lbear

I need a replacement for a carbon fork for a 2003 LeMond Alpe D'Huez. How do I find the correct rake? There is not much info left on line for Lemonds. Is a the axle to crown measurement for a short reach brake 370mm?

Al1943

It's not easy to measure fork rake offset. If you cannot determine what the offset is for your current fork it's a safe bet that a 43 mm offset fork will work well.
Do you know what the head tube angle is? Changing the fork length will effectively change the HTA. Try to avoid that.
I replaced the 43 mm fork in my Trek with a 40 mm for a bit more stability at high speed. What size is your frame?

Al

lbear

The frame is a 55, Lemonds have long tt. Fits like a 57. I would guess the head tube angle is 73-73.5

DaveSSS 

Just to be clear about fork rake, here's a picture.



With the fork off the bike, it's not extremely hard to measure it, once you know the definition. The fork length is also important, since there is not a "standard" length for all forks that use short reach brakes. The length can vary from about 365mm to 374mm and that is enough to change the HTA by about .5 degree.

I have some info from 2002 Lemond frames that is titled Reynolds steel. A 55cm had a HTA of 73.5 and a fork rake of 43mm. All of the smaller sizes used a 47mm rake.

HillRider

One way to meaure fork rake is to place the steerer tube in a couple of V-blocks (you can cut V-notches is two pieces of wood for this) mounted on a bench.

Rotate the fork around the steerer tube until the dropouts point straight down and measure the distance from the bench top to the center of the dropouts' axle slot (you can mount a hub in the dropouts and the center of the hub axle is an easier reference point) .

Then rotate the fork until the dropouts point straight up and measure from the bench top to the center of the dropouts again.

Rake is half of the difference between these two measurements.

Road Fan

That geometry info for the steel LeMond frames should be good for the carbon or carbon-forked ones, too. I recall from the Lemond sites that the geometries of the road bikes were all the same within size.

Road Fan

I measure rake on a flat level surface.

Clamp an unspoked hub in the fork and rest the fork on the surface so it rests on the hub and the end of the steer tube.

Shim up the steer tube with blocks, books, and other stuff until the steer tube is level.

Measure the distance from the bottom of the steer tube to the surface and from the top of the steer tube to teh surface. The average of these two numbers is the height of the steering axis above the surface.

Then measure the diameter of the bare hub and divide by two to get the radius. The height of the wheel axis is this radius.

Subtract the radius from the height of the steering axis to get the trail.

I think this method is very accurate and consistent, as long as your table is level, the steer tube is set level (use a good digital level or something pretty accurate like a FatMax), and the measurements are good. One benefit of this over Hillrider's method is the axle center is located very exactly by the use of the hub. I think the center location is eyeballed in his method. It CAN be done very accurately, but it is a subjective variable.

HillRider

You're right that locating the center of an empty dropout slot is pretty subjective. That's why I recommended placing a hub (I should have said "unspoked hub" as you did) in the dropouts. The center of the qr skewer where it protrudes from the nut is pretty easy to locate accurately.

I do like your method and it's the first time I've seen it described. It's very clever.

Road Fan

Hill, thanks! I've written it here in the past but never quite so well. It's not always the right thing to recommend because it needs the fork to be removed from the bike. I have a method for that, too, but it's much more subjective. It's on Dave Mann's Bike Geometry Project site. I need to rewrite that whole description of measurement, it's all way too subjective.

I must have read your approach too quickly, I blew right past that you were recommending a hub in the dropouts.

Scooper

I can't claim credit for this idea; DannoXYZ suggested it a while back and I sort of refined it. It's still a bit crude, but is extremely accurate. I used a 19mm diameter hardwood dowel (radius is 9.5mm, and that's important). I cut a slot at the bottom of the dowel for the sheet aluminum scale, which is wedged into the slot. The scale edge forms a right angle with the dowel. I use a piece of 3/8" hardwood dowel as the axle since it's a snug friction fit in the dropouts. The scale actually starts at the centerline of the dowel, but the numbered graduations on the aluminum extension start at 9.5mm, the radius of the dowel.

To ensure the dowel is centered in the steerer tube, wrap the dowel with masking tape at both the top of the steerer and down near the fork crown. Also, make sure the dowel isn't warped by twisting it in the steerer and making sure the end of the dowel at the dropouts doesn't move around. The rake (offset) of this Tange 700c chrome plated fork is 45mm.

lbear

All great ideas,thanks!
I think road fans method looks easy and precise.I will give it a try. Scooper nice idea but I will be working with a carbon fork I don't think the dowel will work up the head tube (very small hole in the crown)

Thanks guys

HillRider

Scooper's idea is theoretically quite accurate but has two requirements:

1. You need a fork with the bottom of the steerer tube open. Most carbon forks have this steerer completely closed at the bottom or, at best, have a small drainage hole.

2. The dowel must be very straight. Most of the wood dowels I've seen in hardware and home center stores are less than perfect in this reguard. For routine use, perhaps a length of steel tubing might be more reliable.

If your steerer is open at the bottom and your dowel is straight I guess you could use this to measure the fork rake with the fork still on the bike. Just remove the front wheel and the front brake and stick the dowel into the steerer from the bottom.

JohnDThompson 

There's always the VAR method:

Scooper

That's pretty slick.

HillRider

Again, a clever technique but it won't work with many modern forks with straight legs where the rake is achieved by angling the legs at their juncture with the crown.

And, how do you use that tool on Pinerello forks with all of their waves?