Andrew R Stewart 

Hi all- Sorry being late to this party but I was at NHABS with a lap top that's couldn't connect to the hotel's network.


I got Don's PM and have done an initial read through of this thread. I'll be happy to try various methods and report back. It's not like I lack for forks and measuring devices.


But I'll make a coincidental comment. While at NHABS I sat in on a few seminars, I like the frame design/handling/fit/geometry stuff so took in Damon Rinard's seminar on Saturday. (For those many who don't study this geometry stuff and don't know of him he's done a ton of this and is currently Cannondale's engineering manager of road bikes, after stints with Cervelo and Trek among others). One comment he made fits my own homework, claimed rake (and therefore trail, castor angle, flop...) will vary from intended on production bikes by up to 2mm in either direction commonly. So rake measurements are a not always what they seem and not too important if within a couple mms of the design. Andy. (who found his discussion of trail to be darn close to my beliefs)

Kontact 

I'm surprised molded composite forks could be 2mm off. Bent steel forks, sure.


Or was he just referring to the fact that a bike that should ideally get a 48mm rake fork gets a 45 because that's all that's available from the CF fork factory?

Andrew R Stewart 

It was very clear to us that he was referring to production forks' rakes varying from design. But he also suggested that a couple of MMs of trail change was pretty small a handling effect too.


Here's one of the slides he showed. Andy

Kontact 

If a molded fork's rake can be off by that much, I wonder how much a molded bikes dropout spacing or frame alignment is off by similar amounts?

And I wonder if it is a molding problem or a curing problem?

Sy Reene

Interesting (or understandable?) in that the seminar was from a Cdale guy.. I've always found their geometry chart puzzling (eg. below)
The rake isn't specced any differently across the entire size range, and given the changing HTA's, you see the Trail on a continuous decline as you go up in bike size. There's no way I could see where the handling of a large sized bike seems the same as one on the smaller end of the spectrum.

Kapusta

I am not sure I follow your point/question:

Are you wondering why they use the same fork on all of their frame sizes?

Or are you wondering why they use different head angles (thus giving different trail numbers?)

One thing to consider when comparing the geo numbers across different frame sizes is the fact that with a longer wheelbase, larger frames don't handle the same as smaller frames, anyway.

Kapusta

I am very curious to know more about this as well. It seems odd to me that any bike part manufactured with modern methods could be off by even 1mm, let alone 2mm.

I know with many multi-link full suspension mountain bike frames, pivot points located even 0.5 mm off would mean a bound suspension or a nearly bound one that would eat through bearings in no time. Yet this is very rare.

Andrew R Stewart 

Don't make the mistake of thinking what an engineer thinks and proposes, that is the best design, is actually what's produced. There are a number of reasons why production bikes are the way they turn out. That engineer isn't holding the purse strings to start with... Damon's study of bike handling goes way back and, as example, has included academic sources (Jim Papadopoulos for one) as well as a wide range of industry.


Damon specifically mentioned examples of a brand choosing to use the same fork across a range of frame sizes within a bike model. What was interesting as a hobby builder to listen to the room's reaction to the thought of a "comprised" steering geometry. I got the impression most listening thought that this was just wrong. Yet they were all excited to walk a show where the vast majority of the bikes used off the shelf forks. I never brought up the idea that those bikes they were drooling over likely also had comprised handling if they were at the ends of the bell curve of size.


He also talked about the rider's ability to make nearly all any geometry rideable. The unrideable bike is actually very hard to create. Another idea he offered is that shimmy has little to do with steering geometry. I've found this to be true. All in all I found his seminar to be well worth it. Andy

Kontact 

I'm sorry, you appear to be talking about two different things.


Are forks "off" from "design" because of production tolerances or poor execution? As in a fork that is supposed to be 45mm but just happens to come out of the mold anywhere from 43 to 47mm? Or a fork that was supposed to be 45 but always ends up at 47?

Or are you talking about the tendency to only have a one or two rakes despite 6 different HTAs? So the geometry designer wanted a 48mm rake for the 51cm size frame, but the production manager changed that to 45mm so it used the same fork as the 53cm and 55cm frame size.


The first is a production error that occurs at the factory, while the latter is not a mistake but a design choice based on money. And using only a few rakes departs ideal rake for some sizes by considerably more than just 2mm. The CAAD 12 with a 70.5 HTA and 45mm rake is arguably off by 15mm compared to the trail of a 56mm.

CAAD12 Ultegra | Cannondale Bicycles


So are you talking about production errors, or poor choices?

Andrew R Stewart 

Both, the claimed rake can drift from the published spec and that companies make production choices based on factors that have little to do with best design sometimes. But I don't call a 2mm drift from claimed rake to be a error, but a result of the process and materials used.


A fork moves back and forth by a few MMs due to various forces encountered while riding, like strong braking. But excepting shudder (which is steerer flex, not blade flex) the effect on steering isn't large enough to have riders complaining, there are far larger things going on that overwhelm any slight trail/flop drift. Andy

Kontact 

It is too bad production bikes don't either go to incremental HTAs that are matched to the number of forks they want to pay for, or that they don't go to a Seven type adjustable rake fork.

When Cannondale was using aluminum forks, they had a different rake for every HTA. I can only assume it was relatively easy to bend small lots of SR forks as needed.


But yeah, we can live with some rather less than ideal trail numbers. Long trail doesn't seem to screw people up very much.

Road Fan

I think I agree with the overall points you ascribe to Damon's data and presentation but I'm not sure I get the slide you showed. I think that the raw trail data from BQ has quite a lot of variation, though I could not say there is an error. But I wonder what the point is. He seems to be illustrating a relationship between tire width and trail, without normalizing other variations in the bikes. But width is not directly related to trail, except insofar as it affects wheel radius.

But I agree, the effect on trail should not have a large subjective effect on handling.

Andrew R Stewart 

Damon's idea is that with a wider tire the contact patch (between tire and road) rotate up the tire's side to a greater amount as the bike is leaned then with a narrower tire. This is his reasoning as to why slightly less trail is needed with wider tires (like what Jan/BQ touts).


Damon also suggested that flop (or drop as the wheel is steered off center) is not much of a factor compared to the other forces that the contact patch is seeing. This also coincides with my experience. The practical amount of drop for the steering angles we see typically see when riding is tiny. Andy

Sy Reene

As long as the process doesn't yield a fork with 2 tines that have differing rakes. 😁

Andrew R Stewart 

There's nothing alignment wise that makes a fork that's only twisted be bad. One can match the steerer's axis with the contact patch's center and have the drop outs and crown not parallel. Not talking cosmetics or preconceived expectations, just alignment issues. Andy

Kontact 

As someone who rode a bike with 70.5 HTA for many years, I can attest that wheel flop just isn't that big a deal.

Andrew R Stewart 

Ok, I took some time during today's rain to measure a fork's rake with various methods. The fork is off a "track" (actually mostly rollers ridden) bike I made 20 years ago (that I just replaced with a HOT PINK one). I have a lot of tools for this kind of stuff so I decided to do a 4 method experiment.


As the fork was still in the frame I first did a BB shell to axle with fork forward then reversed manor. I tried to level square up the fork and decided to tale a measurement at both ends of the axle to better cancel out errors. Repeated with the fork reversed. I found this method to be full of handling/rule placement and read off errors. I did this a few times to further insure some averaging of dimensions. What I also did was calculate the correct factor, as the shell to axle dimension is not at a right angle with the steerer, where as the rake is. With a vernier protractor I measured the angle difference at about 12*. The Cos. of 12* is about 0.978, at the lengths in play this resulted in about a 0.75mm correction. The rake calculated to be 33.25mm.


Next up was using Francis's method. I used a yard stick instead of a string. I steadied the yard stick with a Vee block C clamped to the axle, the rule was clamped to this. It was impossible to have the rule's top end, at the steerer's top, to overlap with the steer to intersect with the axis. As the steerer has a .5" radius I positioned the other end of the rule the same .5" above the axle center. I used the crown race bottom as the mid point as it was very close to the rule's lower edge. Measurements told me that the long triangle's length was 500mm, the short triangles respective hypotenuse length was 132mm and the small triangles short side was 9mm. math arrived at about a 34.16mm rake. While this method also has handling errors possible with securing the string/rule reduced some of these.


My late wife's birthday present was used for what I consider to be the gold standard. The fork steerer block and a height gage all on a precision surface plate. This is exactly the same set up as placing the steerer on a raised flat surface (a book/ block of wood) on your kitchen counter top. Well, not really the same as my tooling is very substantial and accurate so measuring errors are far less to happen. I measured the axle's height off the plate's surface with the fork pointing up then down. Each time I leveled the axle ends with the plate then measured at the center to reduce that possible drift. This time the rake came out as 35.0mm, on the nose.


So what do we find? Each method has it's validity and produced only a 1.75mm range of rake results. (About a 5% range). I feel this is acceptable for bike work. Interesting that this is very close to the 2mm rake drift from published spec that has been mentioned. Of course the first two methods are wrought with human error potential. But given with my not so amazing skills and my not wanting to spend too much time repeating each measurement a few dozen times... I think their results are close enough to each other to call it a wash in which is best.


Now my opinion to what is best is the surface plate block/height gage way. I readily accept that using counter tops, enough plywood raisers to clear the reversed fork and a 6" rule will bring the same human errors the first two methods showed.


The epilogue is that the "published" specs for this fork (meaning my build sheets) show a 35mm rake Andy

Ghrumpy

Major Taylor would probably agree.

FBinNY 

Yes, but the except is a big exception. We don't have an expression for radial tire section or height, so we let the context guide us. When speaking of trail, tire width is a proxy for tire height, and/or overall change in wheel radius.

BTW - tire width and height are not the same, but for most purposes, especially when dealing only with a change we can use the width as a proxy for the height. However, we need to be careful when dealing with close vertical clearance because tread thickness can make a critical difference.

Kontact 

I thought the point of looking at width is that the change in trail due to lean angle is less pronounced with wider tires. In other words, when the bike is at an angle, the actual hub to contact patch radius is more consistent with the upright radius when the tire is wider.

And I would imagine that is both a result of the fatter tire's larger cross section, but also the deflected shape of lower pressure/higher volume tires.

FBinNY 

Yes, there's some of this, also including the sideways deflection of the center of forces vs. the theoretical steering axis.

I suspect the point of the discussion isn't to list and calculate every single variable, but simply to demonstrate that there are other factors, who's significance is great enough that a small difference in rake isn't by itself significant in the scheme of things.

This relates to the concept of tolerance in design and manufacturer. These days, many people who aren't directly involved start measuring stuff and finding "problems", ie. the rake is 2mm off spec. Those closer to the process understand that tolerance translates to money, and focus on the sweet spot between cost effectiveness and functionality. For a builder to sweat over 1mm while not knowing what the end user will do that may make a bigger difference is pointless. It's not about what can be done, but what makes sense.

To go back to your early reference to rake precision vs. axle width precision, they aren't comparable. ±1mm is easy to hold on fork end width and exceeding it can make a difference to how smoothly a QR wheel slides into place. OTOH, there's no reason to sweat 1-2mm variation in rake, regardless of how easy it would be to hold a tighter tolerance because that much variation doesn't matter anyway.

As someone who had to design and manufacture parts, and also manufacture other people's designs, a big part of my job involved settling on tolerances, and looking for the sweet spot, namely the "worst" I could get away with before it made a difference. (worst is in quotes, because functionally there was no difference between the worst and better if both were within the specified tolerance). By example. Sometimes we'd set up to manufacture parts at one end of the range of the tolerance -- the worst allowable -- knowing that the process drifted, so over time the parts continued getting "better" until they started getting "worse" in the other direction, at which point we'd stop and reset.

Kontact 

It really isn't a question of which dimension is more important, the point I was making is that precision of dropout location comes from exactly the same process that would insure that rake is on spec since both happen during the same molding process. In other words, it would be difficult to design a mold and layup that is dimensionally reliable laterally but not in pitch. If the process locates the dropout in one axis with great precision, it should also have similar precision in other axes. Especially when we are talking about thermosetting resins.

Sy Reene

The original impetus for me asking about how to measure a fork's rake was because I found an old geometry chart for my model bike. However, the chart I found doesn't indicate the model year so I'm not 100% positive on it being accurate for my bike. Anyway, this geo chart indicates a 74-degree HTA, and 40mm rake. 40mm rake forks are not terribly common if I have to replace, and a 43mm fork rake (more common), seem to yield a pretty low trail figure of 53mm (40mm rake fork gives me 56mm of trail). Maybe nothing to worry about but would prefer not to change the handling if avoidable.

Kontact 

That's probably a similar trail you'd find on a Merckx. But 40s aren't that uncommon, and a steel fork can be turned into a 40.


There's a lot of things that go into fork replacement, which is why we mostly treat frames and forks as a set. Most crashes that will total a fork aren't too gentle on the frame.

FBinNY 

Yes, but OTOH you may be arguing the wrong point.

I wasn't there, and couldn't ask questions, so I can only guess at the point the speaker was making.

However, given that it was at the handbuilt show, I suspect that it was more about functional vs. manufacturing considerations. It might be that he was thinking of hand built steel forks which a builder can tweak to match the blades while still being slightly off from the original design. Or he might have been telling builders that it's OK to modify their original design so as to accept a stock, readily fork, vs. obsessing about something either unavailable, or only availability as an expensive custom version.

As I hinted at earlier, fork rake, like any design and manufacturing decision is about finding a balance between functional and manufacturing considerations.

The point I'm making, which may not be what the speaker intended, is that one has to learn not to obsess over stuff that may not matter anyway. Or, as was told to me by an old timer some 45 years ago; "never sacrifice good on the alter of perfection".