if having to buy a bike on frame measurements alone, whats MOST important?

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De Sisti
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby De Sisti » 29 Jul 2020, 3:55pm

Seat tube angles:
Dolan, 74.0
Spa, 72.5
xacd, 76.3.
Different (effective/actual) top tube length, different stem lengths. Same saddles, pedals, bars, levers, results in that they 'fit me' the same (as close as one can get).

*Edit. CORRECTION. As I'm not an expert on these things I had a look at the engineering drawings I received
from xacd.com. I was measure what I thought was the seat tube angle from the wrong place. The correct
seat tube angle for the xacd titanium frame is 72.5. Apologies for inavertently misleading you.
Last edited by De Sisti on 29 Jul 2020, 6:14pm, edited 1 time in total.

alexnharvey
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby alexnharvey » 29 Jul 2020, 5:30pm

It would be interesting to see how you have adapted these different bikes to fit.

Manc33
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby Manc33 » 29 Jul 2020, 5:51pm

OK sorry PH/reohn2, I see what you mean now - the saddle height could be low (or high) and that's going to affect the reach more on a seat tube with a smaller angle, than one with a larger angle.

I think I just realised why my MTB fits me and no other bike ever has, but I've never known quite why. :lol:
When two cyclists get married, they should throw anodized cable crimps instead of confetti.

reohn2
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby reohn2 » 29 Jul 2020, 5:54pm

Manc33 wrote:OK sorry PH/reohn2, I see what you mean now - the saddle height could be low (or high) and that's going to affect the reach more on a seat tube with a smaller angle, than one with a larger angle.

I think I just realised why my MTB fits me and no other bike ever has, but I've never known quite why. :lol:

The point is that you almost always have enough scope to move the saddle forward including an inline seatpost in necessary,but seldom have enough to move it back far enough,escpecially for tall riders.
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De Sisti
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby De Sisti » 29 Jul 2020, 6:15pm

alexnharvey wrote:It would be interesting to see how you have adapted these different bikes to fit.

CORRECTION. As I'm not an expert on these things I had a look at the engineering drawings I received
from xacd.com. I measured what I thought was the seat tube angle from the wrong place. The correct
seat tube angle for the xacd titanium frame is 72.5. Apologies for inavertently misleading you.

Manc33
Posts: 1659
Joined: 25 Apr 2015, 9:37pm

Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby Manc33 » 29 Jul 2020, 6:24pm

Is it true then that there's three measurements needed to work it out:

- Seat tube angle
- Effective top tube
- Inside leg

With those three and some maths, maybe I could make a calculator where you can input the three things above and it will spit out like a percentage, where 98% means the bike nearly fits but is slightly too small, or 102% means it's slightly too big for you, but in both cases the bike should fit you.

The inside leg would be in the calculator because then it can roughly guess at the saddle height, which in turn lets it know the true reach, assuming you also put the effective top tube in along with the seat tube angle.

It would need to have a separate spreadsheet for each bike frame type, due to the difference between drop bars and flat bars (affecting reach).

It's all possible but it would take me a while working all that out.

If such a calc existed, the problem then is it's almost like you'd be telling an Excel spreadsheet to make a subjective decision. :lol:

I mean some people like being stretched out and some can't stand it. Also some saddles might be 40mm above their own rails while others might be 20mm. There's so many variables, if you want a perfect fit.

It can't be any harder to make that calc than it was making the spoke length calc :mrgreen:

EDIT: That's the easy part done :)

Image

I just tried this one and it doesn't tell you anything at the end, it also seems so think your age affects the size of bike you need, which is just marketing nonsense "Please fill in all fields" :roll:

Code: Select all

https://www.jensonusa.com/bike-fit-calculator


EDIT 2: This is tough in Excel because I think the seat tube angle needs to be treated as if it was a negative number, because the smaller that angle is, the bigger the bike frame is (multiplying by ETT elsewhere). I can divide by that seat tube number and get a smaller number, but that's no use because the calc would be telling you a bike is 95% (too small for you) when it should say 105% (too big for you). I can multiply by the seat tube angle but that's a problem too, because the smaller that angle is, the smaller the resulting number will be, telling you the bike is 98% when it should be 102%.

Perhaps the way to do it is to have a separate thing going on in Excel where it does something for the seat tube angle like "180 -STA" and the number remaining can be used. For example if you put in 72° for the seat angle, it could use "108" (180 -72) but if you increased the STA to say 85°(meaning the reach on the frame is less, meaning the bike is smaller) it could use "95" (180 -85). These remaining numbers are what would be multiplied. This way, a smaller (remaining) number means a bigger angle was input, ergo a smaller frame.
Last edited by Manc33 on 29 Jul 2020, 7:33pm, edited 2 times in total.
When two cyclists get married, they should throw anodized cable crimps instead of confetti.

alexnharvey
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby alexnharvey » 29 Jul 2020, 6:48pm

De Sisti wrote:
alexnharvey wrote:It would be interesting to see how you have adapted these different bikes to fit.

CORRECTION. As I'm not an expert on these things I had a look at the engineering drawings I received
from xacd.com. I measured what I thought was the seat tube angle from the wrong place. The correct
seat tube angle for the xacd titanium frame is 72.5. Apologies for inavertently misleading you.


That makes more sense. Are the two similar ones in their set up compared to the Dolan or did another factor like type of riding complicate things.

De Sisti
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby De Sisti » 29 Jul 2020, 7:43pm

alexnharvey wrote:
De Sisti wrote:
alexnharvey wrote:It would be interesting to see how you have adapted these different bikes to fit.

CORRECTION. As I'm not an expert on these things I had a look at the engineering drawings I received
from xacd.com. I measured what I thought was the seat tube angle from the wrong place. The correct
seat tube angle for the xacd titanium frame is 72.5. Apologies for inadvertently misleading you.


That makes more sense. Are the two similar ones in their set up compared to the Dolan or did another factor like type of riding complicate things.


I had a bike fit in 2011 at Adrian Timmis (near Derby) with the xacd titanium bike (and another frame, which I subsequently sold).
From the measurements (tip of saddles to stem centre, height of saddles from floor*, width of bars, etc) I've managed to get the bikes virtually identical in how my body feels on them. As I mentioned earlier, they all have same pedals, seat, handlebars, gear levers,(even bottle holders and brand of stem).

Type of riding; well I don't race. The Spa is used from October to March and the other two are ridden during the Spring and Summer.


*works for me.

Manc33
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby Manc33 » 29 Jul 2020, 8:04pm

I have made a bike fitting calculator that covers MTB/Road bikes, but it's not meant to be taken seriously, I mean it might be accurate, but might not. I have only gone off the measurements of bikes I know fit me in the past and made my inside leg part of the equation.

I'll post it in a thread of it's own at some point, still polishing it off.

EDIT: It's polished off :mrgreen:
viewtopic.php?f=5&t=139684
When two cyclists get married, they should throw anodized cable crimps instead of confetti.

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531colin
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby 531colin » 30 Jul 2020, 10:02am

De Sisti wrote:Seat tube angles:
Dolan, 74.0
Spa, 72.5
xacd, 76.3.
Different (effective/actual) top tube length, different stem lengths. Same saddles, pedals, bars, levers, results in that they 'fit me' the same (as close as one can get).

*Edit. CORRECTION. As I'm not an expert on these things I had a look at the engineering drawings I received
from xacd.com. I was measure what I thought was the seat tube angle from the wrong place. The correct
seat tube angle for the xacd titanium frame is 72.5. Apologies for inavertently misleading you.


So, thats 2 bikes with 72.5 and one with 74 degrees.

De Sisti wrote:....
I had a bike fit in 2011 at Adrian Timmis (near Derby) with the xacd titanium bike (and another frame, which I subsequently sold).
From the measurements (tip of saddles to stem centre, height of saddles from floor*, width of bars, etc) I've managed to get the bikes virtually identical in how my body feels on them. As I mentioned earlier, they all have same pedals, seat, handlebars, gear levers,(even bottle holders and brand of stem).

Type of riding; well I don't race. The Spa is used from October to March and the other two are ridden during the Spring and Summer.

*works for me.


Saddle setback measures how far behind the BB axle the saddle is. You don't list saddle setback in your set-up criteria, so if all 3 bikes have the same saddles and seatposts with the saddles in the exact same position on the post, then 1.5 degrees difference in seat tube angle will give you about 15mm difference in saddle setback (for an average male height). I couldn't do that; apart from anything else, moving the saddle 10mm produces a very obvious shift in the workload of different muscle groups. (Move it back 10mm loads the hamstrings and un-loads the quads.)

De Sisti
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby De Sisti » 30 Jul 2020, 5:21pm

Different length top tubes, so seats postions are adjusted accordingly.

Manc33
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby Manc33 » 30 Jul 2020, 6:55pm

The title says "a bike".

The difference between flat bars and drop bars is enormous.

The seat tube angle and top tube length that's correct on flat bars, is going to have you too stretched out on drop bars.
The seat tube angle and top tube length that's correct on drop bars, is going to have you too cramped on flat bars.

If I had a road bike setup normally on drop bars, that frame would need to be something like a 48 or 51.
If I was on a road bike and put flat bars on it, that frame would need to be something like a 57 or 59.

These are not small differences!
When two cyclists get married, they should throw anodized cable crimps instead of confetti.

fastpedaller
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby fastpedaller » 30 Jul 2020, 11:21pm

Manc33 wrote:The title says "a bike".

The difference between flat bars and drop bars is enormous.

The seat tube angle and top tube length that's correct on flat bars, is going to have you too stretched out on drop bars.
The seat tube angle and top tube length that's correct on drop bars, is going to have you too cramped on flat bars.

If I had a road bike setup normally on drop bars, that frame would need to be something like a 48 or 51.
If I was on a road bike and put flat bars on it, that frame would need to be something like a 57 or 59.

These are not small differences!


The above isn't correct IMHO....... Surely the object of drop bars is (and has always been) to achieve a more aerodynamic position, otherwise there is no point in fitting them (ok I'll concede they give a 'variety' of different hand positions) but my point is that they are not intended to replicate a straight bar!

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531colin
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby 531colin » 31 Jul 2020, 8:54am

De Sisti wrote:Different length top tubes, so seats postions are adjusted accordingly.


This is the bit that I (and many others) can't do.
I need to sit a fairly precise distance behind the bottom bracket (although in reality I suspect plus/minus 5mm is close enough, as I move about on the saddle anyway).
Having set the saddle setback, reach is adjusted with different stem lengths.
Saddle setback is critical for me because it governs how much weight is on my hands, and what proportion of pedalling "work" is done by quads vs. hamstrings.
I'm sure I'm much fussier over this in my seventies than I was in my thirties; in my thirties I think I could jump on just about anything and ride it.

reohn2
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Re: if having to buy a bike on frame measurements alone, whats MOST important?

Postby reohn2 » 31 Jul 2020, 9:35am

Manc33 wrote:The title says "a bike".

The difference between flat bars and drop bars is enormous.

The seat tube angle and top tube length that's correct on flat bars, is going to have you too stretched out on drop bars.
The seat tube angle and top tube length that's correct on drop bars, is going to have you too cramped on flat bars.

If I had a road bike setup normally on drop bars, that frame would need to be something like a 48 or 51.
If I was on a road bike and put flat bars on it, that frame would need to be something like a 57 or 59.

These are not small differences!

The difference between a frame for flats and drops is reach(ETT length) and possibly height depending on the rider.
The saddle to BB relationship remains constant(optimum)consequentially so does the seatube angle.
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