Technical FAQ with Lennard Zinn: Carbon forks
VeloNews technical writer Lennard Zinn is a frame builder, a former U.S. national team rider and author of several books on bikes and bike maintenance. Zinn's VeloNews.com 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. Zinn’s column appears each Tuesday on VeloNews.com. Question:I have a question on the durability of the RockShox World Cup carbon steer tube. I am very comfortable on my MTB with a low front end but it is causing problems on steep descents. I am
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VeloNews technical writer Lennard Zinn is a frame builder, a former U.S. national team rider and author of several books on bikes and bike maintenance. Zinn’s VeloNews.com 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. Zinn’s column appears each Tuesday on VeloNews.com.
I have a question on the durability of the RockShox World Cup carbon steer tube. I am very comfortable on my MTB with a low front end but it is causing problems on steep descents. I am planning to raise and lower my stem (Ritchey WCS) using normal headset spacers, depending on the course.
From your experience, do you think the fork can handle that? I’m a fairly large rider (180 lb.) and an aggressive XC racer. –Joe
Answer #1 from RockShox:
The Carbon Crown Steerer is designed to withstand the rigors of XC racing, and Joe will have no issues running it. Re-reading the question it almost sounds as if he is worried that about the durability of the Carbon on repeated stem swaps, depending on the course. Care needs to be taken to remove all burrs from both the ID of the stems and that the cut surface of the steerer be finished correctly with 400grit sandpaper as instructed in the owner’s manual.
I would also confirm that the fork is set up in the 80mm format, and not the 63mm.
Answer #2 from RockShox:
I had a slightly different interpretation of Joe’s e-mail. My guess is that he’s concerned about using multiple headset spacers, sometimes above, sometimes below the stem. Besides the obvious sternum concerns that come with running a stack of spacers above a stem, he should have no problems, as long as he follows Rene’s instructions above.
I just purchased a new Bianchi Carbon Fork. It is new although made a couple of years ago. It is a one-inch size. I love it but was told by a bike shop owner who sold Bianchi Bikes at one time that it is prone to failure.
Is this true? Any good or bad feedback? Please help!–Steve
Response from Bianchi:
The questioner doesn’t say what the steerer material is. The writer also doesn’t say if he bought it new at a dealer or on Ebay. People need to know that if they buy from a private party, who claims it is new, that they have no assurance that it is really new. Also, they have no warranty. We never sold the forks separately, from the frames and bikes.
All forks with carbon fiber steerers have to be assembled correctly. We have seen a couple carbon fiber steerers crack due to overtightening of stems or stems with sharp edges that score the carbon fiber. We have seen dealers who honestly, no kidding, have forced a star washer into a carbon fiber steerer. We have never seen carbon fiber blades fail, under normal use. By that I mean not crashing into a car or driving your bike under something with it on the roof rack.
Comment on carbon forks for big riders:
Thanks for the in depth article on Carbon Forks. I am very “old school” having ridden since 1956. I have always been big (225lbs) and when I raced (215lbs) was a serious sprinter who could torque any frame. I have a prejudice about anything not steel, but your fork article has given me some food for thought (no pun intended) when I build up my next bike.
I like the new Campy record parts which have some measure of carbon and for my riding these days is pure sex appeal. Structural parts have been another matter. Steel gives me the security (and comfort) I have always enjoyed. Last fall in a decent in the Rocky Mountains my speedo’ hit 61mph and my bike was very stable. This is a rare event, but I felt very comfortable. I will keep your article research carbon further by corresponding with some of the builders. Cinelli and Columbus especially.
On the shimmy, I have always been able to reduce or eliminate any degree of shimmy by making sure that the frame alignment is good. I have a Scapin built LeMond which I eliminated a little shimmy at moderate speeds by taking a frame align tool to the rear stays. It also fixed the RD shifting problem I was having between 2 gears. I think the bike had crashed at one time and was never checked before I had the bike. Anyhow, my “two bits.”
Thanks again. I will be getting your book on MTB Maint. 3rd Ed.I am building a steel Scapin MTB up and will research the parts (components) with the help of your Lit. –Don
I recently bought a new road bike, 853 steel that came with a carbon fork. Can you tell me what the benefit is of having a carbon fork? I really like the bike, helps me climb better. So I am not complaining, just wondering. –Bill
High Gee-Whiz Factor (GWF). –Lennard
I recently purchased a two-year old Serotta CSI frame, 67cm center to center. The owner is also interested in selling the fork he used with the same bike. The True Temper Web site quotes, in part, a laudatory review written by you in VeloNews too long ago for me to find, and without specifying exactly which of the several forks they make that you reviewed. The original owner of the fork claims to weigh about 170 lbs. and be in his early thirties. I am 205 lbs. and 43 years old. I find that my tolerance for taking shocks in my hands and neck is decreasing as I age. My current bike has a curved steel fork. Can you give me any advice on whether I should buy this fork? Thanks, in advance, for your advice. –Anon.
In general, I think these (Alpha Q) forks are great. I have them on two of my own road bikes (one curved and one straight). They seem to behave similarly, and I think either one would give you at least as much (and probably more) shock damping as a curved steel fork. –Lennard
I am having a problem with a used Bianchi Carbon fork that I am trying to mount onto a frame, and I was wondering if you had ever encountered the same problem. The fork has a carbon steerer tube and when I try to tighten the plug assembly designed to pull the top cap downwards toward the fork, I find that the ribbed split-collar, which is designed to expand when the wedge-shaped plug inside it pulls up, does not establish enough friction to hold in the steerer tube. Hence when I tighten the bolt in the top cap, the plug and expander collar just gets pulled upwards – rather than holding and allowing the bolt to tighten downwards on the cap. As a result, without enough downwards force exerted on the threadless stem I cannot tighten it to the steerer tube without leaving play in the headset.
Two possible solutions come immediately to mind.
1) maybe the ribbing on the present collar is worn and if I replace it with a new one, it would grab in the correct fashion. But then again maybe the carbon fiber steering tube is just too slippery – especially since it is a used fork.
2) I could try putting a sleeve in between the ribbed split-collar and the top cap, which would stop the collar from sliding upwards. Then when the expander wedge is pulled up by the bolt, the split-collar would not be able to do anything other than expand.
Anyway, I just thought that I would see whether you have a recommendation or have ever encountered the problem and seen a solution. If you have, I would love to hear what you have to say. — Vinny
I have encountered the problem and have always managed to solve it with another plug. The sleeve would not work, since it would also prevent the cap from pushing down on the stem. You could try roughing up the inside of the tube with fine sandpaper — you do this anyway on Alpha Q forks to prepare for gluing in an insert, which could also be a solution. Below are some further suggestions (and cautions) from fork manufacturers.–Lennard
Here are some ideas one may try if their preload device is moving.
1) Is it the correct device for the fork? There are slight differences in the steerer tube bores from mfg to mfg and a plug that’s too small may not expand enough.
2) Is the inside of the steerer tube clean and dry?
3) Disassemble the preload device and inspect the components for nicks or burrs that may cause binding while the sliding parts are trying to expand. De-burr if necessary then lightly grease the contact points of the moving parts and reassemble. Do not grease the outside of the plug assembly.
Usually this will do the trick. If not they’ll probably have to try a different preload device or perhaps go the bond-in route.
Fork, steerer and expander.
You can trust Deda, expander plug “Spadca” is available for most popular carbon steerer:
1-1/8”, nominal inside diameter 23.5 mm
1”, nominal inside diameter 20 mm
For all other type, please ask the fork manufacturer. Just for your info, team Bonjour, 2002 season, Time fork, carbon steerer; what was inside the steerer? Standard Taiwan star nut. Requested, paid and suggested by Time!As for the Bianchi used fork: Where is the original plug and cap?
Here is what Bianchi had to say about it:
We ALWAYS recommend people go to a bicycle dealer, as we cannot trouble-shoot most mechanical problems on the other end of a computer screen. We especially recommend that with carbon fiber parts, as torque limits have to be respected.
We cannot ever recommend that people buy USED carbon fiber components. There are too many possible scenarios for disaster, as you can’t ever know how the original owner treated or used the component. –Sky
Here is Kestrel’s answer:
To the question at hand, it could be that the inside of the steerer is worn or almost “polished” smooth by the expander from previous use. And/or the expander could be worn. Dificult to say what to do without being able to see it, but I would suggest he take it to a shop and see what other types of expander plugs they might have. (But definitely don’t use a “star-fangled” type device.)
More importantly, I would want to make sure he knows the history of the fork. Where did he get it? How has it been used and by whom? Can he be absolutely certain it has never been crashed/damaged? I would have these concerns with any used fork (or frame) regardless of material, but since carbon cannot bend or dent the way metals can, the signs of damage are different and may not be noticed by someone who is more familiar with metal ones. For the price of a fork, it’s just not worth taking any chances.
It seems the plug diameter does not fit the inner diameter of the fork. Nor does the plug offer enough travel to make up the difference. There are many solutions and he has mentioned some of them.
1) He could try and find a different plug that fits correctly with the I.D. of his fork.
2) He could try to shim the plug. I would use tape either black or duct tape to add diameter to his plug, although this might make it hard to re-insert the plug.
3) After he fits the fork and is absolutely sure he has the steering tube correct, he could glue the plug into the steering tube using JB Weld (epoxy glue) available at better hardware stores.
The issue is the design of most compression plugs is poor. Most of them expand from the bottom of the plug only, thereby limiting the amount of surface contact you get with the plug and the I.D. of the steering tube. Most of the designs have limited travel (ability to expand) and therefore the fit between the plug diameter and the I.D. of the steering tube is critical.
The ridges on the clamp do not really offer much help as the groves actually reduce the amount of friction between the plug and the steering tube by reducing the surface area contact. In addition the ribs are vertical and therefore in alignment in the direction of the pulling force.
Thanks for the quick replies and the forwarded disclaimers from Bianchi and Kestrel. The possibility of damage in a used carbon fork is always a concern. As an aside to Bianchi’s comment, last year I took a forkwith a threaded steel steerer tube to one of the local bike shops in order to have the threads extended down the steerer tube an inch. I checked with them that they had the dye that could do this and they assured me they did. When I got the fork back, they had cut the wrong threads on the fork. The threads they cut were .030″ smaller diameter than the original. This left the headset nut still able to engage – but barely, and left the steering tube wall dangerously thin. I recognized the problem whenI felt the play in the headset nut, but I wonder if the regular bike customer might have wound up using the fork and have later crashed when something gave way. Anyway, so much for Bianchi’s putting the local bike shop up on a pedestal.
You are of course correct that a sleeve will prevent the top cap from tightening down on the stem. Last night I tried using a sleeve to temporarily keep the plug/collar from sliding and thus allowing the collar to tighten enough so it no longer slips. Then the sleeve was removed and the cap re-tightened on the stem to take out the play in the headset. Finally the bolts clamping the stem were tightened, and they do the major job of holding everything together. I have yet to try riding the bike. The sandpaper (or perhaps a brake cylinder grinding stone) is a good idea, as is getting a new plug with sharper-edged ribs. Ordering and waiting for the latter to come just takes patience. –Vinny
I have a “new” frame with a nice, bladed carbon fork. Only problem is that my toe overlaps my front wheel by a couple of millimeters when I am clipped in. I can’t adjust my cleats due to biomechanical limitations. Is this a big issue, or should I just ignore it?
I am 5’10, and wear a 44 shoe. The frame is a proper fit for me in all other respects. –Jorin
It’s up to you. If it doesn’t bother you, ignore it. It will only happen during very low-speed turns. –Lennard
I own a custom built bike that was built around the geometry of Kestrel EMS carbon fork 4 years ago. I know the Kestrel fork then had a high axle to crown measurement. I am thinking of replacing my fork but would like to get one with the closest axle to crown measurement as on the original Kestrel EMS fork. Do you have axle to crown measurements of most of the carbon forks out in the market? –Mike
A good source is the Quality Bicycles catalog, which your local bicycle shop should have. It lists fork lengths for all of the forks Quality sells, which is an extensive selection. –Lennard
I have a quick question about fork rake for my Merlin road bike. It’s a 56cm with 73.5-degree head angle. I am thinking about a Reynolds 45 degree carbon fork to help toe overlap problems and to increase wheelbase. Good or bad idea? –Wes
If you mean 45mm, it sounds fine.–Lennard
VeloNews technical writer Lennard Zinn is a frame builder, a former U.S. national team rider and author of several books including the pair of successful maintenance guides “Zinn & the Art of Mountain Bike Maintenance” and “Zinn & the Art of Road Bike Maintenance.”