Cycling UK Forum

I'd like to get to grips with a little bit of geometry.

(1) If I compare the Yukon to the Surly, I see
the same reach
basically the same head & seat angle
the same seat tube
....but there is a 23mm difference between top tubes. What? How can this be?
(2) Then if I compare the Surly with the Kona, I see
the same top tube
more relaxed head tube angle on the Kona
....but the reach is the same?

Please enlighten me!
Thanks in advance.

chocjohn9 wrote: ↑18 Jan 2023, 8:18pm I'd like to get to grips with a little bit of geometry.

(1) If I compare the Yukon to the Surly, I see
the same reach
basically the same head & seat angle
the same seat tube
....but there is a 23mm difference between top tubes. What? How can this be?

Screenshot 2023-01-18 at 20.56.37.png

(2) Then if I compare the Surly with the Kona, I see
the same top tube
more relaxed head tube angle on the Kona
....but the reach is the same?

It's probably down to the wheel base and seat angle of each model. On a piece of graph paper you can produce a scale drawing of each frame and then laying the papers on top of each other the differences could be seen... I guess there could already be a CAD program hiding in the Internet which will allow you to do this

I think somewhere on the web there’s a tool that does the comparison and shows it as a drawing of one overlaid on the other. Bike Geometry Geek maybe???

I think the difference between Yukon and LHT is the stack height, determined by the length of headtube, the taller that is, the longer the TT for the same reach. I can't get my head round the Kona, as others suggest you could draw it or use one of the geometry websites.
It is a minefield, every difference has a corresponding consequence, which riders are sensitive to in different degrees, including not at all. What matters of course is your own stack and reach, where that is on a bike rather than on a frame, for example - the steeper ST on the Yukon means pushing the saddle back for the same position, effectively increasing the reach and decreasing the angle, again drawing that out is helpful.
EDIT - I find it annoying that the one measurement nearly always missing from these charts if the front centre, I know you can work it out from the others but the likelihood of toe overlap is higher on my list when comparing frames than some of those always included and it's the one I can't manipulate.

I guess this is what Nearholmer referred to. Very handy.
https://bikeinsights.com/

That’s the one!

Thankyou.

Define some terms

front centre is the distance between the BB axle and the front axle, measured directly.

effective top tube is the horizontal distance between the top of the head tube and a line drawn up the middle of the seat tube (and above), measured centre to centre.

reach is the horizontal distance between the top of the head tube and a vertical line drawn through the centre of the BB...

stack is the vertical distance between the top of the head tube and a horizontal line drawn through the centre of the BB


IT FOLLOWS THAT

reach is independent of head tube angle

If you have the same reach and vary the head angle, you will vary front centre, not effective top tube.

Comparing Yukon and Surly
Surly head tube is 25mm taller
Surly stack is 58mm greater, ( the Surly forks are longer, BB height different? )

If the seat tube and head tube are parallel, then the height of the frame doesn't alter effective top tube (ETT from now on!)
23mm difference in ETT between Surly and Yukon seems a lot as head and seat angles are similar within a degree or so, but i find this stuff difficult to visualise and estimate

There is a free bike CAD program on the web from "The bicycle forest" which you can use to check the geometry you get in manufacturers lists.....they aren't always right! Operating the program requires more understanding of tech and more diligence than I can muster at my advanced age.

There is also a website called "geometry geeks" where you may find the geometry of the bikes you are interested in has already been added, or failing that you can add it yourself to make comparisons.

Edited for colour !

Correct me if I’m wrong but isn’t the Yukon is a road specific Randonneur style frame and the LHT a rough-stuff style tourer? That was certainly the case with the LHT’s I have seen in my workshop and I am also the owner of a Van Nicholas Yukon.
If we are talking about the same bikes here I would volunteer the suggestion that the BB height on the LHT is higher to provide crank clearance off-road.

60 cm Van Nich. Yukon....BB drop is 70mm (same across all sizes, which I think is poor)
https://www.vannicholas.com/touring-bikes/yukon

60cm LHT 700c wheels ....BB drop is 78mm (again, same across all sizes) https://surlybikes.com/bikes/legacy/lon ... 7#spec-geo

Perhaps the OP (or a helpful soul) could put up the geometry charts?

531colin wrote: ↑19 Jan 2023, 11:42am Perhaps the OP (or a helpful soul) could put up the geometry charts?

Here we are - sorry it doesn't seem to showing as a picture here, you'll have to follow the link. Use the 'Swap bikes' button to switch between the two, and select 'Shadow bike' to get an overlay of the other one:

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

Well done.

Shadowed and overlaid. Click the image to enlarge.

Jonathan

Sorry, I got this all wrong
the numbers in chocJohn's table and Bombers diagram are the same within 1mm.....ie they are the same!
....and I still find it difficult to visualise and understand !

Hang on a minute......

In chocJohn's table, ETT is; Surly 605mm; Yukon 582mm.

On bomber's diagram, Surly is 581mm; Yukon 576mm
Actually 605 Yukon 581

I have to say a 5mm difference in ETT with the same reach is more easily believable than a 23mm difference.

TheBomber wrote: ↑20 Jan 2023, 2:45pm Here we are - sorry it doesn't seem to showing as a picture here, you'll have to follow the link. Use the 'Swap bikes' button to switch between the two, and select 'Shadow bike' to get an overlay of the other one:

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

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.

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.