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hi everyone. i am a long time lurker and first time poster. i need your guys' input on frame geometery. i know, know....chat with the frame builder....i will but i wanted to see what answers you guys have to my question.

i am getting ready to order a custom steel ss 29er. the frame will be built around a 440mm fork. yes, short for 29er use. i am looking at the pace rc31 (i am heavily leaning towards this fork) or vicious cycles forks. no need for suspension correct geometry....my arms are the all suspension i need.

my frame will be a xl frame, it will have a 24.75 to 25" effec tt. i see most 29er frames that are large or xtra large have a 72* ht angle. but, most of these frames are built with a 465mm fork.

my question is; would a 71 or 71.5* headtube angle be ok to compensate for the 440mm fork??? i definately want neutral handling....not something super quick or something what will plow thru turns. i think roughly 30mm of fork length equals about 1* of headtube angle so i think 71* will work OK since the fork is shorter.

oh....and i know some people think 1* in headtube angle doesn't make a huge difference. to me it does. i ride many switchbacks and I CAN feel a difference. this is my personal opinion.

thanks for any help/input
 

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Squalor
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1,573 Posts
As I understand it, the length of a given fork actually has very little to do with how a bike steers unless the shorter/longer fork is being retrofited to a frame that was designed for a longer'shorter fork. (eg 440mm fork being used on a bike designed for a 470mm fork like a Kelly rigid fork being used on a Monkey frame, in which case you would quicken up the steering)

If you have your frame built around a 440mm fork and a 72deg HTA with a 150mm HT then it will steer very similar to a bike with a 470mm fork and a 72deg HTA with a 120mm HT (given that the forks have the same rake).

I think the measurement you are really trying to wrap your mind around is called "trail".

Here are a few links for you regarding trail:

http://www.phred.org/~josh/bike/trail.html

http://forums.mtbr.com/showthread.php?t=54646&highlight=trail

http://forums.mtbr.com/showthread.php?t=4951&highlight=trail

This is fun stuff to learn about, so read some articles, search the archives, and ask more questions. There are plenty of people on the board that love to discuss this stuff.

Also keep in mind that the way the bike actually handles is influenced by many other things such as wheelbase, bb drop, STA, and chainstay length (can o worms on this board BTW), it is not all in the front end.

I'll step aside and let the more knowledgeable take it from here.

Good luck on your new bike, and welcome to the forum!

LP
 

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featherweight clydesdale
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1,385 Posts
1speed1der said:
hi everyone. i am a long time lurker and first time poster. i need your guys' input on frame geometery. i know, know....chat with the frame builder....i will but i wanted to see what answers you guys have to my question.

i am getting ready to order a custom steel ss 29er. the frame will be built around a 440mm fork. yes, short for 29er use. i am looking at the pace rc31 (i am heavily leaning towards this fork) or vicious cycles forks. no need for suspension correct geometry....my arms are the all suspension i need.

my frame will be a xl frame, it will have a 24.75 to 25" effec tt. i see most 29er frames that are large or xtra large have a 72* ht angle. but, most of these frames are built with a 465mm fork.

my question is; would a 71 or 71.5* headtube angle be ok to compensate for the 440mm fork??? i definately want neutral handling....not something super quick or something what will plow thru turns. i think roughly 30mm of fork length equals about 1* of headtube angle so i think 71* will work OK since the fork is shorter.

oh....and i know some people think 1* in headtube angle doesn't make a huge difference. to me it does. i ride many switchbacks and I CAN feel a difference. this is my personal opinion.

thanks for any help/input
What lanlope said. You don't want/need to adjust you HTA just because you're going with a shorter or longer fork. Have the frame built around your desired HTA and the fork(s) you intend to use. You may want to consider adjusting the HTA if your rigid fork has an offset more or less than that offered by the current crop of suspension forks.

I currently have a 71.5 on my hardtail and 71 on my fs. Both have 25" TT. Adjusted for about a 1/2" of sag probably puts both bikes at 72 and 71.5 respectively. I'm quite happy and can ride all the tight stuff on either bike, but I notice I need a little more input on the bars with the fs in slow & tight sections. Since you'll be going rigid, you should be happy around 72 degrees if that Pace fork has 38-43mm of offset. That said, I just read that Voodoo is spec'ing their L and XL frames with a 73 deg HTA, so who knows what the optimum number is....

You can always do a CAD drawing and compare what you have now to what you're proposing.

http://www.bikeforest.com/CAD/bikeCAD.html
 
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