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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

Sounds like you are measuring the stem diameter, not the steerer. You would have to take the headset apart to get at the steerer.

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.

On a sorta related note.. is there any easy way to measure the fork's rake somewhat precisely?

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.

An Italian steerer will still be 1" x 24tpi, just with Whitworth thread form instead of BSC.

Then again it's a Raleigh. How great would it be if it were a 1" x 26tpi?

First define "easy" and "precisely." They kind of work against each other here.

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.

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.

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?

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.

This assumes that the fork blade has a long section that is parallel to the steerer tube. It might a little off parallel, or fully off parallel, and you won't be able to tell in the first case. But your measure won't come out right.

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.


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.

True - I must plead old-school brain - that's the way we used to do it - does not apply with straight blade forks.

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?

There would be lots of ways to do it, the primary object being to measure the distance between a line drawn down the middle of the fork column and the center of the dropout. Actually, I just thought of a method while typing this which I believe would work.

1. Place the fork on a level surface, pointing the rake down.
2. 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.
3. Lift the fork column up until it is level.
4. Measure the distance from the bottom of the column where the level is to the table/surface (at the end OK if unthreaded).
5. Add 1/2 of the fork column O.D. where the level is, subtract distance from the dropout center to the table.

I would think this method would be accurate within 1 mm.

I may be old-school, but my mechanic's brain is still working!

Clamp the steerer to something, measure to a fixed surface nearby. Rotate the fork in the clamp 180 and measure again. Difference divided by 2 is rake.

This is what I was thinking.

I would make sure the steer tube is parallel to the surface you are measuring to.

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.

...and that you do turn it 180 degrees, and measure to the same point on the wall.

Not quite following you, but with such a method any error made in the initial measurement will be multiplied by the ratio applied.

Easiest way is endorsed by Richard Sachs.

This is good too.

Keeping mind that we're measuring fork rake, not something that needs to be measured to within 0.001mm.

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.

It doesn't matter that much as long as you take the measure at that looks to be a right angle to the steerer. If you are off a few mms either way it won't change the answer by more than a fraction of a mm. The most important thing is that you are measuring from the same point both times.

Geometry says this is an unworkable idea. It relies on you getting the fork so perfectly vertical that when you measure down to the table (which must be perfectly horizontal), you will have created a near perfect right angle. This is critical because the rake is such a tiny fraction of the fork length, so if you get your vertical leg off by even 0.5mm (which is 0.6° of tilt) your rake is going to be off by 11mm.


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.

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.

Sorry, I just re-read, and I misunderstood what you were measuring at each part, even if I got the overall idea.

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.

Those are the same links, and a basic way to do my first suggestion.