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Fork geo and handling

Old 01-23-22, 02:36 PM
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sdimattia
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Fork geo and handling

I have a Trek Domane AL 3 with a 53mm offset and 380mm a-c. HT Angle is 70.3 and ST angle is 74.6 I'd like to switch it out for a fork with more mounting holes, namely the Ritchey WCS Carbon Adventure Gravel Fork. This fork has a 50mm offset and 393 a-c. Assuming the HT Angle will increase about .5d to 71d instead of 70.3d to account for the 13mm raise in a-c length, the trail with the Ritchey fork will decrease from 67mm to 65mm and wheel flop changes from 21mm to 20mm.

Provided my math and my understanding is correct, would this Ritchey fork be a suitable replacement for the stock Domane? I'm considering taking this bike bikepacking but I ride a 44cm frame so I'm a bit size constrained for bags. I'm quite happy with the current handling but the added mounting holes on the Ritchey are ideal and the increased tire accommodations would be a bonus. Is this going to mess up the handling by a lot?
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Old 01-23-22, 05:49 PM
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Don't forget the added bar height WRT the seat and the greater seat set back when the front end is lifted. What this all means for handling, especially when those added mounts are employed I challenge any one here to truly know. Oh, there will be opinions and some backed by some of the math... but the only one that matters and that is what is so hard to predict. Andy
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Old 01-23-22, 07:56 PM
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Originally Posted by Andrew R Stewart View Post
Don't forget the added bar height WRT the seat and the greater seat set back when the front end is lifted. What this all means for handling, especially when those added mounts are employed I challenge any one here to truly know. Oh, there will be opinions and some backed by some of the math... but the only one that matters and that is what is so hard to predict. Andy
I've got 25mm of spacers under the bar right now so I do have room to lower them and mimic their original position. I also have room to adjust the fore/aft of the saddle and I'm confident I'll be able to achieve my current fit with the new fork. I do understand what you are saying about the theoretical changes and the difficulty of knowing what will actually be affected. That being said, I wouldn't mind some opinions before I drop $500 on a new fork . . .
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Old 01-23-22, 08:09 PM
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Originally Posted by sdimattia View Post
I've got 25mm of spacers under the bar right now so I do have room to lower them and mimic their original position. I also have room to adjust the fore/aft of the saddle and I'm confident I'll be able to achieve my current fit with the new fork. I do understand what you are saying about the theoretical changes and the difficulty of knowing what will actually be affected. That being said, I wouldn't mind some opinions before I drop $500 on a new fork . . .
Unsure if you caught all my points. When you move the seat rearwards and want to maintain the torso/leg relationships you will raise the bars and bring them back towards the seat. That you mention lowering the bars to get back to some set up that we didn't know of just reinforces my wondering about your fit and your understanding of it. Again, it's your call, I can only try to open your eyes to the various relationships at play. Andy
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Old 01-23-22, 08:20 PM
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Originally Posted by Andrew R Stewart View Post
Unsure if you caught all my points. When you move the seat rearwards and want to maintain the torso/leg relationships you will raise the bars and bring them back towards the seat. That you mention lowering the bars to get back to some set up that we didn't know of just reinforces my wondering about your fit and your understanding of it. Again, it's your call, I can only try to open your eyes to the various relationships at play. Andy
I appreciate you reminding me of all the factors I need to consider. I guess I should add I'd drop the bars to keep the bar height the same AND increase the stem length to make up for the small amount the increase in bar height which also brings the bars closer as well. If anything, wouldn't I need to move the saddle forwards to account for the slight increase in ST angle? And if I said I'd move the saddle backwards, I apologize.

I had a bike fit at the shop I purchased the bike from. Future bikes and changes will be aimed at recreating something similar.
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Old 01-23-22, 08:34 PM
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By raising the front end close to 1cm the seat will shift rearward WRT the BB about 1cm too. So, of course you might want to reposition the seat. Which will likely want a longer stem with a bit more drop.

There's another thread that I got confused with yours re the stem aspect, sorry. Andy
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Old 01-23-22, 11:29 PM
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700c wheels are absurd on a 44cm frame. Hence the silly ST and HT angles. I assume there's also massive toe overlap?

I would recommend getting a bike that isn't so poorly designed. But having said that, I don't expect that fork is going to make anything worse.

I put a much taller suspension fork on a MTB and it rode pretty much the same. The only real difference I noticed was when putting a foot down. The ground is further away now...
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Old 01-23-22, 11:54 PM
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Originally Posted by FastJake View Post
700c wheels are absurd on a 44cm frame. Hence the silly ST and HT angles. I assume there's also massive toe overlap?

I would recommend getting a bike that isn't so poorly designed. But having said that, I don't expect that fork is going to make anything worse.

I put a much taller suspension fork on a MTB and it rode pretty much the same. The only real difference I noticed was when putting a foot down. The ground is further away now...
As I have sadly found, no one makes a 650c bike in that frame size anymore, getting tires is ridiculous especially with the pandemic. Sadly not many brands have looked to 650b as an alternate solution deciding its cheaper for them to stock all the same wheels. The ones that have aren't the brands you typically find on a shop showroom floor and have to be searched for, something the average buyer doesn't do. Its actually a crap process getting a decent, small frame bike.
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Old 01-24-22, 05:11 AM
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Your assumption about the head tube angle is incorrect. A longer axle to crown fork will reduce the head tube angle. reduce the seat tube angle and raise the bottom bracket. Trail will be increased, as will wheel flop. I wouldn't describe it as "messing up" the handling, but it will be different. This is very similar to how I design my gravel frames, so I know the handling won't be bad, but it will have more wheel flop at low speed.

As Andrew noted, your fit on the bike will change and you will have to make some saddle and bar height/reach adjustments to match the position you have now.
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Old 01-24-22, 06:54 PM
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Originally Posted by Andrew R Stewart View Post
By raising the front end close to 1cm the seat will shift rearward WRT the BB about 1cm too. So, of course you might want to reposition the seat. Which will likely want a longer stem with a bit more drop.

There's another thread that I got confused with yours re the stem aspect, sorry. Andy
All good, I appreciate the notes you have provided thus far.

Originally Posted by FastJake View Post
700c wheels are absurd on a 44cm frame. Hence the silly ST and HT angles. I assume there's also massive toe overlap?

I would recommend getting a bike that isn't so poorly designed. But having said that, I don't expect that fork is going to make anything worse.

I put a much taller suspension fork on a MTB and it rode pretty much the same. The only real difference I noticed was when putting a foot down. The ground is further away now...
Surprisingly, even with fenders, instances where the toe overlap is problematic are very minimal.*** Even when turning, I don't find myself (consciously at least) remembering to position my lead foot opposite the wheel's turn. Not sure what funky voodoo magic Trek managed to work up to avoid this issue but it does work . . . I wear a size 8/8.5 W's which I guess it considered average. The next bike I buy (gravel most likely) will definitely need to take 650b and/or 700c though.

*** When my feet are in the 3 o'clock position, my toes very clearly overlap the front wheel. With the crank positioned at 3 o'clock there's maybe 2 inches between the front of the Shimano SPD pedals and the front fender (with 28mm tires). See picture. And yes, I know the fenders ruin the aesthetics but it is snow/salt season for me.

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Old 01-24-22, 07:00 PM
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Originally Posted by dsaul View Post
Your assumption about the head tube angle is incorrect. A longer axle to crown fork will reduce the head tube angle. reduce the seat tube angle and raise the bottom bracket. Trail will be increased, as will wheel flop. I wouldn't describe it as "messing up" the handling, but it will be different. This is very similar to how I design my gravel frames, so I know the handling won't be bad, but it will have more wheel flop at low speed.

As Andrew noted, your fit on the bike will change and you will have to make some saddle and bar height/reach adjustments to match the position you have now.
I'm not trying to argue with you, just trying to understand your thinking: wouldn't the extra a-c raise the front end, therefore increasing HT and ST angles? Or does the slightly smaller offset of the fork counteract the increase in height and decrease the angles as you said?

I did find a cheaper alternative to how/where I can store my gear while bikepacking so I might just go with that and leave the fork swap for a future date. Thanks everybody so far for your insights!
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Old 01-24-22, 07:37 PM
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Originally Posted by sdimattia View Post
I'm not trying to argue with you, just trying to understand your thinking: wouldn't the extra a-c raise the front end, therefore increasing HT and ST angles?
The frame angles are calculated based on a line drawn between the front and rear axles, or in most cases that can be represented by the ground. A line drawn straight up from the ground would be 90 degrees. As you lean that line back toward the rear of the frame, that decreases the angle. In the case of using a longer axle to crown fork, you are raising the front of the frame with the frame rotating around the rear axle. This leans the head tube and seat tube back and decreases the angle.

Below is a crude demonstration of the effect. As you can see, the bottom angle is 90 degrees. When you lift the front of that, with the rear staying in the same location, the angle decreases.

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Old 01-24-22, 08:48 PM
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Originally Posted by dsaul View Post
The frame angles are calculated based on a line drawn between the front and rear axles, or in most cases that can be represented by the ground. A line drawn straight up from the ground would be 90 degrees. As you lean that line back toward the rear of the frame, that decreases the angle. In the case of using a longer axle to crown fork, you are raising the front of the frame with the frame rotating around the rear axle. This leans the head tube and seat tube back and decreases the angle.

Below is a crude demonstration of the effect. As you can see, the bottom angle is 90 degrees. When you lift the front of that, with the rear staying in the same location, the angle decreases.
Got it! For some reason I was thinking of the angle backwards. Thanks for clarifying. Found this nifty calculator, plugged some numbers in and it spit this out.


Given these new angles, the trail would increase from 69mm to 74mm, wheel flop from 21mm to 25mm. Since I'll be riding the bike loaded at slower speeds around <15mph, this is probably not ideal then. Now that I figured out a cheaper alternative that involves a lot less math and changing of parts, I'll keep my stock fork for now.

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Old 01-24-22, 09:47 PM
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And you're using fenders? Oof. I don't know how much that bike cost, but I'd seriously look into a custom frame for next time. It might be less expensive than you expect. As someone else mentioned, the selection of bikes for people that aren't very tall is basically non-existent. If you're like me and want to use wide tires, fenders, actual decent geometry, and not have huge toe-overlap, custom is unfortunately the only option. At 5'9" I can do all of those things with 26" wheels. At your height, you'll have to compromise on one or more areas because there aren't any decent 24" tires on the market that I'm aware of.

Having two custom-frame bikes, I could never go back. There's nothing being made today off-the-shelf that fits my needs or is even remotely interesting to me.

Here's my titanium commuter. It was something like $1200 for the frame and fork. Full custom, everything I wanted.


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Old 01-25-22, 09:51 AM
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Originally Posted by FastJake View Post
And you're using fenders? Oof. I don't know how much that bike cost, but I'd seriously look into a custom frame for next time. It might be less expensive than you expect. As someone else mentioned, the selection of bikes for people that aren't very tall is basically non-existent. If you're like me and want to use wide tires, fenders, actual decent geometry, and not have huge toe-overlap, custom is unfortunately the only option. At 5'9" I can do all of those things with 26" wheels. At your height, you'll have to compromise on one or more areas because there aren't any decent 24" tires on the market that I'm aware of.

Having two custom-frame bikes, I could never go back. There's nothing being made today off-the-shelf that fits my needs or is even remotely interesting to me.

Here's my titanium commuter. It was something like $1200 for the frame and fork. Full custom, everything I wanted.


Beautiful bike you got there. The Trek was ~$1200 but I'm very happy with it. Fits me great and I rarely notice the toe overlap. It might look problematic but it's not enough of an issue to make me change bikes. My other bikes have 26", 650c, and 20" wheels. At 5'1", the cycletruck with the 20" wheel in the front is really the only bike that doesn't have any overlap. Overlap is something I've just accepted. Unlike you taller folks who know what it's like not to have overlap while still using more common wheel sizes, I notice it less because it always exists I know there are work arounds but at the moment, I'm happy with what I've got. I'll definitely consider a custom bike in the future but for now I plan on riding that Trek until the frame gives out.
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Old 01-26-22, 04:38 PM
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The wife is 5’3”. When we got together her longtime number one bike was a custom built for her in 1975. Approximately 4cm of overlap. She had become accustomed to that. I asked the original builder why it was like that. His response was he had been 24 years old at the time and it was frame #63. He was 5’11” and what did he know about bikes for short people? Asked him to replace the 48mm rake fork with a 70mm. He did that at a very reasonable cost. Then we replaced the 170 cranks with 150mm. And had room for fenders.

After riding that bike bike for almost forty years with overlap she had completely stopped thinking about it. She likes it a whole lot better without. And loves fenders.

Since then we have acquired a 51cm 1973 Colnago Super. 71.5 head angle and 55mm rake. 52.5cm top tube. No overlap with the original 165mm cranks and 700x28 tires. 57mm of trail, correct for a road bike. This was a production bike, not a custom.

There is no reason for anyone to suffer an alleged road bike with a 70 head angle. They could be designed in 1973 and they can be designed now. The Colnago mentioned above has a level top tube and it has a head tube (head lugs just meet, there is space between top tube and down tube). The fork has enough clearance a 700x32 could be used if desired and there would still be enough air under forkcrown to not worry about random debris.

Do not spend money until you see a bike that is properly designed. You won't get one from any of the big name brands. They do not care about you.
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