29er geometry geeks... « Singletrack Forum

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29er geometry geeks...

56 posts & 18 voices | Started 11 months ago by pedalhead | Latest reply from pedalhead

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pedalhead - Member

I'm designing a custom 29er frame primarily for endurance racing & multi-day stuff (so, all-day comfort is important), but also something that's still going to be fun for short singletrack blasts and useful for XC racing. I'm pretty sure about most of the dimensions, but thought I'd sanity check (is there any sanity on STW?!) my idea on head tube angle. I don't like a really twitchy ride that I think some 29ers can suffer from. In particular I don't want to feel like I'm going to be pitched over the bars on steep technical descents & small drops, but of course I still want it to go round corners. With that in mind, I'm looking at a 70.5 degree HA with 470mm axle-crown rigid fork (45mm offset) or sagged 80mm Fox F29 suspension fork. Also thinking something like a 105mm head tube (this is on a 17.5" / medium type frame size).

70.5 degrees seems pretty slack by 29er standards (or perhaps it isn't?), but from what I can tell I *think* it's what I want. Any thoughts before I make an expensive mistake? Cheers!

Posted 11 months ago #
clubber - Member

Head tube angle alone won't determine the handling. That's my thought...

Seat tube angle, top tube length, chainstay length - all these will affect it too by changing weight distribution, wheelbase, etc.

Posted 11 months ago #
pedalhead - Member

fair enough, though everything else is pretty unremarkable...

-- Seat tube angle = 73 degrees
-- Effective top tube length = approx 600mm
-- Seat tube = approx 450mm
-- Chainstays = approx 450mm
-- Head tube = approx 105mm
-- BB drop = 50 to 60mm

Posted 11 months ago #
thomthumb - Member

i don't know a lot about geometery but being STW i thought i'd get my tuppence out

70.5 sounds slack. It's closer to a lenz behemoth (69.5) than a swift or on one (72)
however the singular gryphon (rigid only) has a 70.5 HA and this has been reveiwed as making it more capable than one would expect.
also although a niner air (small) with 100 mm fork is listed at 70.5. (there does seem to be a trend to have the smallest size 29ers with slacker HAs - which maybe to reduce toe overlap)

HA, fork offset and trail are all closely related and shouldn't really be chosen independently of one another.

add in to that the the top tube length will to some extent determine stem length which will also effect steering geo.

sorry for the draindump and lesson in geometery!!

Posted 11 months ago #
rootes1 - Member

Pedalhead:

what trail measurement does you headset, fork offset and length give you?

Clubber

changing weight distribution

Weight distribution is something that no one ever seems to talk about, despite slack angles, short stems etc etc dramatically affect f-r distribution

Posted 11 months ago #
Ben_Haworth - Moderator

Seeing as your head tube's nice and short and BB drop is healthy, I'd steepen it a tad to 71° and go for the low-slung approach.

Posted 11 months ago #
thomthumb - Member

50mm bb drop would feel high would it not?

is that a personal preference?

EDIT:

In particular I don't want to feel like I'm going to be pitched over the bars on steep technical descents & small drops, but of course I still want it to go round corners.

i would have thought a little more BB drop 60-70mm would help achieve that.

please don't ruin your bike on what i say though

Posted 11 months ago #
Mostly Balanced - Member

Work out what wheelbase your measurements give you. 1075mm or less should give good manoeverability. for the record an 18 inch Inbred 26er is 1100mm, my Dean Ti 26er is 1040mm and it flick-flacks in and out of the trees beautifully. If you want to keep your weight back on the bike you might want to consider a curved seat tube at a slacker angle to allow the bottom bracket to be as close as possible to the rear wheel to avoid toe overlap. Also see the Canfield Nimble 9 for another way of doing it with straight tubes.

Posted 11 months ago #
hugor - Member

Rather than just punch out a whole lot of numbers why don't you ride an existing frame, decide how you'd like to change the handling, and make the necessary modifications onto your custom frame.
I ride a 2010 Niner RIP9. In 2011 they have slackened most of the angles by 1 degree which is massive. Personally I don't think the 2010 geometry could be more dialled but clearly they think it needed changing.
I don't think you'll get a good answers to your question here or anywhere as its a matter of personal preference.
I would ride an On-one or something, decide what you don't like about it and adjust the geo to achieve your preference.

Posted 11 months ago #
clubber - Member

1075mm or less should give good manoeverability. for the record an 18 inch Inbred 26er is 1100mm, my Dean Ti 26er is 1040mm and it flick-flacks in and out of the trees beautifully

But this is the whole problem. Single measurements don't tell you anything - yes, check the wheelbase isn't silly long/short but don't base the whole thing on it. You could have a bike with short wheelbase that still handles like crap.

Posted 11 months ago #
pedalhead - Member

Thanks all, useful stuff. I currently get toe overlap on my 29er (I'm 5' 9" with mahoosive size 11 feet), but this design increases the top tube by approx 1cm and slackens the HA which may alleviate that.

I wish I knew how to work out the trail figure...I'll google it

Ok, I googled it, and according to this site, with a 2.2 inch tyre, I'm looking at a trail of 83mm. I have no idea how to interpret that though!

The BB drop is something I'm still working out so I've estimated for now.

I've been riding a 29er exclusively for nearly a year now, and am currently on a 2nd (borrowed) 29er. It's the difference in the ride between the two that's got me thinking of designing my own ideal solution. I wish there were more opportunities to test 29ers...and sadly I can't make Charlie's beach party on the 25th!

Posted 11 months ago #
Ben_Haworth - Moderator

Ignore wheelbase. It really doesn't tell you much. Knowing the front centre is useful. And knowing the chainstay length is useful.

Posted 11 months ago #
pedalhead - Member

cheers Ben. Front centre will indeed be useful from a toe overlap point of view as well.

Posted 11 months ago #
rootes1 - Member

just for comparison these are the swift dimensions:

http://www.singularcycles.com/swift.html

Posted 11 months ago #
clubber - Member

That's what I'd be basing it on but then I love my Swift. I'd probably keep it as is except shorten the head tube.

Posted 11 months ago #
pedalhead - Member

cheers guys, been riding a Pegasus for the past year or so

Posted 11 months ago #
clubber - Member

So what are the characteristics of that that you'd like to tweak?

Posted 11 months ago #
pedalhead - Member

as above really, slightly longer top tube, less twitchy steering...yes I know everyone else seems to love the Swift/Pegasus handling, but I don't find it very confidence-inspiring on steep techy downs in particular. It is possible that part of the issue is the Lefty I'm using...gonna look closely at that before making any final decision on figures.

Posted 11 months ago #
IanB - Member

It is possible that part of the issue is the Lefty I'm using...gonna look closely at that before making any final decision on figures

Interesting that. I like the handling with the On-One carbon rigid on the front. All very subjective, but I've been OK with it on some steep/ techy stuff in the Beacons. If you wanna pop over my way for ride, you're welcome to try my set up to save splashing out on kit "just to see".

Slightly off topic, but on the lefty front the LBS here has started stocking Cannondales, and I'm seeing more lefty's about now. Saw a guy lent hard into a tight left hand turn in town - saw it front on and was really surprised to see how much his front wheel was out of line with the back one. Didn't look good.

Posted 11 months ago #
clubber - Member

Fork dive with the lefty when braking on steep stuff? As you say, I'd check that it isn't the fork. From my memory, when I had a headshok, it was quite noticeable that because it didn't bind when cornering in the way that a telescopic fork does, it actually dived more and made the bike feel quite squirrely (technical term!) in fast corners, particularly if bermed.

Posted 11 months ago #
triple_s - Member

I don't think 70.5 sounds too slack, I had demo on a Scott Scale recently and that claims to have a 69.5 degree head angle and I thought it was fantastic, I found it very confidence inspiring!(not that I rode any steep techy downs) Might be worth seeing if you can find a demo one to have a play on...

Posted 11 months ago #
pedalhead - Member

Hi Ian, kind offer thanks, and one that I may well take you up on! Good excuse for a ride in some proper hills again too :-). Interesting on the Lefty thing...they're allegedly super stiff...I'm pretty sure mine was, though I haven't been riding with the best wheel builds and I know I've been getting some flex there so it's difficult to say for sure. Given the hassle & expense of the Lefty, I don't think it's worth it overall. The low weight is nice though of course. What I do know though is that I don't like the feeling I get where trail obstacles feel like they want to push the front wheel in & under the bike. Just doesn't feel right. I have a suspicion that bearing migration in my Lefty may have been causing an overly steep head angle...though I'm yet to confirm that.

Posted 11 months ago #
pedalhead - Member

clubber, yeah the Lefty is definitely prone to fork dive. Putting a spacer in the air chamber to correct for the reduced 29er travel helped enormously with that, as did a bit more pressure in the chamber. It's still more prevalent than on something like a F29 though.

Posted 11 months ago #
jameso - Member

sound like sensible numbers to me.. the front-centre and CS length should be nicely balanced and the tt / SA seem right with that, middling-ok.

that HA will give a relatively long trail but if you account for an 80mm stem and a less-than-xc narrow bar i expect it'd feel good. it won't be twitchy, but it shouldn't be floppy - i get 83mm trail there - a 50mm rake fork would give you 78mm.

a sagged 80mm fox will be less than 470mm though, so there may be a difference handling between the 2 forks. depends how you set them up. an 80mm 29er sus fork has an a-c of about 480-485mm so you're accounting for not-alot-of-sag.

i'd be tempted to go for a 460mm fork if you were getting the fork made and will only use 80mm sus forks. shorter rigid forks are better rigid forks.

Posted 11 months ago #
pedalhead - Member

thanks James, useful info, good point on the sag

Posted 11 months ago #
thomthumb - Member

shorter rigid forks are better rigid forks.

in what ways better?

Posted 11 months ago #
rootes1 - Member

Slightly off topic, but on the lefty front the LBS here has started stocking Cannondales, and I'm seeing more lefty's about now. Saw a guy lent hard into a tight left hand turn in town - saw it front on and was really surprised to see how much his front wheel was out of line with the back one. Didn't look good.

front wheel will always track different to the back when cornering - might just 'look' worse on a lefty as it looks odd to start with visually..

I rode one and thought it was very good.

Posted 11 months ago #
jameso - Member

almost every way, if designed well ) long rigid forks are not a mechanically sound idea.

Posted 11 months ago #
jameso - Member

leftys - one large tube will be stiffer than 2 slimmer ones for a fixed amount of material..

i've ridden a lefty and as odd as it looks, it's a pretty stiff fork. way beyond the other forks at the time (ie pre 20mm axles and taper steerers)

Posted 11 months ago #
Ben_Haworth - Moderator

Singular Pegasus is a great bike. That is all.

Posted 11 months ago #
thomthumb - Member

a long rigid fork will give more flex though and take out vibrations better for a given tube. (maybe?)

i guess if the tube is as thin as possible it needs to be thicker when longer and my point is void.

Posted 11 months ago #
IanB - Member

I rode one and thought it was very good.

Yeah, me too a looong time ago on a Scalpel. My recent sighting did look very much like the whole wheel was flexing under the weight of the bike/ rider though, even taking into account the difference between front and rear tracking. Didn't clock just how big the bloke on it was though, or where he had his weight.

Posted 11 months ago #
IanB - Member

Good excuse for a ride in some proper hills again too

I can certainly take you up a few around here

Posted 11 months ago #
pedalhead - Member

I can certainly take you up a few around here

have some pity for the chap from the flatlands though, ok?

Posted 11 months ago #
Clink - Member

So what was the outcome of this?? what geo did you settle on pedalhead?

Posted 5 months ago #