Framebuilders - odd question #283

How much and how badly woud the handling change on a Niner One9 medium if I changed the stock for for a full carbon one (20 - 25mm shorter crown to axle)?

http://www.ninerbikes.com/images/geometerypageONE9.jpg


here's the original steel fork - future rust problem?
http://www.ninerbikes.com/ninerparts2.html (Yes, I watched the video)


Carbon forks - no rust

on-one 470mm
http://www.on-one.co.uk/index.php?module=pagemaster&PAGE_user_op=view_page&PAGE_id=162

white bros 465mm
http://www.whitebrotherscycling.com/product-rigid.php?specs=rocksolid29

pace 465mm
https://www.pacecycles.com/product.asp?catID=2&subcat=460


Anyway, the $64 nub question (please bear with the nub) is would I compromise the bike's handling so much that it would become a twitchy, annoying ride or would I barely notice it?

29 views and no replies - no frame building, bike handling or bike geometry gurus here?

OK - - if you go down 25mm from a 490mm a-to-c, you will reduce your a-to-c to 465mm, or by a bit over 5%. According to my quick-n-dirty overlay, this should steepen your head angle by 1 degree, from 71 to 72. It's up to you if you can live with that to avoid a 'possibility of rust.'

http://i160.photobucket.com/albums/t162/dminorwa/29er.jpg

I would personally look at going this route (with a correct 490mm a-to-c):

http://www.rstmtb.com/m29.html

OK - - if you go down 25mm from a 490mm a-to-c, you will reduce your a-to-c to 465mm, or by a bit over 5%. According to my quick-n-dirty overlay, this should steepen your head angle by 1 degree, from 71 to 72. It's up to you if you can live with that to avoid a 'possibility of rust.'

[.IMG]http://i160.photobucket.com/albums/t162/dminorwa/29er.jpg[/IMG]

I would personally look at going this route (with a correct 490mm a-to-c):

http://www.rstmtb.com/m29.html

Thanks vey much for that - I'll be sticking with rigid though (the One9 comes with a Rockshox Reba Race 80/100mm option).

How is the handling going to be affected? I can tell the difference between ride feel of a dutch bike, a touring bike and cyclocross bike. However, I'm not really sure what, if any, difference the ~1degree change would make.

I'm not the best person to answer that one, as I'm not sure what a 29er does to other geometry factors like trail and whether or not there are offset differences between the forks. I know that I myself would not want to go steeper by a degree on something that steep already but that's just me. Anyone?

We did the math on this about 3 weeks ago. Every 11mm = .5 degree change. You would actually go from a 72 degree to over 73 degrees of head tube angle. That seems way steep. I shortened the forks on my stumpjumper 10mm ( 1/2 a degree ) to sharpen up the steering and felt the difference.

Good luck let us know how it works if you try it.

Brian

...You would actually go from a 72 degree to over 73 degrees of head tube angle. That seems way steep.I agree. I got the same results and then typed it wrong (now corrected).

thanks for the answers folks - any uber framebuilder dudes care to comment on how ~1 degree steepening affects handling and stability please?

I ran the numbers on one of my touring designs, this one has a 70 degree head angle (sorry), and it has 62mm of fork offset, with one degree added to the stearing angle, the trail shrank about 5 mm. That would be an increase in the twitchier more responsive scale. It should only be really bad if your current trail is inadequate. 5 mm represents a fair proportion of total trail range, which is oging to be in the 10mm range (not total trail, but the range in a given design likes say 60-50).

If you wanted me to actually model your situation, I need the real wheel size of your design, which is the axle height when loaded, x2. I also need your fork offset number, is that above?

I ran the numbers on one of my touring designs, this one has a 70 degree head angle (sorry), and it has 62mm of fork offset, with one degree added to the stearing angle, the trail shrank about 5 mm. That would be an increase in the twitchier more responsive scale. It should only be really bad if your current trail is inadequate. 5 mm represents a fair proportion of total trail range, which is oging to be in the 10mm range (not total trail, but the range in a given design likes say 60-50).

If you wanted me to actually model your situation, I need the real wheel size of your design, which is the axle height when loaded, x2. I also need your fork offset number, is that above?

thanks for the reply

wheel diameter is 622ETRTO(700c/29") with a 700x28c clincher - not sure how to work out axle height (unless it's divide by 2 and add tyre depth?)
? axle height 311 + ~25 = ~336mm ?

The pace and white bros. forks are out beacause I'm over the weight limit nekkid.

That leaves the on-one fork which is 470mm with 47mm rake
"29in forks have 470mm axle-crown dimension, with 47mm rake"

the original fork is 490mm with a 40mm rake

angles and dimensions are as per the medium niner one9 frame with the ebb at 6 o'clock

I just get it by measuring the wheel, I guess maybe you don't have it yet? There is a dbase of real wheel and tire sizes somewhere.

I just get it by measuring the wheel, I guess maybe you don't have it yet? There is a dbase of real wheel and tire sizes somewhere.

Thanks for the reply, ok and no, not yet - the old bike got stolen (check my sig) and I'm currently accumulating parts for the new bike

I'll see if there's anything on Sheldon Brown's website for calculating wheel radius.

http://www.sheldonbrown.com/cyclecomputer-calibration.html#iso

ETRTO number + 2x tyre width = wheel diameter

so axle height should be ~339mm

That is giving me a number that is 5/8" smaller than a 700c with 38 mm touring tire...

Ok, so I ran the number on the basis of a wheel .4" greater in radius than the 700C 38, I don't think there is that much difference in the change given the steep stearing set up, even if we are a little off on wheel diameter. What we are looking at here is the difference, not absolute numbers, though I think we are pretty close on either.

So with the 72 degree angle I get a trail of 2.93" with the 40 mm offset, and 2.64" with the 47mm offset. Shift that to 73 degrees, and trail shrinks to , 2.67", and 2.38.

So a fork change from a fork setting 72 degrees and with 40mm offset, to one setting 73 degrees with a 47mm offset, would shrink trail 2.93-2.38, or 14 mm. However your trail is still 60 mm, which doesn't sound too bad.

Here is the 72 degree trail diagram, where the two tightly spaced diagonals are the 40 and 47 mm offsets. Trail is measured at the baseline between either of these offset lines and the perpendicular line dropped from the center of the wheel. You can see that rotating the headtube angle to nearly 90 degrees would eventually get you a trail of zero, and you can see the effect on trail of changing the offset.

thanks

So, in summary, even though the crown drops 20mm with 7mm greater rake the net effect is virtually nothing to the bike's handling?

14 mm is a lot, and while I don't know what the normal trail is on a 29er, it does not sound like too little to end up with a remaining 60 mm, so it may be a handling improvement as far as the trail goes.