jambon

Hi all ,

I'm looking into getting an LHT . I'm considering a 60 or 62cm frame 700c (I'm tall) based on the effective toptubes of my current bikes .

When I look at the geometry of the two frames ;

Long Haul Trucker | Bikes | Surly Bikes

I can see that the 60 has a reach of 403 , slightly longer than the 62 at 401

So although the 62 has an effective top tube of 610 mm to the 60's 60mm toptube , The reach of the smaller frame is actually longer .

So..... If my saddle is set in the same position relative the Bottom brackets on both sizes then the bikes are effectively equal in size and the only difference is in standover height . Am I correct or am I missing something ?

J

CliffordK

Not Quite.

A lot can be adjusted with seat and stem adjustments, but the frames are slightly different

From the chart:
60cm:
ST Angle: 72.5, HT Angle: 72, TT Length: 594.8, HT Length: 189, Reach: 403, Wheelbase: 1080.8

62cm:
ST Angle: 72, HT Angle: 72, TT Length, 603.9, HT Length: 210, Reach: 401, Wheelbase: 1085.3

So, the head tube of the 62cm bike is 21mm longer.

A bit of old math: SOH CAH TOA, so Cos(angle) = Adjacent/Hypotenuse.
COS(72) = A/21, A = 6.5mm

So, if you raise the stem on the 60cm bike by 21mm, you effectively shorten the reach by 6.5mm, so the reach is actually effectively about 4.5mm shorter on the 60cm frame when adjusted for HT height as would be expected.

Note the 60cm frame has a slightly steeper seat tube, which can be adjusted for by using more setback with the seatpost.

So, the 62cm frame has a slightly longer wheelbase (4.5mm), which I think is all in the TT length, and thus less toe overlap.

Hmm... do all the angles and lengths add up?

jambon

But the difference in the head tube sizes only becomes a factor if you plan to have your stem set lower on the steerer than the 21 mm difference would allow , the head tube angles are equal , the head tube size difference just means 21 mm less steerer will show on the larger size ?

CliffordK

Oops, I copied the wrong wheelbase above... and the numbers now add up a little better.

If you laid the two frames on top of each other, and aligned the bottom brackets, then the rear dropouts should line up, I think (same chainstay length and BB drop).

The dropouts on the fork of the 62cm frame will be about 4.5mm in front of those on the 60cm frame.

Likewise, the head tube on the 62cm will be about 4.5 mm in front of the HT on the 60cm frame.

The top of the seat tube will be set back another 5mm or so on the 62cm frame due to less of a seat tube angle, and the effective top tube from the seat tube to the head tube will be about 10mm (1cm) longer on the 62cm frame.

The "Reach" is an odd number.

If you set the stems at the exact same height/angle on the two frames (2cm higher on the 60cm frame), then the effective reach will be longer on the 62cm frame.

On the other hand, say you slam the stem on the 60cm frame, then you'd have to use a more angled and longer stem to get the bars of the 62cm frame to the same spot.

ThermionicScott 

I'm not entirely sober atm, so I may edit/delete this post tomorrow, but with identical head tube angles for the 60 and 62cm LHTs and a 10mm longer ETT for the 62cm, I'm wondering if the reach figures weren't accidentally switched for the two. Maybe email them and get it from the horse's mouth?

CliffordK

I don't think so.

From my (corrected) notes & calculations above.

Wheelbase is 4.5mm longer for the 62
HT is 21mm longer for the 62 at the same angle.

With the 72 degree angle of the headtube, it brings it about 6.5 mm further back in those 21mm.

So, 4.5 - 6.5 = -2mm, and the "reach" should be about 2mm shorter as in the specs.

The TT length is still longer, because, the seat tube goes parallel to the HT, so the longer HT doesn't change the TT length. However, the angle on the 60 is 1/2 degree steeper, effectively giving a shorter TT.

ThermionicScott 

/shrug, maybe the numbers are right. I hate Surly geometry regardless.

jambon

If you lay the two frames on top of each other and align the bottom brackets then the head tube of the smaller frame would be slightly further away than the larger frame as the reach (distance from centre of bb to centre of head tube ) of the smaller frame is longer ? The extra length of the 62cm top tube is behind the bb so as far as a riders cockpit is concerned both sizes are nearly equal , no ?

Darth Lefty 

I CADded it up and the numbers are right enough*. The longer seat tube takes up some of the longer top tube; the slacker seat tube angle takes up the rest. Why they are different is probably a more interesting discussion.




* The geometry is overconstrained and so some things are off by a tenth of a mm or less, but it's not important, the jigs are probably not that accurate.