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-   -   What if I put on a shorter fork? (http://www.bikeforums.net/hybrid-bicycles/843253-what-if-i-put-shorter-fork.html)

PatrickGSR94 08-31-12 11:46 AM

What if I put on a shorter fork?
 
Just wondering how bad or good would it be to replace my front rigid fork that is the same length as my old suspension fork fully extended, with a shorter fork that was meant to replace a rigid fork in the first place. It would be a change from 453mm axle-to-crown length to 385mm. Obviously the front end would lower down, but my old fork did that anyway as it rode bottomed out the whole time I was riding. So basically the whole bike frame would tilt downwards from the rear axle. Steer tube would be slightly more vertical. Top tube slightly more horizontal, bottom bracket a bit lower.

I did a quick little CAD sketch using a photo of my bike, and by my estimates the frame would rotate down by about 6 to 6.5 degrees. The bottom bracket center would go from about 12" off the ground to about 10". Don't most road-oriented bikes have about 10" from ground to BB anyway? Of course I would adjust the seat tilt angle to compensate, and I have an adjustable stem.

Here's my bike:
http://img.photobucket.com/albums/v3...e/IMG_8190.jpg

Fork I was looking at: http://www.nashbar.com/bikes/Product...0#ReviewHeader

I used the pic of the bike to make a simple stick-figure of the bike, to scale. Then I copied it and rotated the frame down about the rear axle and drew in the shorter fork:
http://img.photobucket.com/albums/v3...tickFigure.jpg

Thoughts? Not saying I would actually spend $150 to do this, but would still probably look for a different bike. But if I happened to come across a fork for cheap... what would be the good/bad/ugly?

clarkbre 08-31-12 12:17 PM

From your cad drawings, the front looks too high as it isbut the other option looks too low. Ithink splitting the difference might be better to shoot for. Maybe look at your bike and take some anglemeasurements from where it stands now and see how that would change the steertube and seat tube angle. It seems thatthe average seat tube angle is around 74 degrees while the steer tube angle isabout 71 degrees. See what yours are atnow, take of the front wheel, determining how many MM the front needs to belower and buy that much less fork than you have now.

dr1445 08-31-12 12:25 PM

a shorter fork will quicken the streering, might get a bit twitchy at speed.

IthaDan 08-31-12 01:08 PM

... and you'll have to kick your bars up even higher relative to the top tube, with an even steeper stem.

I'm amazed with all the modifications you've done to this bike, but it might be time to move on, you could have a bike much more suited to your riding style for the money you've put into this one.

PatrickGSR94 08-31-12 01:48 PM

Yeah I'm just kicking it around, say if I did get another bike later but still kept this one and wanted to change it out or something.

The left drawing is traced directly off the photo, so those angles are what they are, or extremely close. I have the adjustable stem pretty flat right now, so I could raise it up some if needed. Actually a bit more saddle/bar drop might be good. I just flattened the stem and laid the bar ends a bit more flat last night to let me get a bit more aero.

It's funny, when I first started riding again back in the Spring I thought bolt-upright was where it was at in terms of comfort. But now I'm seeing the benefits of leaning forward a bit more, both in terms of pressure on my hind side and for aerodynamics.

Slaninar 10-28-13 10:49 AM

Quote:

Originally Posted by PatrickGSR94 (Post 14677403)
Yeah I'm just kicking it around, say if I did get another bike later but still kept this one and wanted to change it out or something.

The left drawing is traced directly off the photo, so those angles are what they are, or extremely close. I have the adjustable stem pretty flat right now, so I could raise it up some if needed. Actually a bit more saddle/bar drop might be good. I just flattened the stem and laid the bar ends a bit more flat last night to let me get a bit more aero.

It's funny, when I first started riding again back in the Spring I thought bolt-upright was where it was at in terms of comfort. But now I'm seeing the benefits of leaning forward a bit more, both in terms of pressure on my hind side and for aerodynamics.

I know it's old. I did this on my hybrid. An accident destroyed suspension forks. The only rigid forks I could find were some 5 cm shorter ones. It was good - more aero, quicker steering is not too bad, but my seat is too far forward (compared to cranks).

http://sheldonbrown.com/images/kops1.gif

The KOPS position at 9 is now some 2 cm more forward from the pedal axle than it used to be. Having moved the seat as far to the rear as possible. Not sure how much this affects riding, but that's what has happened.

If I could find a suspension corrected threaded fork, I'd put it, but don't want to spend too much and definitely don't want suspension forks.

bwilli88 11-07-13 06:20 AM

If you could find a fork with a bit of a forward curve it would help with the trail and reduce the twitchiness of a shorter straight fork with that head angle.

fietsbob 11-07-13 12:43 PM

It is a frame designed to use a suspension fork , so the blades are longer to keep the head and seat angles the same .

if you like the looks of a bike that was designed to use a shorter blade, non suspension fork ,
Go shopping and get one of those.

PatrickGSR94 11-07-13 02:39 PM

Yeah this thread is REALLY old now and I'm pretty certain I will not be getting any other fork for this bike.

BaseGuy 11-15-13 08:10 AM

Quote:

Originally Posted by dr1445 (Post 14677066)
a shorter fork will quicken the streering, might get a bit twitchy at speed.

+1.

You'll change the rake of the whole bike. You'll end up with a steeper steering head angle (more perpendicular to the ground), and less trail as well (the distance from the front tire's contact patch forward to the theoretical point where a line extended down the fork would penetrate the ground).

Trail creates self-centering force in the steering. Less trail = less on-center stability, more wandering feel. Rake determines steering quickness. Less rake (lower front, steeper head-tube angle) = quicker steering.

I used to roadrace motorcycles. We would sometimes lower the front about 1/4" to quicken steering on tight tracks. It was amazing how much a 1/4" drop would do to steering quickness, and that was on a 370 pound machine.

I suggest you use your CAD drawing to measure the steering-tube angle with the shorter fork, then compare that angle (measured in degrees from horizontal) to published geometry figures for some new bikes. If you're anywhere near road bike steepness, don't do it.

PatrickGSR94 11-15-13 01:05 PM

As I said, it's almost for sure never going to happen.

But my thinking was, the head and seat tube angles look to be a good bit slacker than what's on my road bike. The seat tube angle is something like 70 and I'm not totally sure what the head tube angle is, but it looks less than what my road bike is (72.5 HA and 73.5 SA). A shorter fork would increase both those angles to something closer to what my road bike is.

However I'd rather put future money towards building up dedicated commuter from scratch rather than putting more money into this bike, trying to make it into something it's not.


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