Tech Q & A: Will Changing The Fork Ruin Jo’s Ride?
Jim’s Tech Talk
By Jim Langley
An interesting question arrived in last week’s mail from a cyclist named Nick who lives and rides in the New York area. It’s about bicycle forks and bike handling. Fork replacement isn’t a frequent repair but it can be required after a crash, so I’m sharing this question and answer since it could come in handy in the future (luckily, Nick’s replacement has nothing to do with crashing and hopefully you won’t crash either!).
Nick asked,
“Good day, Jim. I hope this message finds you well! During the pandemic, I purchased a 50cm size Felt ZW carbon frameset for my wife, Jo. Unfortunately it’s a bit too big for her, so last week I purchased a 47cm Felt ZW frame only, no fork. My plan is to use the old fork on the new frame.
Now, from the geometry chart, I learned that the 50cm frame’s head tube is longer than the 47cm frame’s.
My question is, will using the fork from the 50cm adversely affect the handling of the new build? Maybe there’s a way for a home mechanic to take some rake measurements? I’ve sent this question to Felt but haven’t received a response, and Jo is eager to get back to riding her bike. Thank you very much for your help. And please let me know if you have any questions.”
My reply
Yes, Nick, you can take rake (also referred to as “offset”) measurements. The measurement is taken from an imaginary line bisecting the steering axis to another imaginary line that’s parallel to the first line and that passes through the center of the dropouts. It’s tricky to get this measurement with the fork on the bike.
But, if you were to make a rake measuring template, such as a posterboard with lines on it, you could rest it against your bicycle as a reference, align the frame’s headtube with one line, mark the dropout’s center, and then you’d be able to take an accurate measurement. You can also do it with the bike in a repair stand by using plumblines, but I find the posterboard quicker, easier and more accurate.
Since you’re swapping forks, you don’t need to measure in the frame though. You can check the fork itself and not have to deal with the frame. This makes it easier because you can hold the fork on your homemade template. Or with the fork in hand you could lay it on a table with the dropouts facing up and measure from the centerline of the steering axis to the center of the drops that way.
But, you actually don’t need to do either of these because the geometry chart you found and provided (thanks!) shows the rake right on it (I marked up the chart to point out the important specs). And what it shows is that Felt used forks with a 50mm rake on both those frame sizes.
So that means that they felt (pun intended), that it was the correct fork for both size bikes.
The difference is that the fork on the smaller frame puts the wheel slightly further forward. This happens because the 47cm frame has a 72 degree head tube angle instead of the 72.5 degree angle on the 50cm frame she had before. 72 is a half a degree shallower, which is why the wheel moves forward.
Felt probably did this to provide a bit more front wheel toe clearance, which can become tighter as frames get smaller. As you can see from the wheelbase, which is 12mm shorter on the 47 than on the 50.
Rake and trail
Fork rake alone doesn’t define the way a bike handles. Instead it’s the combination of how the fork, head tube angle, wheel size and tire size work together, which is described with another measurement called “trail.”
Trail is measured from where the tire contact point is on the ground to where the imaginary line running through the steering axis would meet the ground. You can measure this too, or calculate it using the head tube angle, fork offset plus wheel and tire size.
For this answer, I simply used AI to calculate and found that on the 50cm frame, the fork provided 53.2mm of trail (assuming Jo’s on 25mm width tires). On the 47cm frame, the trail changes to 56.6mm of trail or plus-3.4mm.
Overall, in terms of handling the frame change should make the 47cm bike more stable than the 50cm. Some riders might notice it others might not, but it should be easy to get used to either way and feel fine. Otherwise Felt would have put a fork on with a different rake.
So I’m confident that Jo will love the new frame with the old fork.
Fast forward a few days: Apparently Jo does love her new Felt because here she is on a ride in Valley Forge. Thanks for the awesome photo Nick and Jo!
More on fork rake, trail and handling changes
To learn more about fork and frame geometry and how changes affect how a bicycle handles, here’s a helpful reference from Craig Calfee at Calfee Designs (Craig was among the first to begin building custom carbon frames so he knows a thing or two about frame geometry): Geometry of Bike Handling
Your turn
Readers, it would be interesting and informative to learn of fork replacements gone right or wrong. So, if you have any experience with this please share your story in the comments. Thanks!
Have a tech question for Jim?
Jim Langley has been a professional mechanic and cycling writer for more than 40 years, and he answers reader questions every week in Tech Talk. No technical question about bikes or bike gear is too simple or too complex. We publish questions using your first name and last initial only.
Jim Langley is RBR’s Technical Editor. A pro mechanic & cycling writer for more than 40 years, he’s the author of Your Home Bicycle Workshop in the RBR eBookstore. Tune in to Jim’s popular YouTube channel for wheel building & bike repair how-to’s. Jim’s also known for his cycling streak that ended in February 2022 with a total of 10,269 consecutive daily rides (28 years, 1 month and 11 days of never missing a ride). Click to read Jim’s full bio.
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