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I'm in the process of ordering a custom frame. They are recommending a 72 degree or maybe a 72.5 degree HTA. I asked them about 73 degree and they are willing to try it but they sound a little unsure. I will be using a Pace rigid fork 468mm length and 43mm rake. I am currently riding a large Surly KM and I would like to quicken up the handling a little. I'm kinda leaning towards the 73 degree but I don't want to blow it. Any info would be appreciated.
 

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Not an O2 thief.
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Make sure the tt is right

If you run the 73, make sure they make the tt longer so you run a shorter stem. If you run the same length stem with the 73 as you would with a 72, then you can get "pitched over" sooner.

My FS race 29'r has a 71.4. It took a while to get used to, but it didn't slow me down any. I think I'll have a hardtail built with a 72....as that is what almost everyone else is using.

You also need to consider fork knob clearance. It's harder for a frame builder to get that right if you run a real steep angle like 72.5 or 73. They'll have to bend the down tube or use a gusset to give you clearance.

The distance from the front axle to the center of the bb should stay the same no matter what angle you use....therefore, the change in the tt length.
 

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Harmonius Wrench
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It seems that you are aware of how fork offset will be a factor. Try plugging in your numbers here. This will calculate your fork trail, which is the main factor in consideration when talking about handling. Too much trail and you may find that you suffer wheel flop and poor tight handling characteristics. Too little trail and you will end up with an overly twitchy, nervous front end.

You can compare to your favorite mtb and have your builder work around that trail figure and get the proper head angle using your fork of choice.

FWIW: I have ridden a 29"er with a 73 degree head angle/ 38mm fork offset and I can't imagine wanting anything quicker than that. Very little rider input needed to change your line. I would think fast, steep downhills would be a handfull on a bike like that. YMMV.
 

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And He was Not
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I've got a Soulcraft built with a 72.5 with a 38 offset. The calculater showed 35.3 trail. It rails around tight single track, but it doesnt inspire as much confidence when bombing down hills.I rarely take this bike to the Mtns. because of this. I'm amazed at how much .5-1.0 degree makes on a 29.
 

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drev-il, not Dr. Evil!
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Guitar Ted said:
FWIW: I have ridden a 29"er with a 73 degree head angle/ 38mm fork offset and I can't imagine wanting anything quicker than that. Very little rider input needed to change your line. I would think fast, steep downhills would be a handfull on a bike like that. YMMV.
I rode my Inbred with a short fork that gave the bike a 74 degree head angle while the rigid fork had 50mm rake. I enjoyed the ride very much, but the bottom bracket was too low. Even did steep fast downhills and didn't feel out of control. :) Maybe the resulting low BB (11.25") affected handling in a good way?

As Ted says, your mileage may vary.
 

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Englehardt said:
I'm in the process of ordering a custom frame. They are recommending a 72 degree or maybe a 72.5 degree HTA. I asked them about 73 degree and they are willing to try it but they sound a little unsure
I had my Black Sheep made with the same trail as your proposed 73 HTA and 43 mm offset Pace fork (~65 mm), but I got there with a 72 HTA and a 50 mm rake. That's one of the things I insisted on when I spoke with the builder (James) and he had no problem with it -- it was based on my experimentation and real world experience. I may be slow to respond to trail obstacles, but when I want the bike to move it darn well better move right then.

While I agree with your desire to have a quick handling bike, two things concern me about the set up you propose. First, there is potential for toe overlap with the front wheel -- not a concern on a road bike, but bad news on a mtb. Second, the steep front end that may increase trips over the bar. The Pace is a nice enough fork, but for near the same price you could probably get a nicer/lighter/stronger Ti fork in a custom rake from Black Sheep (or other people for that matter) that would allow you to go with a more moderate HTA (say 72) and place the wheel further out in front to eliminate toe overlap issues and prevent excessive trips over the bar. With the right rake (say 48 to 50 mm), you would still get the quick handling you desire.

Finally, I wouldn't worry about the bike being too quick and nervous with 65 mm of trail. If it becomes a concern, just slap a Hopey Steering Damper on there and you can have instant response to steering inputs and all the stability and control you could ever desire on fast downhills. Yes, you can have your cake and eat it too!
 

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Guitar Ted said:
It seems that you are aware of how fork offset will be a factor. Try plugging in your numbers here. This will calculate your fork trail, which is the main factor in consideration when talking about handling. Too much trail and you may find that you suffer wheel flop and poor tight handling characteristics. Too little trail and you will end up with an overly twitchy, nervous front end.

You can compare to your favorite mtb and have your builder work around that trail figure and get the proper head angle using your fork of choice.

FWIW: I have ridden a 29"er with a 73 degree head angle/ 38mm fork offset and I can't imagine wanting anything quicker than that. Very little rider input needed to change your line. I would think fast, steep downhills would be a handfull on a bike like that. YMMV.
what could be the best "trail" value for all mountain use?
 

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Harmonius Wrench
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Depends

Obelix2 said:
what could be the best "trail" value for all mountain use?
What is your definition of "all mountain"?

You could do the slack head angle, bigger offset thing, end up with a trail figure of 80+ ala Mike Curiaks single speed and love it, or maybe not.

That's the beauty of this geometry soup 29"er folks find themselves in. You can season it to taste! ;) (As long as we're talking rigid forks, that is. Suspension forks are still limited to a "degree". hehhehheh!)
 

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Feet back and spread 'em!
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PeT, why do you say that a steeper head angle results in more trips over the bars? if all else is the same, like wheelbase and rider CG relative to wheelbase...why would this be true?
 

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bikecop said:
if all else is the same, like wheelbase and rider CG relative to wheelbase...why would this be true?
While it may just be the fat on my head that causes me to experience more over the bar (OTB) events than others, I based my claim on experience with various "cross" setups. I've adapted several old road frames with steep HTA (as steep as 74 degrees!) and small rakes for cross and had issues with OTB. That's in comparison to my current cross and 29er bikes with 72 HTA and 50 mm of rake -- push that wheel out in front (longer front end measurement) and it makes it harder for my fat head to rotate over the top of it. So, it was an emperical discovery, but also one that would be supported by theory -- there was no significant difference in the weight distribution between front and back wheels with the converted road bikes and the current cross and 29er rigs. But in essence, my COG is further behind the contact patch of the front wheel, hence I seldom get my rear wheel off the ground when my front wheel is still contacting terra firma.

I also find no basis in the claim that increasing wheelbase slows handling. Even with an inch or two added to the front center measurement because of a slack HTA and compensating large rake, a small trail measurement (e.g. 65 mm) will assure a quick handling bike. In my opinion, there is no downside to a longer front center than is commonly found on most mtbs as long as it has you're still weighting the front end (longish top tube and stem) and have a smaller trail number. YMMV, and I know it does for lots of other people...
 

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And He was Not
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CBaron said:
Enoch- Your trail numbers looks off to me. I've been playing around with trail #'s and if I plug your data in to the trail calculator I actually get 75.6. Go here http://www.anvilbikes.com/images/1064634020.xls
I used the same one Shiggy posted. I'm not sure how the calculator thingy works so it could very well be off , :rolleyes: I do have my blue prints of my bike and it IS a 72.5 head angle with a 38 offset.

All the angles and #'s can be overwhelming. I understand incorrect trail can cause wheel flop, but you also have wheel base and rider postion thank can dramaticly affect the handling on the bike. My seat angle is 73, I wished it was more like 72 or even less. That way I could be a little further over the rear wheel,
 

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Harmonius Wrench
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Enoch said:
I used the same one Shiggy posted. I'm not sure how the calculator thingy works so it could very well be off , :rolleyes: I do have my blue prints of my bike and it IS a 72.5 head angle with a 38 offset.
The site referred to by Shiggy only takes English measurements, not metric. If you input the metric offset figure, you would get weird results. Not sure if you saw that or not.

All the angles and #'s can be overwhelming. I understand incorrect trail can cause wheel flop, but you also have wheel base and rider postion thank can dramaticly affect the handling on the bike. My seat angle is 73, I wished it was more like 72 or even less. That way I could be a little further over the rear wheel,
Hey, if a knuckle head like me can figure it out, I'm sure you can too! :p

As far as seat tube angle goes, I have always favored a bit of saddle set back, effectively giving you the same thing, but yeah, slacker seat tube angles can be cool. I had a Park Pre with parallel 72/72 degree angles in my 26"er days that I just loved.
 
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