Correct way to measure a fork
#1
Newbie
Thread Starter
Join Date: Aug 2017
Posts: 3
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 1 Post(s)
Likes: 0
Liked 0 Times
in
0 Posts
Correct way to measure a fork
1
down vote
favorite
I've acquired a Raleigh Suntour (circa 1990s) frame which I am re-fitting. I need to refit the headset and so I need to know the dimensions of the steerer column. The frame had an threaded stem.
I've measured the external diameter as about 22-23mm (so about 85-86% of an inch). I measured it by applying measuring calipers to the outside of the steerer column i.e not the cups for the ball bearings.
I've looked at various websites (such as Parktool) about standard threaded headset sizes and they say they range from one inch to one and a quarter. So even the smallest standard is quite a bit larger than my measurement.
I've checked my measurement several times and I'm sure it is about right. Do I have a steerer with an obsolete measurement or have I made some silly mistake.
Thanks in advance
Chris
down vote
favorite
I've acquired a Raleigh Suntour (circa 1990s) frame which I am re-fitting. I need to refit the headset and so I need to know the dimensions of the steerer column. The frame had an threaded stem.
I've measured the external diameter as about 22-23mm (so about 85-86% of an inch). I measured it by applying measuring calipers to the outside of the steerer column i.e not the cups for the ball bearings.
I've looked at various websites (such as Parktool) about standard threaded headset sizes and they say they range from one inch to one and a quarter. So even the smallest standard is quite a bit larger than my measurement.
I've checked my measurement several times and I'm sure it is about right. Do I have a steerer with an obsolete measurement or have I made some silly mistake.
Thanks in advance
Chris
#2
Senior Member
Join Date: Apr 2011
Posts: 7,082
Mentioned: 41 Post(s)
Tagged: 0 Thread(s)
Quoted: 4420 Post(s)
Liked 1,568 Times
in
1,030 Posts
Sounds like you are measuring the stem diameter, not the steerer. You would have to take the headset apart to get at the steerer.
#3
Old fart
Join Date: Nov 2004
Location: Appleton WI
Posts: 24,792
Bikes: Several, mostly not name brands.
Mentioned: 153 Post(s)
Tagged: 0 Thread(s)
Quoted: 3591 Post(s)
Liked 3,401 Times
in
1,935 Posts
If you are replacing the headset, the pertinent measurements are stack height, thread spec, and fork crown race seat diameter. A 1990s Raleigh almost certainly has an English thread (1" x 24tpi) steer tube. Depending on when and where the frame was made, it may have a JIS (27.0mm) or ISO (26.4mm) fork crown race seat. The only way to be sure is to remove the crown race and measure the race seat. Stack height is a measure of the space taken by the various headset components. Your replacement headset should either be the same or shorter than the original headset. The difference in distance between the top and bottom edges of the head tube, and distance from the fork crown race seat and very top of the threaded steer tube will tell you the maximum stack height your frame/fork will accept. Typical measurements of this distance will be 30mm to 40mm. The shorter this difference, the more limited your selection of replacement headsets.
#4
Advocatus Diaboli
Join Date: Feb 2015
Location: Wherever I am
Posts: 8,641
Bikes: Merlin Cyrene, Nashbar steel CX
Mentioned: 14 Post(s)
Tagged: 1 Thread(s)
Quoted: 4739 Post(s)
Liked 1,533 Times
in
1,004 Posts
On a sorta related note.. is there any easy way to measure the fork's rake somewhat precisely?
#5
Senior Member
Join Date: Apr 2009
Location: New Rochelle, NY
Posts: 38,729
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 140 Post(s)
Tagged: 1 Thread(s)
Quoted: 5793 Post(s)
Liked 2,594 Times
in
1,437 Posts
It's almost 100% certain that you measured the stem which was 7/8" OD. It fits a fork with the same ID, and therefore it's 99.99% certain the steerer is 1" OD, and since it's not from Italy, we can be equally sureit's a 1"x24tpi threaded steerer.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
#6
Senior Member
Join Date: Jul 2009
Posts: 786
Mentioned: 14 Post(s)
Tagged: 0 Thread(s)
Quoted: 384 Post(s)
Likes: 0
Liked 6 Times
in
6 Posts
Then again it's a Raleigh. How great would it be if it were a 1" x 26tpi?
#7
Senior Member
Join Date: Jul 2009
Posts: 786
Mentioned: 14 Post(s)
Tagged: 0 Thread(s)
Quoted: 384 Post(s)
Likes: 0
Liked 6 Times
in
6 Posts
Without removing the fork from the frame? Not really. Some sort of jig is the only way to measure precisely. But that isn't easy.
Lots of CFRP forks have the rake printed on the serial number tag, or can be looked up on mfrs. web sites. Other materials generally do not.
#8
Senior Member
Join Date: Apr 2011
Posts: 7,082
Mentioned: 41 Post(s)
Tagged: 0 Thread(s)
Quoted: 4420 Post(s)
Liked 1,568 Times
in
1,030 Posts
2. Pick a tooth on the bottom of the crank to use as a measuring point. Mark it.
3. With the fork pointed straight forward (like while riding straight), measure from that tooth to the center of the dropout.
4. Turn the fork backward in the frame. You'll have to remove the front wheel and do something about the handlebar.
5. With the fork pointed straight backward, measure from that tooth to the center of dropout.
6. Divide the difference between first and second measure by two. That's rake.
7. Put you bike back together. Don't try to go for a ride with your crank zip tied.
#9
Advocatus Diaboli
Join Date: Feb 2015
Location: Wherever I am
Posts: 8,641
Bikes: Merlin Cyrene, Nashbar steel CX
Mentioned: 14 Post(s)
Tagged: 1 Thread(s)
Quoted: 4739 Post(s)
Liked 1,533 Times
in
1,004 Posts
1. Zip tie a crank arm to the chainstay to keep the crank from moving.
2. Pick a tooth on the bottom of the crank to use as a measuring point. Mark it.
3. With the fork pointed straight forward (like while riding straight), measure from that tooth to the center of the dropout.
4. Turn the fork backward in the frame. You'll have to remove the front wheel and do something about the handlebar.
5. With the fork pointed straight backward, measure from that tooth to the center of dropout.
6. Divide the difference between first and second measure by two. That's rake.
7. Put you bike back together. Don't try to go for a ride with your crank zip tied.
2. Pick a tooth on the bottom of the crank to use as a measuring point. Mark it.
3. With the fork pointed straight forward (like while riding straight), measure from that tooth to the center of the dropout.
4. Turn the fork backward in the frame. You'll have to remove the front wheel and do something about the handlebar.
5. With the fork pointed straight backward, measure from that tooth to the center of dropout.
6. Divide the difference between first and second measure by two. That's rake.
7. Put you bike back together. Don't try to go for a ride with your crank zip tied.
#10
Mechanic/Tourist
Use a T-square or level (or really any long piece of metal, plastic, or even a piece of stiff cardboard, at least 2 inches wide with a 90 degree corner) and lay the long edge along the center of the fork blade (and head tube if long enough) so that the end intersects the dropout center. Measure from the long edge to the center of the dropout. Done.
Last edited by cny-bikeman; 02-12-18 at 08:05 AM.
#11
Senior Member
Join Date: Apr 2011
Posts: 7,082
Mentioned: 41 Post(s)
Tagged: 0 Thread(s)
Quoted: 4420 Post(s)
Liked 1,568 Times
in
1,030 Posts
As the head tube is at an angle I don't think it important to worry about the much smaller angle between chainring tooth and dropout, especially as it will change when you rotate the fork. For an easier, more accurate method without removing the fork try this:
Use a T-square or level (or really any long piece of metal, plastic, or even a piece of stiff cardboard, at least 2 inches wide with a 90 degree corner) and lay the long edge along the center of the fork blade (and head tube if long enough) so that the end intersects the dropout center. Measure from the long edge to the center of the dropout. Done.
Use a T-square or level (or really any long piece of metal, plastic, or even a piece of stiff cardboard, at least 2 inches wide with a 90 degree corner) and lay the long edge along the center of the fork blade (and head tube if long enough) so that the end intersects the dropout center. Measure from the long edge to the center of the dropout. Done.
You can also try to use the head tube, but the fork crown is going to get in the way of holding something against the side of the head tube to use its center. It jst isn't an easy thing to measure directly.
Originally Posted by Sy Reene
Thanks! Sounds good if the chainring tooth seleted is inline with the fork dropouts. Do you think precise enough to distinguish a 40 from 43 or 43 from 45mm rake?
#12
Mechanic/Tourist
#13
Advocatus Diaboli
Join Date: Feb 2015
Location: Wherever I am
Posts: 8,641
Bikes: Merlin Cyrene, Nashbar steel CX
Mentioned: 14 Post(s)
Tagged: 1 Thread(s)
Quoted: 4739 Post(s)
Liked 1,533 Times
in
1,004 Posts
Probably 40 from 43, but a 2mm difference is going to be difficult. The bottom of the chainring is a point pretty close to a right angle from the dropouts, but it isn't perfect, so that's a small error. (Something like 2mm for every 5° off of 90°.) And then you have the measuring error problem - did you measure to dead center of the dropout, was the fork perfectly centered. And you can refine the process and calculate out the geometry errors if you have a very repeatable measuring process. This method is just the best one I have heard that doesn't fall victim to all the minor curves and incompatible shapes that make more direct measuring methods more useful.
#14
Mechanic/Tourist
- Place the fork on a level surface, pointing the rake down.
- Take a short level that has a groove for placing it on a pipe (common on 9" levels) and place it on the fork column, tape it if you like. If a threaded fork make sure the level is secure against the unthreaded portion.
- Lift the fork column up until it is level.
- Measure the distance from the bottom of the column where the level is to the table/surface (at the end OK if unthreaded).
- Add 1/2 of the fork column O.D. where the level is, subtract distance from the dropout center to the table.
I may be old-school, but my mechanic's brain is still working!
Last edited by cny-bikeman; 02-12-18 at 06:17 PM.
#15
Senior Member
Join Date: Apr 2011
Posts: 7,082
Mentioned: 41 Post(s)
Tagged: 0 Thread(s)
Quoted: 4420 Post(s)
Liked 1,568 Times
in
1,030 Posts
So, if I do have the fork off of the bike, what's the method of determining the rake? Long story short, if I go into an LBS and tell them I want a new fork, but don't know the current fork's rake, what should they tell me they'll do to ensure they order the correct matching replacement?
#16
Advanced Slacker
Join Date: Feb 2017
Posts: 6,210
Bikes: Soma Fog Cutter, Surly Wednesday, Canfielld Tilt
Mentioned: 26 Post(s)
Tagged: 0 Thread(s)
Quoted: 2762 Post(s)
Liked 2,537 Times
in
1,433 Posts
#17
Senior Member
Join Date: Apr 2009
Location: New Rochelle, NY
Posts: 38,729
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 140 Post(s)
Tagged: 1 Thread(s)
Quoted: 5793 Post(s)
Liked 2,594 Times
in
1,437 Posts
One can use a bit of geometry to measure rake.
Place an axle in the fork, tie a loop in a piece of string and and run it to the top of the steerer. Measure the distance to the steerer's base, along with the steerer length. You now have a right triangle established. Now measure the entire length of the fork top to axle, and apply the same ratio of length to rise, and you have the rake.
BTW, don't forget to correct for the radius of the steerer.
Place an axle in the fork, tie a loop in a piece of string and and run it to the top of the steerer. Measure the distance to the steerer's base, along with the steerer length. You now have a right triangle established. Now measure the entire length of the fork top to axle, and apply the same ratio of length to rise, and you have the rake.
BTW, don't forget to correct for the radius of the steerer.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
#18
Mechanic/Tourist
#19
Mechanic/Tourist
One can use a bit of geometry to measure rake.
Place an axle in the fork, tie a loop in a piece of string and and run it to the top of the steerer. Measure the distance to the steerer's base, along with the steerer length. You now have a right triangle established. Now measure the entire length of the fork top to axle, and apply the same ratio of length to rise, and you have the rake.
BTW, don't forget to correct for the radius of the steerer.
Place an axle in the fork, tie a loop in a piece of string and and run it to the top of the steerer. Measure the distance to the steerer's base, along with the steerer length. You now have a right triangle established. Now measure the entire length of the fork top to axle, and apply the same ratio of length to rise, and you have the rake.
BTW, don't forget to correct for the radius of the steerer.
#20
Senior Member
Join Date: Jul 2006
Posts: 3,065
Mentioned: 63 Post(s)
Tagged: 0 Thread(s)
Quoted: 1217 Post(s)
Liked 187 Times
in
118 Posts
#21
Senior Member
Join Date: Apr 2009
Location: New Rochelle, NY
Posts: 38,729
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 140 Post(s)
Tagged: 1 Thread(s)
Quoted: 5793 Post(s)
Liked 2,594 Times
in
1,437 Posts
With a minimal amount of diligence, this method will be accurate to within 1-2mm.
So, I wish I could draw, but again you're running a string from the center of the axle to the top of the steerer. Then measuring at a right angle from the steerer to the string at specific distance along the way. That gives you the sign of the angle (opposite /hypotenuse). Now measure the total length of the string,top to axle, and apply the rule for similar triangles and add the radius of the steerer to get the rake.
You're right that whatever error you made on the first measurement will be multiplied, but other methods also have errors, including a large one if the fork isn't perfectly squared up when rotated.
In any case, I don't claim this is THE way to measure rake, or even the best way. I was just offering another method as an option for those interested.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
#22
Senior Member
Join Date: Apr 2011
Posts: 7,082
Mentioned: 41 Post(s)
Tagged: 0 Thread(s)
Quoted: 4420 Post(s)
Liked 1,568 Times
in
1,030 Posts
One can use a bit of geometry to measure rake.
Place an axle in the fork, tie a loop in a piece of string and and run it to the top of the steerer. Measure the distance to the steerer's base, along with the steerer length. You now have a right triangle established. Now measure the entire length of the fork top to axle, and apply the same ratio of length to rise, and you have the rake.
BTW, don't forget to correct for the radius of the steerer.
Place an axle in the fork, tie a loop in a piece of string and and run it to the top of the steerer. Measure the distance to the steerer's base, along with the steerer length. You now have a right triangle established. Now measure the entire length of the fork top to axle, and apply the same ratio of length to rise, and you have the rake.
BTW, don't forget to correct for the radius of the steerer.
Triangulation can work very well if the angles used are fairly broad. When you deal in tiny fractions of total leg length or angle the error on the short leg gets enormous.
#23
Senior Member
Join Date: Apr 2009
Location: New Rochelle, NY
Posts: 38,729
Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
Mentioned: 140 Post(s)
Tagged: 1 Thread(s)
Quoted: 5793 Post(s)
Liked 2,594 Times
in
1,437 Posts
It calls for a string and a ruler (for forks off the bike) and there's no triangulation. Just two easy length measurements, and a simple application of ratios, ie rake/axle to top = measured side/measured short section.
Feel free to knock it, but it might be fairer to actually understand or try it first.
BTW - as far as accuracy goes, it's an analog of the method that surveyors and others have used for well over a century to measure the heights of mtn peaks and buildings.
__________________
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
FB
Chain-L site
An ounce of diagnosis is worth a pound of cure.
Just because I'm tired of arguing, doesn't mean you're right.
“One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN
WARNING, I'm from New York. Thin skinned people should maintain safe distance.
#24
Senior Member
Join Date: Apr 2011
Posts: 7,082
Mentioned: 41 Post(s)
Tagged: 0 Thread(s)
Quoted: 4420 Post(s)
Liked 1,568 Times
in
1,030 Posts
Table? What table? I guess my shorthand explanation was too shorthand, but I just re-explained it above.
It calls for a string and a ruler (for forks off the bike) and there's no triangulation. Just two easy length measurements, and a simple application of ratios, ie rake/axle to top = measured side/measured short section.
Feel free to knock it, but it might be fairer to actually understand or try it first.
BTW - as far as accuracy goes, it's an analog of the method that surveyors and others have used for well over a century to measure the heights of mtn peaks and buildings.
It calls for a string and a ruler (for forks off the bike) and there's no triangulation. Just two easy length measurements, and a simple application of ratios, ie rake/axle to top = measured side/measured short section.
Feel free to knock it, but it might be fairer to actually understand or try it first.
BTW - as far as accuracy goes, it's an analog of the method that surveyors and others have used for well over a century to measure the heights of mtn peaks and buildings.
BUT, it still suffers what CNY and I pointed out - that you are trying to get accurate hypotenuses from very acute triangles measured with crude tools. With angles of just a few degrees, the amount of error even a slight inaccuracy in length measuring or angle measuring produces is giant. Too much to get a useful rake out of.
#25
Senior Member
Join Date: Apr 2011
Posts: 7,082
Mentioned: 41 Post(s)
Tagged: 0 Thread(s)
Quoted: 4420 Post(s)
Liked 1,568 Times
in
1,030 Posts