top tube slope

[tuscanyswe]

I have a bike with following dimenstions:

fork lenght: 370mm
headtube: 180mm
seattube: 550mm
bb drop: 70mm
sta: 73
hta: 74

This frame has a 58 toptube with a 6degree slope according to manufacturer.

Thinking of getting this frame built:

forklenght: 385mm
headtube: 165mm
seatube: 545mm
bb drop: 72.5mm
sta: 73
hta:73

This frame would have a 57.5 toptube with a 4.3 degree slope according to drawing.

Now how can this be? The 15 mm shorter headtube on new frame should negate the 15mm longer fork axle to crown lenght it has. They have very similar bb drop and seattube lenghts so if anything the shorter toptube of the new frame would mean it has more slope than the frame i allrdy have, yet im getting information it would actually have 2 degree less slope.

Can the 1 degree difference of hta mean this much slope difference even tho the toptube is longer and the seattube is half a centimeter higher?

Im confused, help me out! :)

[Smiley]

you are not stating the amount of HT extension above the TT so you really don't know the slope yet. and the fork spans are different as you state so one HT will be shorter than the other too.The spans will make up for a longer to shorter HT. The BB drops are so small I would not factor that in, just advise what the HT ext are first and don't assume they are both 0.7 cm.

lastly all this is kosher as long as both TT diameters are equal too, not going oversized etc or ovalized.

[tuscanyswe]

Ht extension on old frame is 20mm and on new frame is 15mm so again the new frame would have more slope than the old the way i think (obv not correct)

so old frame:

sta: 73
hta: 74
st: 550mm
ht: 180
ht above tt: 20mm
fork lenght 370
tt lenght: 580mm effective with a 6degree slope

New frame:

sta: 73
hta: 73
st 545mm
ht:165mm
ht above tt: 15mm
fork lenght: 385mm
tt lenght: 575mm effective with a 4.3 slope

Theres something im not getting : /

[Smiley]

so are tube diameters the same too? I am not trying to mess with you but I get my slope number from Serotta when we check fits based on Stand Over at HT, Stand over at mid point and Stand over at Seat tube as this eliminates the tube diameter issues. I think both your old and new bike should have the head set at the same spot but with more HT ext on the old bike. Lastly one can't assume that your seat tube is not extended at the collar either so these tow bikes need stand over distances to be 100 % accurate. Smiley

[tuscanyswe]

so are tube diameters the same too? I am not trying to mess with you but I get my slope number from Serotta when we check fits based on Stand Over at HT, Stand over at mid point and Stand over at Seat tube as this eliminates the tube diameter issues. I think both your old and new bike should have the head set at the same spot but with more HT ext on the old bike. Lastly one can't assume that your seat tube is not extended at the collar either so these tow bikes need stand over distances to be 100 % accurate. Smiley

The tubing will actually be larger on the old bike (new not made yet but thats the idea) so that would again further decrease the slope compared to new frame.

Now how can then the new frame that has the TT slightly higher on the HT and also has a shorter ST and shorter toptube have 2 degree less slope.

I will fit fine on either as the standover will not be much of concearn. However im trying to find the most pleasing slope and my point of reference is the old frame with 6 degree slope. Id like a little less. So 4 would be good but im having trouble seeing that the dimensions discussed here are adding up to 4 degree slope considering the old has 6 and logically should have less slope.

what am i missing?
Or is perhaps the 4 degree calculated with a 370mm fork rather than the 385mm specified?

[nicrump]

***

[tuscanyswe]

Aha.

The moots (old frame) site specifies seattube at center to top. I thought it was ctc. So ST on moots is actually 52 ctc. And that would explain the larger slope.

Thanks for help Smiley!

[Smiley]

Aha.

The moots (old frame) site specifies seattube at center to top. I thought it was ctc. So ST on moots is actually 52 ctc. And that would explain the larger slope.

Thanks for help Smiley!


see the math never fails :)

[retrogrouchy]

The tubing will actually be larger on the old bike (new not made yet but thats the idea) so that would again further decrease the slope compared to new frame.

Now how can then the new frame that has the TT slightly higher on the HT and also has a shorter ST and shorter toptube have 2 degree less slope.

I will fit fine on either as the standover will not be much of concearn. However im trying to find the most pleasing slope and my point of reference is the old frame with 6 degree slope. Id like a little less. So 4 would be good but im having trouble seeing that the dimensions discussed here are adding up to 4 degree slope considering the old has 6 and logically should have less slope.

what am i missing?
Or is perhaps the 4 degree calculated with a 370mm fork rather than the 385mm specified?

This is a non-issue, since road bicycle frames should never have sloping top tubes at all, but should have TTs that are level with the ground, as God intended them to be made! :p

[nicrump]

This is a non-issue, since road bicycle frames should never have sloping top tubes at all, but should have TTs that are level with the ground, as God intended them to be made! :p

if this is what your god intended then your TT should actually curve to a radius something like 6,377,830,992mm.

[michael white]

if this is what your god intended then your TT should actually curve to a radius something like 6,377,830,992mm.


I'll ask Him for the formula, and we'll be up to code.