[smoo]

Am I correct in thinking that a 1 degree increase in seat tube angle approximately equates to a 1cm increase in reach if top tube length is the same, assuming the saddle is set the same distance behind the bottom bracket?

Current bike has straight 73/73 angles, 57cm effective top tube and 100mm stem.

I'm looking at a frameset that has a 74.5 degree seatube, 72 degree headtube, and a 53.5cm effective top tube. Seems like a massive decrease in top tube length, but if the extra 1.5 degrees on the seat tube means I can think of the top tube as equivalent to 55cm, I should be able to use a 120mm stem and get about the same reach, right?

Assuming this is correct, what sort of differences could I expect in handling from a bike with such significantly different geometry? Head tube length and chain stay length are identical.

 

[Kerry Irons]

Correct

Am I correct in thinking that a 1 degree increase in seat tube angle approximately equates to a 1cm increase in reach if top tube length is the same, assuming the saddle is set the same distance behind the bottom bracket?

Current bike has straight 73/73 angles, 57cm effective top tube and 100mm stem.

I'm looking at a frameset that has a 74.5 degree seatube, 72 degree headtube, and a 53.5cm effective top tube. Seems like a massive decrease in top tube length, but if the extra 1.5 degrees on the seat tube means I can think of the top tube as equivalent to 55cm, I should be able to use a 120mm stem and get about the same reach, right?

Assuming this is correct, what sort of differences could I expect in handling from a bike with such significantly different geometry? Head tube length and chain stay length are identical.

Your assumption of 1 cm per degree is generally correct. It's strange that a frame that large would have such a steep STA. Is this a tri-bike or a TT bike? The thing I would be worrying about would not be reach, but that there were some other strange features to a frame with those angles. Examples might include super short chain stays, strange front-center dimension, etc.

 

[PJ352]

Going from a STA of 73* and ETT of 570mm's to a STA of 74.5* would mean an equivalent ETT of about 555mm's (not your posted 550mm's), but my math may be off. That aside, I agree with your assessment that a 120mm stem will get you close in reach.

Regarding any potential differences in handling, it would be helpful if you posted the complete geo of both bikes, or at least links to where the geo can be compared.

 

[smoo]

Many thanks for the replies.

The current bike is based on a custom titanium frame from the UK based Enigma. I bought this about 3 years ago when I was coming from MTB / commuting and was much less experienced. I was recommended to go for an effective top tube of 56cm, but insisted on 57cm for my own rather uninformed reasons... I wanted to point that out before posting the geometry - any peculiarities in the top tube to stem length ratio are my fault! In fact, the bike fits perfectly with a 100mm stem and the ride is sublime. Many thousands of happy (mostly fast) miles and recently upgraded to full Record. I love this bike...

Here is the geometry

Actual saddle height currently is 73.5cm and the drop to the bars is 7 - 8cm. I'm 5' 9" with good flexibility, light (63kg), good in the hills and a fairly aggressive rider with ambitions to start racing after I relocate later this year.

I want to try out a full-on, race-orientated carbon frame while I am still young enough to appreciate it (43 now). I was looking at the Dedacciai Scuro RS, which gets good reviews and can be had for a very decent price as it is often re-branded and sold with 3rd party stickers. In the UK at least it's widely regarded as a high quality, very good value frame. Here is the geometry. I was looking at the medium size. It's noticeable that the seat tube angle changes a lot between the medium and large sizes...

Assuming it would fit with a 120 or 110mm stem, my main concern is how such a different geometry would feel on the road. I guess a bit more twitchy? I like bikes that feel light and instantly responsive, although despite the relatively long TT the Enigma already feels pretty responsive to me. Would a shorter tt / longer stem help with climbing? Maybe a bit less stable on the descents though?

All advice much appreciated.

 

[Hank Stamper]

Am I correct in thinking that a 1 degree increase in seat tube angle approximately equates to a 1cm increase in reach if top tube length is the same, assuming the saddle is set the same distance behind the bottom bracket?

Current bike has straight 73/73 angles, 57cm effective top tube and 100mm stem.

I'm looking at a frameset that has a 74.5 degree seatube, 72 degree headtube, and a 53.5cm effective top tube. Seems like a massive decrease in top tube length, but if the extra 1.5 degrees on the seat tube means I can think of the top tube as equivalent to 55cm, I should be able to use a 120mm stem and get about the same reach, right?

Assuming this is correct, what sort of differences could I expect in handling from a bike with such significantly different geometry? Head tube length and chain stay length are identical.

Wouldn't you want to keep your same position relative to KOPS thus the different angle wouldn't really accomplish what you're thinking?

 

[smoo]

Wouldn't you want to keep your same position relative to KOPS thus the different angle wouldn't really accomplish what you're thinking?

Yes, I'd keep the same saddle position relative to the BB, so the saddle would have more setback to offset the steeper angle. This is what would make the 53.5 effective top tube equivalent to a 55 or 55.5 effective top tube on my current bike. So in fact, despite the big difference in actual tt length, the stem would only need to be 1 or 2cm longer, and my weight distribution over the front wheel wouldn't be so radically different. Perhaps it wouldn't make much difference after all, and the biggest difference would be the seat tube angle? But maybe that wouldn't make much difference either if my contact points were the same?

 

[PJ352]

Unless my eyes deceive, your current bike has a zero setback post and (from the pic) it looks like you've adjusted the saddle back (clamped near the front of the rails). If that's correct, be careful going with a bike with a steeper STA, because you may have trouble maintaining your KOPS (+/-) position.

If I'm wrong here, feel free to correct me.

 

[smoo]

Unless my eyes deceive, your current bike has a zero setback post and (from the pic) it looks like you've adjusted the saddle back (clamped near the front of the rails). If that's correct, be careful going with a bike with a steeper STA, because you may have trouble maintaining your KOPS (+/-) position.

Yes, that's correct, but the Scuro has an integrated seatpost with (I think) at least 2 or 3 cm setback, so it should be fine.

 

[PJ352]

Yes, that's correct, but the Scuro has an integrated seatpost with (I think) at least 2 or 3 cm setback, so it should be fine.

Ok, you'll need about 1.5cm's of that. Also, am I correct that the new size is a 51cm with a STL of 73 measured center of BB to top? You said your saddle height is now 73.5, so that seems close, but I don't know your points of measurement.

I compared the geo of the two bikes and saw that the HTL has a 1mm variance and CS lengths are the same. You lose about 1 cm at front-center, but I think you anticipated that with the change in STA/ saddle position, thus the need for the longer stem.

Trail on the 'old' bike is 58mm's, and the new one (with a recommended 45mm fork rake) will be about 62.5mm's. That will slow steering some, but you can bring trail back to the high 50's with an increase in fork rake to 49mm's, but that may be difficult to find.

 

[smoo]

Also, am I correct that the new size is a 51cm with a STL of 73 measured center of BB to top? You said your saddle height is now 73.5, so that seems close, but I don't know your points of measurement.

Yes that's right, but it's fine, my current saddle height is measured from the center of the BB to the top of the saddle, and with the saddle rails, plus the clamp / mini-seat post thing that sits on top of the seat mast, there will be plenty to spare.

Trail on the 'old' bike is 58mm's, and the new one (with a recommended 45mm fork rake) will be about 62.5mm's. That will slow steering some, but you can bring trail back to the high 50's with an increase in fork rake to 49mm's, but that may be difficult to find.

Yes, the frame is usually sold together with the matching 45mm fork (same rake for all frame sizes I'm afraid..) I will probably just have to go with that. Will that much increase in trail be really noticeable? Ironic that after all my worries about twitchyness due to the shorter top tube, it may actually end up being more sluggish due to increased trail... Also, will the longer stem contribute towards less responsive steering...?

Thanks for the advice.

 

[PJ352]

Yes that's right, but it's fine, my current saddle height is measured from the center of the BB to the top of the saddle, and with the saddle rails, plus the clamp / mini-seat post thing that sits on top of the seat mast, there will be plenty to spare.

Yes, the frame is usually sold together with the matching 45mm fork (same rake for all frame sizes I'm afraid..) I will probably just have to go with that. Will that much increase in trail be really noticeable? Ironic that after all my worries about twitchyness due to the shorter top tube, it may actually end up being more sluggish due to increased trail... Also, will the longer stem contribute towards less responsive steering...?

Thanks for the advice.

If you're confident that saddle setback and height requirements can be met, IMO the geo/ fit issues are settled.

I can understand your concerns re: twitchy handling, but I don't think going from a trail of 58 to 62 is going to result in sluggish steering. Maybe slightly slower/ more predictable. Whether you'll notice or not is IMO an unknown until you ride the bike, because handling is the sum total of geo, not fragmented parts of it. And your preferences enter into it as well.

The longer stem will do nothing but compensate for the change in geo and reestablish your reach requirements. Even if you ended up with a 120mm stem (and I'm not sure you will), you'd still be in a more than acceptable range.

 

[smoo]

Thanks again for the comments. I suspect I'll end up with a 110mm stem if the equivalent effective tt length is nearer 555 rather than 550, as on my current setup I've got more room on the bars to extend the reach by moving the levers forward than I do to shorten it. I wouldn't buy the new stem until I've got everything else set up and measured the reach however...

So now it's just a question of whether the extra trail is less than ideal, and if so if I would be better off with a different frame.