Menu

Technical Q&A with Lennard Zinn – Funky Disco Drop-out

  • By VeloNews.com
  • Published Jan. 13, 2004
  • Updated Sep. 28, 2010 at 4:48 PM EST

By Lennard Zinn

Technical Q&A with Lennard Zinn – Funky Disco Drop-out

Photo:

Funky disco drop-out
Dear Lennard;
Last Saturday I installed a new Kelly rigid mountain bike fork on aGunnar Ruffian. Friends at the shop were a little miffed by the drop-outson the fork. Seems that they are, in a sense, forward/upward facing. Somephone calls yielded some vague information about problems with wheels runningdisc brakes coming loose from ‘standard’ forks. Can you comment on thisproblem?

Can you tell me if you’ve seen Kelly’s solution to the problem and,if so, what you think of the design?
Judd

Dear Judd;
Well, you can just barely see that the dropouts face forward on thatKelly fork in the tiny photo at www.kellybike.co(It’sthe black one in the photo.) The problem “with wheels running disc brakescoming loose from ‘standard’ forks” is a real one, albeit perhaps overstatedin bicycle chat rooms. Here is the issue:

If you think of a disc brake setup, you know that the front brake caliperis above and behind the front dropout. Now think of the wheel turning,with a lot of mass and momentum behind, namely a heavy rider rolling downa trail at high speed. When the rider applies the front brake, it grabsthe caliper, and that point above and behind the dropout where it is grabbingthe rotor now becomes the fulcrum, or center of rotation, if you will,for a force about that point.

Since the wheel is spinning forward, the force created on the wheelto rotate it about the point at which the brake is grabbing makes the directionof the force at the hub axle close to straight down. (Actually, the hubaxle’s direction relative to the bike will only be straight down at thebeginning of its travel, in the unfortunate circumstance where it is allowedto continue rotating about the caliper!) Clearly, if you have a dropoutthat opens straight down, as most forks have, you had better have thathub clamped tightly in the dropout or it will come straight down out ofthe dropout slot! Now, not all QR skewers are created equal. I have noticedon my own mountain bikes, all of which have disc brakes, that if I usea skewer where the lever is aluminum with an off-center hole at its roundedend to create the cam, I get downward movement of the axle in the dropout.After riding, I notice this by flipping the front skewer open when thebike is standing on its wheels. If the fork
drops down a bit to clunk back down onto the axle, I know that thathub moved down in the dropout while riding.

Now the Nader hooks (wheel retention devices) built into suspensionfork dropouts are designed to stop the head of the skewer from coming outwithout being unscrewed a number of turns. However, if you get this typedof downward axle motion happening on a regular basis, over time the Naderhook (made of soft magnesium, like the rest of the fork lower leg) willwear out around the edges of the dropout slot. Furthermore, the metal atthe end tips of the dropout will be pushed and worn at the edge, makingthe dropout thickness decrease closer to the end of the slot. This meansthat, as the axle moves down, the skewer’s clamping force further decreases,because the dropout thickness is less. Obviously, enough of this type ofwear, and you can imagine a situation where you slam on your front brake,and the wheel pops straight down and out!

The solution I have adopted for my own bikes is to only use front skewersthat have a high clamping force. An example would be Shimano and Campagnoloskewers where the end of the lever is formed into a cam bent at 90 degreesfrom the lever arm and inserted into a round hole in the end of the skewerrod. These and the type of skewers on Mavic Ksyrium or CrossMax wheelsseem to create a higher clamping force for the same amount of force usedpushing the lever over when installing the wheel. After riding with skewerslike these, if I flip my front lever open, the bike does not clunk downonto the axle; it is still fully seated into the dropout. No matter what,you need to tighten your front skewer about as hard as you can with a frontdisc brake.

All this is a long way of saying that the Kelly front-opening slot designis well-designed to counteract this wheel-removal force created by a frontdisc brake. That said, I imagine it would take some time to get used tothat geometry when installing your wheel. You would not be able to justdrop the fork down onto the axle.
Lennard

Chilled carbon
Dear Lennard;
We here in northern New England are currently enjoying one of our seasonalbloody cold snaps. My bike storage area is a semi insulated, unheated room.During weather like this those bikes are subjected to sub zero temps fordays at a time. I’m wondering if climate like that might be damaging, particularlyto carbon fiber components and frames. The storage alternative is my dirtfloor, field stone basement, which remains a constant 50 degrees but isprobably a little damp. Which is the bigger threat moisture or cold?
Paul

Dear Paul;
I’ll defer to an expert on that one.
Lennard

Answer from Easton:

Dear Lennard;
That’s a good question from your reader about cold and moisture.He need not worry about the cold or moisture as they relate to his bicycle components made from carbon fiber. Bikes equipped with composite components are generally high-end and one wonders why they aren’t stored in their rightful place in the bedroom?It appears his ride is consigned to a dark cold room in the dead of winter. I know how cold it gets as I lived in Massachusetts for a couple of winters. In all seriousness, he asks a great question. Does exposure to sub-zero temperatures have a negative effect on carbon fibers? The short answer is, “No.”Now for the long answer: Carbon fibers were invented for use in the Aerospace industry in both aircraft and vehicles destined for outer space. The fibers were designed to work in an extreme range between hot and cold. The fibers are tested down to a chilly -67F and can then be brought back to normal temperature without degradation. There would be a small amount of performance loss if one actually tried to use the component at -67 degrees. Not only would the cold weld your hands to the
bar but the stress induced by riding the bar at this temperature could cause some level of distress to the resin system that holds the fibers in place.In regard to the question of moisture, there is no issue for the composites here either. Water cannot damage the components.
John G Harrington
Vice President- Bicycle Products
Easton Sports, Inc

We have the technology…
Dear Lennard;
About nine months ago, I had my hip replaced, for various reasons.Long story short, all my bikes previously were set up with 175mm non-driveand 165mm drive side cranks due to the leg length discrepancy. Now thatthings are basically even (less than 1/4 inch), I switched back to a “normal”crank set on all my bikes. I have no pain in my hip and back anymore andthe leg is getting stronger with each pedal stroke.

However, one problem has arisen; pain on the outside of my left knee(the operative leg was the right one). Now, I did change my set up a bitfrom what it was “pre-bionic man operation”; I adjusted the seat positionand height accordingly. My brother and I are both very experienced mechanicsand have fit bikes for numerous people. From that standpoint, things arefine. I can’t lean over too far but I never get in the drops anyway andmy mountain bike has a riser bar. I’m wondering if it has anything to dowith my cleat position on my left (non-operative leg)? And if so, wouldmoving the cleat inboard correct this problem?

I read the email and response about orthotics and hip/knee pain. Wouldit be to my advantage to invest in an orthotic?
Aaron

Dear Aaron;
If you still have a leg-length discrepancy, you may need to use a cleatshim. Otherwise, if length is even, maybe it’s a tight IT band. If youare favoring the bionic side, you could be pulling it more on the otherside. In other words, if you are shifted over toward the bionic side ofthe saddle, whether that leg is shorter or not, it would pull the musclesand tendons on the other leg more. These would include the IT band on theoutside of the non-bionic knee and the sartorius muscle on the inside ofthat knee.

Furthermore, you may not be religious about stretching your IT bandand your hamstrings (which cooperate with the IT band) after your operation.It sounds like your hamstrings are tight by the fact that you can’t leanover. You could see if you get any relief by putting a Big Meat wedge underyour cleat that raises the little-toe side of your non-bionic foot up relativeto the big-toe side, which would relieve some IT band tension. If thathelps, then check out some custom orthotics, and do some stretching andphysical therapy on that IT band.

Also, keep in mind that I am by no means a health-care professional,and you ought to consult one of those when you have knee pain.
Lennard

Feedback

Re: sticking a 26-inch fork into a 700C frame (see “Whatthe fork?!“)

Lennard;
I am in the same position as Marvin, I am a short rider and would liketo build a TT bike but can’t seem to get my bars low enough. Problem issolved by purchasing a high rise stem from the local bike shop and turningit upside down. I am ordering a 130degree rise, 120mm length, turning upsidedown should gain me a inch or two in drop.
Bob
Dutch Wheelman Cycling Team

Hey Bob,
You might check out the stem calculator on my Web site. It is designedfor xactly this purpose. If you put in the specs of your new stem relativeto your old stem, it will tell you exactly how much lower it drops themand how it changes the horizontal reach. It gives the position of the barin space relative to the top of the headset. Check it out: www.zinncycles.com

Re: white carbon. (see “Howdo they make it white?“)
Dear Mr. Zinn;
I know you are a very smart man, but I feel compelled to comment about”White Carbon” question a reader posed to you. There is no such thing aswhite carbon. All carbon is black. You cannot make it white unless it isreally fiberglass.

White carbon is not carbon at all, it is fiberglass, and the propertiesof fiberglass are not even close to those of carbon.

Please inform your readers of the truth!
Clark

Dear Lennard;
Of course the carbon fibers remain black. Perhaps Selcof has addedan exterior ply of some other fibers? Texalium is one possibility. Theseare aluminized glass fibers, easily laminated together with carbon structuralfibers below.

Do a “Google“for references on Texalium.
Dianne

Dear Lennard;
Re the white carbon fiber discussion: All carbon is black but perfluorinationof carbon will make it white, e. g. graphite can be made white by exposingto fluorine gas under special conditions.
I’m not sure what Selcof uses, but just FYI.
Jason
Ph. D., Research and Technology Department
High Energy Materials Division, Naval Surface Warfare Center

A final word:
Andrew put up Christmas photo of himself on a motorcycle inBaja. So I thought I’d put up my own Christmas photo… though I seem to have a little trouble adjusting to winter life in Hawaii! Also, the Hilo Bike Hub fortunately has a tall owner, who loaned bikes, shoes and helmets to me and my brother for a cool ride with some friends on some great trails near Hilo covered with roots, holes dug by rooting feral pigs, as well as several man-made obstacles.


VeloNews technical writer Lennard Zinn is a frame builder (www.zinncycles.com), a former U.S. national team rider and author of several books on bikes and bike maintenance including the pair of successful maintenance guides “ Zinn & the Art of Mountain Bike Maintenance” and “Zinn & the Art of Road Bike Maintenance.”Zinn’s regular column is devoted to addressing readers’ technical questions about bikes, their care and feeding and how we as riders can use them as comfortably and efficiently as possible. Readers can send brief technical questions directly to Zinn. Zinn’s column appears here each Tuesday.

FILED UNDER: Bikes and Tech TAGS: / /

Stay updated on all things VeloNews

Subscribe to the FREE VeloNews newsletter