Geometry understanding. The basics.

[TheBomber]

Where do those numbers come from? I'm pretty sure a LHT fork isn't 416mm and if that's wrong then at least some of the other measurements are likely to be as well.

I didn't include anything for the lower race of the headset. The height of that would be deducted from the fork:

https://www.bikegeocalc.com/#12Surly+LH ... 2.5G30H30Z

[PH]

Where do those numbers come from? I'm pretty sure a LHT fork isn't 416mm and if that's wrong then at least some of the other measurements are likely to be as well.

I didn't include anything for the lower race of the headset. The height of that would be deducted from the fork:

https://www.bikegeocalc.com/#12Surly+LH ... 2.5G30H30Z

Thanks, that would explain it and of course the VN's is integrated.

[531colin]

Those are the actual top tube lengths you're quoting from the Bike Geometry Calculator Colin - the ETT numbers are up above them. The Surly came out exactly as specified, while the Yukon appears as 581mm - only 1mm less than the spec.

Thank you....corrected!

[531colin]

So the 23mm (24mm) difference in ETT is because the LHT head tube top is much higher than the Yukon?

I'm no mathematician!
There is half a degree difference between seat angle and head angle in the LHT.
There is 58mm difference in stack between the 2 bikes.
If I draw a triangle with one side 58mm and one side 23mm, there is no angle of half a degree, i would guess its more like 20 degrees?

[TheBomber]

The difference in seat tube angles will only be worth about 5-10mm on the ETT.

The big difference is the stack - although they both appear to have the same reach, because the reach of the Surly is measured at a point much higher than the Yukon, it is actually a much longer bike. If I set the rider position on the Surly to be the same as the Yukon it would look like this:

https://www.bikegeocalc.com/#20Surly+E ... 2.5G30H30Z

Has anybody ever ‘slammed’ the stem on a Surly before?!

[nomm]

I am having this exact dilemma with a new bike purchase. I test rode a sonder colibri in a medium (for Riders between 5ft 6in and 5ft 10in) with a slammed 120mm stem and it felt ...great?!

But when it came to checking the geo later on against bikes I have recently test rode, the ETT on it is 14mm longer than my last bike + the 120mm stem...?

So I ended up going down some dark rabbit holes (https://blog.bikeinsights.com/how-uprig ... ke-design/) trying to get my head around stack and reach measurements, stack and reach ratios and headtube lengths. In fact the reach on the medium Colibri is the same as an XL in another 'endurance' road bike I have been looking at... the Tifosi Rostra Disc.


And I am still none the wiser as they vary so wildly and with some 'expert' opinions stating ETT measurements are almost pointless.

[TheBomber]

Do you have a riding position that you are happy with and would like to replicate on a new bike? You can use the Bike Geometry website to do that. Probably also the one linked back on page 1 that I’m not familiar with. Then you can see what stem, spacers etc are required.

ETT is only really useful when comparing bikes that are very similar in most other ways. The steepening of seat angles and slacking of head angles in more recent years, along with options for longer head tubes makes ETT a bit irrelevant. Stack and reach get round the problem, but I for one can’t make sense of them by just looking at the numbers, I have to code them into that website. Which is what I have been doing recently for an imminent bike purchase. If I’m wrong about that I’m about to make an expensive mistake!

[531colin]

The difference in seat tube angles will only be worth about 5-10mm on the ETT.

The big difference is the stack - although they both appear to have the same reach, because the reach of the Surly is measured at a point much higher than the Yukon, it is actually a much longer bike. If I set the rider position on the Surly to be the same as the Yukon it would look like this:

https://www.bikegeocalc.com/#20Surly+E ... 2.5G30H30Z

Has anybody ever ‘slammed’ the stem on a Surly before?!

Thank you for your patience!
I'm sure i had all this stuff at my fingertips a few years ago, but its gone!

I think the penny finally dropped;
The stack varies the reach because reach is measured from a vertical line through the BB to a line through the head tube: these lines converge at an angle of 18 degrees with a 72 deg head angle.

One of the first things I look at is seat tube angle. I like to sit behind the pedals, I can't really manage much steeper than 72 degrees.
I like stack and reach because they are centred on the BB, which you can't move; however, as we see in this thread stack influences reach.
Effective top tube is useful (provided the seat angle suits you) and doesn't vary much with stack (frame height) because the seat and head tubes are close to parallel.
Stack (or head tube length and fork length) is important because it governs handlebar height with a carbon steerer. Steel steerers are relatively easy (and neat) to extend with an internal extender; I don't like the external sreerer extenders which you have to use with an alloy steerer, and lots of folk don't like riser stems.

The take home message is that none of these measurements are much use in isolation

Edit; my BB axle to saddle top is close to 700mm. I reckon 1 degree change in seat tube angle moves the saddle about 10mm forward or back.

[531colin]

........... dark rabbit holes (https://blog.bikeinsights.com/how-uprig ... ke-design/) trying to get my head around stack and reach measurements, stack and reach ratios............

I think you would be better looking at comparing the geometry data on one of the websites which are referenced on this thread.....superimposed pictures of 2 frames makes much more sense to me than a ratio between 2 measurements, thats too deep for me!