Piff 

I have a bike that's using a replacement fork. Simply by looking at old catalog pictures, and comparing against my own fork, I can see that my fork has too much rake. I can also feel it when I'm riding, the bike is very twitchy. I'd like to get someone to re-rake it, but am unsure how far to go, and how I would go about calculating the rake of the original fork.

edit: HMMMM, I feel a bit dumb. What If just measured its new wheelbase, and subtracted that from the one in the catalog? I imagine that would give the amount of extra rake. In measuring this, would I take the measurement starting from the back, middle, or front of the rear dropouts?

This is the bike in question:

unterhausen

more likely it has the wrong axle-crown distance. I'm guessing it's too long. So your effective head angle is less, and you have more trail. I think the twitchiness is actually too much flop. You probably would be happier with more rake, not less, with this particular fork.

Andrew R Stewart 

Hopefully the person who is chosen to do the work also knows about and does the investigation to determine what's the best next step.


To Eric's idea- The axle to crown length might be too long. If the front brake is the OEM one you might spot where in the pad mounting slots the pads used to be set at and where they are now. The difference suggests the A-C length difference between the OEM fork and the replacement one. If the caliper has been changed out then that's further evidence of the fork being too tall and the OEM caliper wasn't going to reach the rim any longer.


It's fairly easy to measure a fork's rake, easier and less error prone if the fork is not in the frame. I note that the spec charts don't even list the rake, so I don't see how you have a number to aim for. Even if you did reduce the rake you will then have furthered lengthened the A-C dimension.


Before I would do anything more I would revisit the frame. The reasons why someone would replace a nice OEM fork with a miss matched one can also involve frame alignment (as in a front end impact). I would look very carefully at the frame's condition. Are there any under the TT and/or DT ripples or bulges? Are the HT and the ST parallel? For that mater is the fork it's self straight? Without a full understanding of the complete picture I would not go about changing anything. What I read from the OP at this point suggests a lack of understanding or a macro view. This can change with more effort/time/education.


Last thing is about spec sheets and actual dimensions. They are not always the same. Real frame numbers shift a small bit this way and that way over the duration of a production run. The sales brochure is made at only one moment and with the info at hand at that time. (and not always even well described, depicted or complete). Some companies are pretty good at staying close to published specs (and I put Miyata in this camp) but others less so.


Do you still have the OEM fork (or ft caliper if that was also changed)? I suspect that the OEM fork had about a 365mm A-C length and 43mm rake. The trail would have likely worked out to around 60-65mm. But this is pure speculation, an educated one but... Andy

McBTC

The magic amount of trail is 57 and mechanical trail is the most meaningful so, with a head tube angle of 74° if you go with 28mm tires you're looking at a rake of 38.

Bicycle Trail Calculator | yojimg.net

Kontact 

I doubt you can "see" the rake of the fork. You're just seeing the shape of the curve rather than the offset.

You can measure rake by measuring the distance to the bb with the fork pointed forward, then with the fork pointed aft. The difference divided by 2 will be the rake.

McBTC

Ooops... looks like the head tube is 73 not 74 so... by the logic above that would be an offset of, 43mm

Piff 

You guys are totally right, the fork is longer than the original. When I purchased the bike from Craigslist I made sure to check for cracks/bumps, and when riding without touching the handlebars the bike tracks straight.

Would you recommend instead of trying to get a framebuilder to properly re-rake the to simply buy another fork? Entering the values into the trail calculator gives a trail value of 59 for this fork on ebay:

https://vi.raptor.ebaydesc.com/ws/eBa...=0&cspheader=1

Piff 

Thank you! I was having trouble thinking how I would go about doing this.

So I measured the current crown to axle length, and it's 376mm or so. Haven't measured the rake yet.

Andrew R Stewart 

Why is 57mm the magic amount? Andy

Andrew R Stewart 

Well, you'll be close to the rake with this method. Since the BB doesn't sit at a right angle to the steering axis as the front center line crosses it the offset being measured will be a tad longer then the rake. It's simple trig.


An eye that has spent decades looking at bikes and their specs can see general aspects of design with only a careful look at. So while the exact dimension might not be able to be seen a narrow range of likely size is not hard to discern. Andy

Kontact 

It will be accurate to half a millimeter. If you want more accuracy, measure from the bottom tooth of the small chainring.

57mm is "neutral trail" - which is often the goal with racing bikes.

79pmooney

Huh? I have two issues with this. The optimum trail varies (a little) by head angle. And, much bigger issue - different people have different tastes as to what "feels right". I have tracked the fork rake and head tube angles on the bikes I have owned. Yes, I am doing rake vs headset, not rake vs trail. Until quite recently I wasn't using big tires and all the bikes were ridden 700c so there was good correlation between the two and rake is easier to measure (and spec to a builder).

I have found all the bikes I really liked the handling of fell on a diagonal line on that rake/HS angle chart. Also that the corresponding trails also fell on a diagonal line when plotted trail vs HS angle. In other words, there is not one "magic" trail for me for all HS angles. Also the trail and "feel" I like is quite different from most published
"right" trails. When I order a new custom fork or whole frame, I will spec the rake. I always get questioned/corrected but I do not give the go-ahead to build until the builder sees it my way. (Second time around is much easier after the builder sees how much I like the fork he felt he built "wrong".)

Ben

Kontact 

I'm not understanding your post. Trail is the product of rake and HTA. Regardless of HTA, if you use a rake that produces a certain trail, the bike will handle similarly in terms of what trail does.

23mm tires:
72.5° HTA + 48mm rake = 56mm trail
73.5° HTA + 42mm rake = 56mm trail

Both bikes will have very similar low and high speed handling because they have the same trail. Rake is the tool used to produce a particular trail given the need to vary HTA to produce different reach and wheelbases.

Piff 

Alright, so I did some better measuring and the rake is roughly 45mm, and the crown to axle is about 380mm.

Based off those numbers, is it possible to tell how much extra rake is needed?

GrayJay

One of the best descriptions of trail behavior I have seen that is well worth reading is at;
Spectrum Cycles | Geometry

I would image you could do a complex trigonometry analysis to generate a plot showing how 57mm trail is neutral.
In practice just knowing that it empirically works out is enough.

McBTC

That's interesting... assuming the 380 is maybe ~8mm longer than OEM-- you'd reduce the head angle by ~.44° so--e.g., instead of 73° you'd use ~72.6° for the head angle. With an offset of 45 you're pretty much coming in at the same OEM spec for my rig which is 72.5° with a 45 offset.

GrayJay

Change in fork length has the effect of changing the head tube angle "HTA" (and also changes the STA and BB height). It takes a fork length change of around 20mm to produce 1 degree of change in the HTA. If you want to determine the rake change needed, then you need to know what the new effective HTA is after the longer fork is installed. Then, with the HTA and the desired trail value you can back-calculate the rake needed to get there. A change of +/- 3 mm is probably not going to be perceptible, I wouldn't bother with re-raking it you find your trail is anywhere within the range of 54-60mm.

You can accurately measure the HTA with a phone app, but you can also skip measuring the HTA and fork rake and just measure the fork trail directly using a long strait-edge, a 4' drywall square or a long carpentry level works well for this. Line it up along axis of the heat tube to mark where the head tube axis hits the ground, then measure distance back to a plumb line to ground from the axle centerline. I would definitely get a measure of your existing trail before you go bending the fork.

79pmooney

You are right about the math and probably about neutral trail bikes. I like less trail and I find that optimum trail varies with HTA. (Optimuim being the handling I like. Since I only spec forks and frames to be ridden my me, that is all that matters to me. I have ridden bikes from less than 72 to 75 HTA. The bikes with really sweet handling (in my book, I don't care about anybody else's) all fall on a diagonal line on a HTA-rake plot. The corresponding trails I believe also follow a line. I haven't plotted them; it may be a curve. It is not a constant trail.

Ben

Piff 

Alright, I measured the trail (multiple times of course) and came up with trail of about 62mm. Increase rake by about 4-5mm then?

Andrew R Stewart 

In my world 62mm of trail is about center target. Andy

Piff 

I do appreciate your help thus far, but that isn't exactly the answer to the question I was asking. Considering that the bike is handling badly with too much flop, and that the trail is about 62mm, would increasing the rake by 4-5mm be a good decision?

Kontact 

It would not. I have a bike with 65mm of trail and it does not have excessive wheel flop. 62mm trail is pretty reliable handling and not far off neutral.

Just so we're clear from your previous post, you measured 40mm of rake which translates into 62mm of trail via the 73° HTA, correct?


If you have a 40mm fork and have a problem with handling, look to the frame. That first fork was probably crashed and the frame might be wonky from the crash, too.

Piff 

45mm of rake via 72.X° HTA, for a 62mm trail. X being unknown because I dunno how much longer the replacement fork is compared to the original.

Kontact 

Where did 72 come from? You said you have a 1200 which uses a 73 HTA in most sizes according to the charts you posted.

The difference in fork length likely only accounts for less than half a degree if you went from the longest to the shortest length forks for a 700c.


You started the tread because you thought you had too much rake based on how the fork looks, now you're talking about adding rake. I am puzzled.

Piff 

Well, the 72ish degree HTA comes from a relatively long replacement fork, versus a likely short original fork as the 1200 has tight clearances. Another commenter in the thread equaled 20mm fork length equal to 1 degree for a HTA, so it seems likely that the HTA change is at least half a degree. Yes, no?

And yes, I thought there was too much rake originally by looking at the fork, but, based on others commenting on how an increased fork length leads to more trail, I'd have to agree that what I previously called "twitchy handling" should actually be called floppy. And since there's too much trail, adding rake would increase the HTA and lower trail.