So I’m trying to decide what my next AMHT frame will be. I currently own a standard 456 18″, and my main gripe (except a bad case of newstuffitus) is that the top tube is too long. I’m 5’10 with ~33.5″ inseam. Average arms afaik. I feel too stretched out with the 456’s long top tube even with a 50mm stem.

I was dead set on a BFe 17.5″ as a replacement, but after some measuring I realise that I could get away with a 16″ frame (it would give me about 120mm of insertion with a 400mm post), and it just so happens that the 456 EVO in 16″ looks good geometry wise. Question is, what would the effects be, as opposed to running a frame with approximitely the same ETT but with a 17″-18″ ST? More chuckable/less stable, more standover, lower bars relative to seat, those are the obvious ones I guess. Any really bad downsides? I’d guess there’s a good reason On-One only recommends 16″ for inseams up to 31-ish, and I remember seeing similar recommendations from other manufacturers.

I see lots of people doing this with AM frames though, going a size down from what they would normally run, but would it be too extreme in my case?

It’s going to be my do-it-all bike with 100-140mm forks and other specs aiming to make it climb decently and descend magnificently.

P.S. Please don’t make this into a BFe vs 456 thread. Yes, I do know there’s differences other than ST and ETT to consider. The thing is, except for the ETT and VERY general HA/SA/CS measurements I don’t really have a preference – in my book, they both fit my bill equally good (with economy in favour of 456 and a few other things in favour of BFe).

P.P.S. At the end of the day I want to build a new bike so a new frame is on the cards no matter what, so don’t go there.

not read all that ^^^^^ but

short= more chuckable/ more space to move around on bike

long= better pedaling efficiency

“feel too stretched out with the 456’s long top tube even with a 50mm stem”

How layback has the seatpost and how far back is the saddle on the rails
The adjustment here could be much more than the difference between many a frame size?

imo with that short a stem, to try to balance the cockpit a bit, for up and downhill, I wouldn’t want a layback post, else if it had one I’d be pushing the saddle quite a long way forward

“long= better pedaling efficiency”
also, generally assuming similar head angles/chainstays/forks etc, longer top tube is obviously going to mean longer wheelbase and thus better stability at speed?

“120mm of insertion with a 400mm post”
Obviously depends if the frame or seatpost have a specific minimum insert more than that or even if have some kind of ‘non-standard’ seattube-toptube bracing/flared top tube/elongated seatpost arrangement, but generally that ought to be plenty

No layback, saddle quite a long way forward. Ideally I’d like a 60mm stem but still decrease the length of the cockpit, which is why I’m looking at ~590mm ETT (which is common among AMHT’s in 17-18″ sizes except for the 456 which runs longer). I’d also prefer to be able to set the saddle position based on knee position rather than try to compensate for a top tube that is too long for me.

The length between pedals and saddle will be the same regardless of ST length, so surely there won’t be any difference in pedaling efficiency?

I’m certain that minimum insertion isn’t an issue in this case.

not read all that

You should have, then your reply might not have been utter bollox.

Size the frame by top tube, thats all there is to it really.

A long seat post isnt a problem, the only thing you have to make sure of is that there is enough inserted into the frame. I think Cotic (and probably on one) recommend at least 1cm of post below the bottom of the seattube/top tube weld.

Edit, when I say size the frame by top tube, thats not quite right. Front centre length is just as important.

Saddle position should be placed for optimum pedalling, not compensating for incorrect top tube length.

I also have a 456 (Summer Season) with 50mm stem. I’m also 5.10ish to 11
Was just thinking today how I’m tempted to try and swap my frame for a 456 in 16inch.
I lowered my forks to 100 and it felt much more comfortable on the flat and I’ve moved my saddle just about in the middle. Sitting down I have my arms very slightly bent and it feels ok.
How the bike feels too big for me is when I’m ‘chucking it around’ It’s ok going quite fast downhill but anything a bit nimble and quick or up and down single track and I feel like I’m on a tank. The other thing I don’t like is the stand over which is a bit balls close.
I was huming and haring for ages over whether to get the 16 or 18. Oh yeah, with the on one seatpost I don’t get anywhere close to maximum extension so I think that wouldn’t be an issue on a 456 16

pros: seatpost goes up or down.

cons: you have to do it manually.

I prefer a smaller frame personally but do have one bike the right size. It`s easier to adjust how you pedal than make yourself bigger/smaller.

On one missed a trick with the Evo imo. If they drop the top tubes (especially on the 20″) and added a brace (soverign/alpine style) it would be even better.

I am 6ft and have a medium BFe, its great. One of the reasons I got the medium is because I have been using it, with the forks at 100mm, to learn how to jump and I also take it to the BMX track. I have used it at the Peak with the forks at 140mm and it was fantastic.

To add balance I had a 16″ 456 with virtually the same build and really didn’t like it. To me the 456 feels more like a long travel XC bike while the BFe feels like a proper trail smasher. The BFe climbs well too, no where near as well as a lightweight XC bike but that’s to be expected.

I have the longest Hope Eternity in my 456 16″. In beyond the minimum nice and stiff and plenty of adjustment. However I am on the limit of the seatrails to make the cockpit work without making the stem longer.

Works fine but a very fine line between good and bad.

i run a Base SJ-26 jump frame…its a 14 inch frame with a 150mm fork up front. i’m 5’8″ so really i should maybe have gone for a bigger frame but having said that i’ve always loved having small frames that are more chuckable and give me an increased standover height.
i use a Blackspire Shore seatpost which at 425mm long is long enough for me to pull out of the frame for long distance rides…i’ve used a wide range of other seatposts in the past but i always found that after a period of prolonged use they would gradually start to bend backwards and would then never slide back down into the seat tube. the blackspire seatpost is now 7 years old and still perfectly straight..although it is rather bulky it is one hell of a super strong post. the top tube is a litle on the short side even for me but i’ve compensated for this by pushing the saddle along its rails as far back as possible.
the bike rides really well considering all this lardiness and it even works as a commuter bike….6 mile commute takes 30 minutes including 3 steep hill climbs!!
hope this helps

long seatposts arnt a bad thing.

too long a frame is bad.

on ones are known to be long. a smaller frame may suit your riding better.

Cheers guys, lots of inputs to go by, bottom line seems to be that I shouldn’t discard a 16″ frame. I’ll take that into consideration before I make my decision. Thanks.

Without actually looking at the numbers, my medium Soul felt quite a bit shorter than my medium 456. (it’s one of the things that made me stop obsessing about geo charts tbh, the way it feels isn’t always the way it measures, tiny wee differences add up to a lot)

That said, I’m pretty much an ideal Cotic-medium but I still got on fine with a small BFe. It didn’t set my world on fire but it never felt too small, either. Then again it didn’t really feel better than the 17.5 either.