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Discussion Starter · #1 ·
I am hoping someone could help me determine the rake/offset of the fork in this drawing? I see the headtube and seattube angles, but I'm not certain how to figure the offset. I would happily calculate myself if anyone can refresh my memory of the geometry formula used..... thanks
 

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Recovering couch patato
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Frames don't have offset, at least for what you mean.

Unless all figures are presented in hundreds of mm's, it will be impossible to calculate fork offset based off.

Also, people may be less motivated to help out if you don't tell us what they're helping with.
 

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Discussion Starter · #7 ·
Damn bitmaps! Attached is a jpeg.

Cloxxi: Yes, I know frames don't have offset, just anticipating the next question to those of you with more knowledge and understanding of frame geometry than myself (ie you, Cloxxi :) )

Here's what I want to do, if possible. I do not have the time to get to the trails as much as I wish and have been riding a 1984 Trek Sport 400 which I have setup as a fixed gear running 23Cs. Very light setup, around 12 pounds all up. It is a complete blast to ride, soaks up the bumps and provides a good workout. Unfortunately, my custom Ti29er sits awaiting its turn much too often :( There is alot of money tied up in the 29er, as well as it being a sweet ride in its own right.

What I would like to do is see if it is practicable to set the Ti 29er up with a similar geometry as the Trek for road riding. The Trek is a 56cm, so its numbers are: ST angle-73.5, HT angle-73, Chainstay Length-16.73".

The 29er is equipped with Paragon sliding dropouts. Can you guys help me with what fork offset and a-c measurement I would need to get similar numbers to the Trek? One degree on the seattube angle, which will vary by a-c. I'm not sure about the offset to get the 71.5 degree HT angle even close to 73 deg. Obviously the BB drop and height will be different and can be accomodated easy enough (as long as its reasonable for 170mm cranks). I am also curious how much trail is going to change, more I would guess, but how much I wonder.
 

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Discussion Starter · #8 ·
Thanks 1strong1. I'm surprised it's 45mm.

Would this be considered suspension corrected, or non-corrected? If corrected, to what travel I wonder?

Thanks again.
 

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Discussion Starter · #10 ·
It is non-corrected

OK, thanks. For clarification, what would the HT angle of a corrected bike typically be? Yes, I realize it depends on sag and fork travel to some extent. Curious and am trying to wrap my brain around. Thanks 1strong1
 

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Helibee said:
It is non-corrected

OK, thanks. For clarification, what would the HT angle of a corrected bike typically be? Yes, I realize it depends on sag and fork travel to some extent. Curious and am trying to wrap my brain around. Thanks 1strong1
Frame are made around expected fork lengths, so their HTA's are detirmined ahead of time. Are you asking what typical HTA's are on 29ers? That is also hard to answer because it is really a wide range, just like 26ers. Anywhere from 73* (Spider) to 69* (Jones FIsher), and everything in between.

In other words, if you buy a bike non-corrected it might have the same HTA as if you bought on built for a corrected length.

If you are talking about production bikes and switching forks, then that is a whole different set of threads to read.
 
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Helibee said:
It is non-corrected

OK, thanks. For clarification, what would the HT angle of a corrected bike typically be? Yes, I realize it depends on sag and fork travel to some extent. Curious and am trying to wrap my brain around. Thanks 1strong1
Corrected and non-corrected are simply the length of the fork axle to crown. A typical rigid sus-corrected is around 470mm, while a non-corrected is 430mm range.
 

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Discussion Starter · #13 ·
Gotcha. Thanks. By putting a fork of shorter a-c I should be able to change the st and ht angles close to what I'm looking for, then use a prescribed fork offset to get the trail comparable to the road bike for handling purposes. Sound correct? Can I use Bikecalc to do this? Thanks again 1strong1.
 

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A hopped on pop.
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hey Todd, are the olympics going on this?

anyways, the rake looks to be about 48mm on the drawing, denoted by the R1.89 around the front hub. convert 1.89" to mm and you get about 48mm.

also, the fork length (crown to axle) is listed at 16.75", which calcs to 425mm (NON suspension correct).

the angles look good, nothing "out of the ordinary". the fork is not suspension correct though, slap a suspension fork on their and the frame is going to be choppered out pretty good....slow handling and a front wheel that will not stick.

i guess you want to get you ht angle steeper to quicken up the handling? if so, just get a builder to make a fork with more rake. i think 425mm is about as low as you can go in terms of fork length for a 29er, but what the heck do i know.
 

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Helibee said:
Gotcha. Thanks. By putting a fork of shorter a-c I should be able to change the st and ht angles close to what I'm looking for, then use a prescribed fork offset to get the trail comparable to the road bike for handling purposes. Sound correct? Can I use Bikecalc to do this? Thanks again 1strong1.
Yes, you could use a fork of different AC to change the head angle. Note that this will also change your seat angle and bb height. For instance, if you decrease the AC enough that the head angle on your bike changes from 71.5 to 73.0 then your already-steep seat tube angle will go from 74 degrees to 75.5 degrees. The practical effect of this is that you may not be able to get your seat back far enough. It will also increase your effective top tube length a bit.

The other way to do it would be to use the same AC, and adjust rake to get an equivalent trail figure. Your trek has 55 mm of trail; to get 55 mm of trail on your ti bike you'd need 54 mm of fork offset.

You may find this preferable because the front center will be slightly longer, and may thus match up better with the longer rear center of the ti bike. Also, this way bb height doesn't change, hence no problem if you want to try the new fork off road for some quick handling there.
John Bratton
Denver, CO
p.s. I just picked up a 1987 Trek 400 myself. Nice ride! I can't imagine what parts you put on yours to get it down to 12 pounds, though... must be some pretty light stuff. Mine is all stock, and then I added about a half pound by putting on a Brooks.
 

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Harmonius Wrench
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I'm doing a test on Twenty Nine inches that may interest some of you here. For a look at what a short, (430mm axle to crown fork with 43mm offset) does to a typically suspension corrected frame, check out this: http://twentynineinches.com/2008/08/13/an-experiment-in-front-end-geometry-fork-3/

Follow the links at the bottom of the article for more. I am doing four more forks before I am through.

Test riding a 44mm offset/465 axle to crown fork tomorrow. :D
 

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Discussion Starter · #17 ·
Thank you for all the insightful responses.

My hopes are I can use a production fork to get where I want in handling. I'm thinking a road carbon fork. What is the association between a-c length and its affect on ht angle? In other words, if I use a fork of 400mm (arbitrary number), what is the effective ht angle change? (i.e. 10mm change in a-c changes ht angel 1deg....)

Thank you John_Biker. I agree with the advantages you articulate, but finding a production road fork with 54mm of rake is going to be a challenge. While I could go the custom route, I could also justify a custom frame built to these specs too... but that is not a bad thing :) I have wiggle room with the BB height of the mtb to allow some more drop and still have plenty of crank clearance on the road.

Kam, thank you too. The Olympics may find their way to this bike, or the Trek, or the hypothetical new construct of James' ;) While the 425mm a-c fork is short, there is quite alot of space between the crown and wheel. As I'm sure you know, many road forks are sub-400mm for 700c. Getting anything other than a 23 in them is another issue though.

Ted, great read. Your site is awesome and I enjoy reading it. I envy your time to do the testing/reviewing you do. I'm considering building up a jig and buying some lugs and tubing and brazing up various ideas bouncing around in my head. Thanks for your efforts and putting the time into letting the rest of us know your results.

If anyone has a recommended road fork of the specs that can get me where I want, I would appreciate your suggestions. Thanks
 

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Harmonius Wrench
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Helibee: I'm not sure you can get a formula that will work out without adding in a wheel base figure and radius of the rear wheel. So, for every Xmm less of axle to crown you would need to figure in that wheel base/radius figure because of the effect that the distance to your pivot point (ie: rear axle) will vary from bike to bike.

In my example, using an OS Bikes Blackbuck, reducing the axle to crown measurement 30mm yeilded a 2 degree steeper head angle on the bike.

YMMV ;)
 
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