Comparing geometry for a new bike , this look about right?

[cyberknight]

Looking at a new bike on C2W and this is my current geometry.( 2 bikes )

seat tube 51 cm/ 50
Top tube 54 cm/53
Head tube 15 cm/15
Seat post showing 11.5 cm/12.5
Cranks =17 cm/17
stem = 10 cm/10

New bike i am looking at ..
Boardman Race small

Seat tube 51.5 cm
Top Tube 54 cm
Head Tube 13.5 cm
cranks 17 cm
stem 10 cm

Specialized sport. 52 cm

Seat tube 49 cm
Top Tube 53.7 cm
Head Tube 12 cm
cranks 17 cm
stem 90cm

Specialized 54 cm

Seat tube 51 cm
Top Tube 54.8 cm
Head Tube 14.5 cm
cranks 17 cm

Trek 1.2 54 cm
Seat tube 50.6 cm
Top Tube 54.3 cm
Head Tube 15 cm
cranks 17 cm

 

[MacB]

they all seem in the ballpark, but to be fully accurate you need to compare HT/ST angles and the Reach as measured from BB centre to HT, same way as effective top tube is measured. Either of these will tell you how much of the TT is ahead of the BB, ie the amount of reach. Some of the manufacturers show this number on their frame dimensions anyway.

If you imagine a triangle of BB, ST and HT with the BB as the pivot point, if you rotate the triangle forward(steeper ST angle) then more of the top tube is in front of the BB and so more reach, the reverse is true. I think 1 degree is either 5mm or 10mm difference in reach.

This assumes that you'll be keeping your usual saddle to BB relationship.

 

[cyberknight]

they all seem in the ballpark, but to be fully accurate you need to compare HT/ST angles and the Reach as measured from BB centre to HT, same way as effective top tube is measured. Either of these will tell you how much of the TT is ahead of the BB, ie the amount of reach. Some of the manufacturers show this number on their frame dimensions anyway.

If you imagine a triangle of BB, ST and HT with the BB as the pivot point, if you rotate the triangle forward(steeper ST angle) then more of the top tube is in front of the BB and so more reach, the reverse is true. I think 1 degree is either 5mm or 10mm difference in reach.

This assumes that you'll be keeping your usual saddle to BB relationship.

Yes i will be keeping the same set up, unfortunately there are no geometry data for my current bikes so calculating angles is a bit hard , i do use saddle set back as a set up measurement .As you can see my 2 current bikes have slightly different geometry so to compensate saddle position on the rails is different to get the same saddle setback from the BB

Bike i have ..

Bike i am considering..

 

[MacB]

hmmm, I see what you mean, you can use bike cad software here:-

http://www.bikeforest.com/CAD/bcad.php

but, unfortunately the free online version doesn't have a couple of the measurements you'd need. Personally I'd do the following:-

get a bit of string and run it between HT and ST
use a spirit level and then measure for your effective top tube
run a straight edge vertically up from centre of BB and mark the string at that point
measure forward from this point to get the reach number

you can then compare your effective TT and reach with those of the new bikes, you can certainly get close enough this way that fit can be fine tuned via stem length. I also keep a note of saddle nose to horn of hoods measurement as a reference for when setting up a new bike. When I was finalising my custom frame I tried to work to a 110 stem length. I figured this would give me 20mm adjustment, both ways, without going to extremes of stem lengths. For measuring saddle setback I've found backing the bike up against a smooth wall to be easiest. Then just measure wall to saddle nose and BB centre and subtract the latter from the former.

If you exercise some google fu you can find all sorts of weird and wonderful homemade contraptions for measuring up bike positions. I still go out prepared to tweak during a ride and then take detailed measures again when I get back.

By the way I worry less about the ST lengths as long as it's not aesthetically displeasing to me. As long as I can get the reach right, and the bars at the right height I'm fine. So, for me, effective TT, reach and HT length are the 3 key factors. I prefer to get the bars where I want without a highrise stem or a huge stack of spacers.

 

[brockers]

I feel your pain cyberknight! It sounds like we have similar dimensions (170cm tall, 80cm inseam), and I always seem to fall between small and medium, or 52 and 54 cm sizes when I look at frame sizes.

 

[cyberknight]

I feel your pain cyberknight! It sounds like we have similar dimensions (170cm tall, 80cm inseam), and I always seem to fall between small and medium, or 52 and 54 cm sizes when I look at frame sizes.

Yup , sounds like your the same height , i tend to go for a 54 on a french fit.

As for saddle setback MacB i tend to use a long bit of wood up from the BB with a spirit level and measure to the saddle.

 

[cyberknight]

hmmm, I see what you mean, you can use bike cad software here:-

http://www.bikeforest.com/CAD/bcad.php

but, unfortunately the free online version doesn't have a couple of the measurements you'd need. Personally I'd do the following:-

get a bit of string and run it between HT and ST
use a spirit level and then measure for your effective top tube
run a straight edge vertically up from centre of BB and mark the string at that point
measure forward from this point to get the reach number

you can then compare your effective TT and reach with those of the new bikes, you can certainly get close enough this way that fit can be fine tuned via stem length. I also keep a note of saddle nose to horn of hoods measurement as a reference for when setting up a new bike. When I was finalising my custom frame I tried to work to a 110 stem length. I figured this would give me 20mm adjustment, both ways, without going to extremes of stem lengths. For measuring saddle setback I've found backing the bike up against a smooth wall to be easiest. Then just measure wall to saddle nose and BB centre and subtract the latter from the former.

If you exercise some google fu you can find all sorts of weird and wonderful homemade contraptions for measuring up bike positions. I still go out prepared to tweak during a ride and then take detailed measures again when I get back.

By the way I worry less about the ST lengths as long as it's not aesthetically displeasing to me. As long as I can get the reach right, and the bars at the right height I'm fine. So, for me, effective TT, reach and HT length are the 3 key factors. I prefer to get the bars where I want without a highrise stem or a huge stack of spacers.

Yes its mainly effecttive top tube length i am looking at atm , what about a cannondale synapse?

http://www.cannondale.com/bikes/road/performance-road/synapse/2011-synapse-alloy-7-sora-16639

 

[billy1561]

I know nothing about these angles you speak of but i did try a synapse and it felt really comfortable. Better than the comparable Trek 2.1, It's almost certainly going to be my next bike on the ctw scheme. Could you get to try one?

 

[cyberknight]

I live in the sticks, trying a bike is very hard to do as the ranges even the LBS tend to stock are limited , heck to get to try a small boardman the Halfords staff are pointing me towards a 20 mile + drive to hopefully find one as they do not stock them as standard.

I have rode a 56 cm bike and it was too stretched out , sat on a medium board man and it felt to large as well.

If i could try before it would be great but it is a very remote chance of doing so.

Seen a 56 cm trek at work but that looks to big as well .


Funnily enough i was looking at the synapse earlier ....

 

[billy1561]

Well I'm 6-2 but only 31" inside leg. Tried a 58 on advice of LBS as my torso is big compared to my shorter legs and it felt spot on.

 

[cyberknight]

Well I'm 6-2 but only 31" inside leg. Tried a 58 on advice of LBS as my torso is big compared to my shorter legs and it felt spot on.

5 foot 7 " ( on a good day) with a 30.5 " inside leg , so i have long legs for my torso length.