost end right at the top of the seat tube, but what are the benefits of having them end 3 quarters of the way up the seat tube? Like this: http://www.wiggle.co.uk/images/bmc-slx01-09-zoom.jpg
    Seat stays? on the bmc they create a smaller triangle (stronger).
    Bhima wrote:
    Also, what's going on with the top tube on that frame? Why is there a hole? Weight saving gimmick?
    Gimmick.
    Bhima wrote:
    3) Wavy lines: what's the benefit? See here: http://www.nytro.com/prodimages/1775-RED-L.jpg
    No benefit, they're more of a trademark in pinarello's case.
    Bhima wrote:
    4) Are the features mentioned in question 1, 2 and 3 only available in carbon? The frame will be steel/aluminium, but I've only ever seen these features on carbon bikes...
    yep, carbon only. With the old style lo pros, they had to bend the tubes to make the shape they wanted.
    Bhima wrote:
    5) It will be a geared road bike, but could I have track dropouts, so I could convert it to fixed if I wanted?
    Yes, you could have old style horizontal dropouts, or the ones you see on many TT bikes with track dropouts and a derailer.
    Bhima wrote:
    6) Paint: could I make a significant weight saving by not paining it? How many grams are we talking?
    You'll still have to lacquer it, you won't save any weight.
    Bhima wrote:
    7) Why are some bikes made with downward-sloping top tubes? The frame design looks like it could be a lot stronger because it's bigger. Is this true? Here's an example: http://i193.photobucket.com/albums/z96/ ... G_1546.jpg
    That's the old style lo pros I mentioned earlier, they were made for TTs to get the front end lower (which is now down by using carbon). Don't know if it's stronger.
    Bhima wrote:
    These questions can probably be answered by the frame builder, but the guy i'm using is on holliday for a bit and I want to get this design finished today! It's looking really good so far... :D
    Do you really need a custom frame? if you don't know what you're doing (and you don't), you can end up with a rubbish heavy bike that cost you loads and doesn't turn. And also, pretty much all pro riders ride off the peg bikes. You could get a decent alu or carbon frame for the price of a custom steel one.

    If you do get a custom frame, make sure the builder sorts out the important geometry (trail, head angle, seat angle etc) and you just tell him what fancy bits you want.
  • BhimaBhima Posts: 2,145
    Infamous wrote:
    Do you really need a custom frame? if you don't know what you're doing (and you don't), you can end up with a rubbish heavy bike that cost you loads and doesn't turn. And also, pretty much all pro riders ride off the peg bikes. You could get a decent alu or carbon frame for the price of a custom steel one.

    If you do get a custom frame, make sure the builder sorts out the important geometry (trail, head angle, seat angle etc) and you just tell him what fancy bits you want.

    Well, originally I was going to create a different & experimental geometry, which you can't buy but it turned out to be a silly idea in the end. My current bike fits me, but only because i've gone to extreme lengths with bars/stems/seatposts/etc and hours of messing about tweaking stuff - I was going to buy a new frame anyway, for my "good" bike, but I may as well get something which fits perfectly. I have found off-the-peg bikes which might fit pretty well, with the exception of having either really long head tubes, or top tubes that are too long/short. If i'm going to pay for a good frame, I may as well get it perfectly sized... My ideal dimensions don't currently fit perfectly with any frames i've seen.

    By the way, i'm looking at carbon fibre now.

    Although a smaller rear triangle seems stronger, would the exposed part of the seat tube and seatpost not be weaker? If you have all your weight on the saddle, there would be a larger moment, surely? I only see these shorter type seat-stays on high-end expensive frames though, which leads me to believe that it's a superior design...

    With regards to those "low pro" frames - on that photo I posted, the position is the same as a standard road bike, it's just that there's less exposed seatpost... Why not just use a negative stem and/or reduce the length of the head tube instead?
  • If YOU are designing a frame, why all the questions?
  • InfamousInfamous Posts: 1,158
    Bhima wrote:
    Well, originally I was going to create a different & experimental geometry, which you can't buy but it turned out to be a silly idea in the end. My current bike fits me, but only because i've gone to extreme lengths with bars/stems/seatposts/etc and hours of messing about tweaking stuff - I was going to buy a new frame anyway, for my "good" bike, but I may as well get something which fits perfectly. I have found off-the-peg bikes which might fit pretty well, with the exception of having either really long head tubes, or top tubes that are too long/short. If i'm going to pay for a good frame, I may as well get it perfectly sized... My ideal dimensions don't currently fit perfectly with any frames i've seen.

    By the way, i'm looking at carbon fibre now.
    But that's why you can buy different bars/stems/seatposts/etc so you can fit on standard sized bikes. For the money, you will get a better/faster frame by buying off the peg and it will fit you.
    Bhima wrote:
    Although a smaller rear triangle seems stronger, would the exposed part of the seat tube and seatpost not be weaker? If you have all your weight on the saddle, there would be a larger moment, surely? I only see these shorter type seat-stays on high-end expensive frames though, which leads me to believe that it's a superior design...
    I doubt you'd notice a difference either way.
    Bhima wrote:
    With regards to those "low pro" frames - on that photo I posted, the position is the same as a standard road bike, it's just that there's less exposed seatpost... Why not just use a negative stem and/or reduce the length of the head tube instead?
    They did reduce the head tube... and often used negative stems, there were even track lo pros with the bars attached to the top of the forks! All that plus 650c front wheel = very low front. But now they can shape the tubes however they like using carbon.
  • redddraggonredddraggon Posts: 10,862
    Smaller and compact frames generally equals stiffer - smaller tubes won't deflect as large a distance.
    I like bikes...

    Twitter
    Flickr
  • mmacavity wrote:
    If YOU are designing a frame, why all the questions?

    My initial thought, how can you design a frame if you know nothing about design?!!
  • AnonymousAnonymous Posts: 28,799
    I've seen the end result of someone who knows nothing of custom frame geometry doing their own design. And it wasn't pretty -


    BadTrek.jpg
Sign In or Register to comment.