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Technical Q&A with Lennard Zinn: Carbon tubes and lubes; Tour tech redux; crank-length questions

Seat tubes and lube?Dear Lennard,I have been in the bicycle industry for many years and have consistently had conflicting advice about using grease in a frame's seat tube when using a carbon seatpost. With the many materials that frames are using now, are there some frames that should have grease in the seat tube when using a carbon seatpost, or is using grease in a seat tube with a carbon seatpost a big no-no? What do the majority of frame and carbon seatpost manufacturers have to say about this and why isn't it more clear? Also, if grease is a good idea, is there any particular

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By Lennard Zinn

Seat tubes and lube?
Dear Lennard,
I have been in the bicycle industry for many years and have consistently had conflicting advice about using grease in a frame’s seat tube when using a carbon seatpost. With the many materials that frames are using now, are there some frames that should have grease in the seat tube when using a carbon seatpost, or is using grease in a seat tube with a carbon seatpost a big no-no? What do the majority of frame and carbon seatpost manufacturers have to say about this and why isn’t it more clear? Also, if grease is a good idea, is there any particular grease that is better to use than others?
Ian

Ian,
Here are responses from some carbon-seatpost makers.
Lennard

From Easton:
No grease on carbon posts. Grease contains certain minerals that can attack clear coats, can penetrate the resin matrix and could cause swelling of the composite laminate. Can you say “stuck seat post?” Don’t use grease.
John G. Harrington
Vice president, bicycle products
Easton Sports, Inc.

From Campagnolo:
No grease. In some cases it can be dangerous to use grease as the chemical composition can cause a reaction between materials. Besides, it increases the torque required to clamp the post.
Richard Storino
Campagnolo USA

From Deda:
Absolutely no grease on carbon, ever. Also, do not use solvents to get old grease off, or to get old grease out of the seat tube. John Harrington of Easton and I believe that many solvent residues in the seat tube soften the gel coat of the carbon, then bond the gel coat to the inside of the seat tube, freezing the seat post in position for eternity.
Tom Franges
Deda Elementi North America tech support

From Bontrager:
There are some petroleum greases that are slightly acidic. That acidity can attack the resin used in carbon-fiber construction (frames or posts). This same acidity can attack aluminum or steel frames components, too. However, most greases are engineered to be slightly basic. Neither Trek’s nor Bontrager’s carbon resin is affected by this acidity. The main reason we suggest you don’t use grease on any seatpost inserted into a Trek OCLV frame is that there is not enough surface roughness inside the seat tube to ensure your post stays where you put it.
Scott Daubert
Trek team liaison

Tour tech redux
I continue to receive questions about the Tour de France to which I have already supplied answers: the “spare tire” bags under the CSC riders’ saddles; oval chainrings; Armstrong’s down-tube shift lever; and Mikael Rasmussen’s bike. Here is a link to the answers to these and other questions. Of course, I am still happy to answer Tour questions – just not ones that I have already answered in detail.
Lennard

Three questions about crank length
Dear Lennard,
I am confused about crank length in relation to bike size. My mountain bikes are all 175, including my single-speed. My road bike is a 172.5, as is the cyclo-cross bike I recently purchased. However, I hear a number of different things about crank length: all of your bikes should have the same length; single-speed bikes should have a 180 for more pedaling leverage; and a cyclo-cross bike should have a shorter crank due to geometry and overlap of front wheel and toe (which is what brought this question about – the cyclo-cross bike I purchased has major toe overlap). Can you help me? By the way, I usually ride a medium, 18-inch, 54-55cm bike.
Brandon

Dear Lennard,
Recently, I borrowed a friend’s Giant TCR1 composite for a test ride. I set it up with my measurements and checked them twice. I used my own saddle and pedals so the fit was identical to my current bike, an aluminum Giant (or so I thought). After riding down the block I felt that the saddle was too high, so I dropped it a millimeter. But after another couple of blocks I still felt it was too high, so I dropped it another millimeter. I did a quick “on the road” check by placing my heel on the pedal and dropping my foot to the 6 o’clock position. The saddle now seemed set to the correct height. I rode my usual 50-mile group ride and at the end of the ride my legs felt pretty fresh. Suffice it to say that I thought I’d found a really comfortable frame. After the ride I found out that the bike had 172.5 cranks, not the 170s that I use. This, of course, resolved the seat-height issue. Now, I have two questions. First, every fit guide I’ve read says I should use 170 cranks. But I’m wondering whether a 172.5 might help keep my legs fresher. Second, I’m in the process of building a time-trial bike. The past couple of years I’ve heard of riders using longer cranks for their TT bikes than their road bikes. Is this something for me to consider?
Matthew

Dear Lennard,
Why are mountain cranks usually longer than road cranks? I assume it has something to do with increased torque for the longer crankarm, but I am sure it is more complex than that. Do factors like the distance between the pedal spindle and pedal platform do anything to change crankarm-length issues? What are the pros/cons to using the same crank length for both road and mountain? Is bike fit – knee over spindle, correct top-tube length, correct seat to stem/bars distance – more important than the 5cm difference between 165/170?
Ted

Dear Brandon, Matthew and Ted,
I am lumping these three answers together, since there is overlap in your questions. As you may be aware, I am extremely interested in crank length and have been almost since I started cycling back in the 1970s. This interest is largely due to my 6-foot-5-inch height and the feeling that I was somehow being robbed of some available power by the fact that the available cranks were much shorter relative to my 38-inch legs than they are for someone with more average leg length. Frustrated with the short lengths available to me, I started making my own cranks. I have experimented with it quite a bit, and I have a personal stable of bikes with cranks ranging from 175mm to 210mm (and sometimes even 220mm) on them.

In general, I will first say that, if you are riding the Tour de France, you want every bike you ride to be as identical as you can make it, because riding hard for eight hours every day will not allow you the luxury of changing things without problems like tendonitis or backaches showing up. However, if you ride maybe 10-15 hours a week or so, as I do, my experience is that you can ride bikes with different crank lengths, Q-factors, saddle types, bar shapes, etc., without problems.

As for the specific types of bikes, I am quite happy with the same length cranks on all of my bikes, and yet there are some reasons to have different lengths on different types of bikes. For instance, I have a mountain bike and a road bike with 200mm cranks, and I love them both and notice nothing moving from one to the other, other than Q-factor and riding position. It works great, and, as long as the bottom-bracket height of the frame is built for that length, which is the case with these bikes, I have no problem with clonking pedals on rocks with the mountain bike or with cornering clearance on the road bike. I can definitely see, feel and measure the increase in climbing speed, and while descending on the mountain bike, I appreciate the longer stance for stability when standing while going fast downhill over rough stuff.

If there is a reason to have a shorter length on one than the other, I will say that a mountain bike begs for a bit shorter crank, because the variation in cadence can be so large when riding uneven terrain, crossing ravines, and whatnot. A shorter crank not only gives you more clearance pedaling over rocks, it is easier on the legs when changing cadence rapidly, from, say, 40 to 120 RPM. Some of these same things apply to cyclo-cross, to answer that question. As for solving big toe-overlap problems with the front tire by making a 2.5mm decrease in crank length, though, don’t expect too much improvement!

To avoid getting too long-winded here, I first will refer you to a page on my own web site. I go into great detail there about lots of issues related to choosing a crank length for yourself, as well as some formulas for determining it.

Pedal-spindle-to-pedal-platform height has little or no effect on crank length, because it just offsets the pedal circle up or down. As for knee over pedal, the fore-aft seat positioning should certainly be dependent on the position of the forward pedal, but if everybody had cranks that were proportional to their leg length, a lot of these problems would go away.

For instance, most stock-sized tall bikes have really shallow seat angles. This is not because tall people have proportionately longer thighs, but rather because they are generally constrained to using the same very narrow range of crank lengths as everybody else, which places their knee further forward relative to the shorter crank, so their seat angle has to tip back to get the saddle far enough back. The inverse is true for short people, and short bikes have steep seat angles for this reason (as well as to avoid toe overlap).

A related, but less important issue is that the straight section of the seat rails does not increase or decrease in length with increasing or decreasing leg length, so the taller the rider, the less proportional adjustment available. I think that positioning on the bike and crank length are equally important.

I got a kick out of seeing that Czech rider Ondrej Sosenka used 190mm cranks to set the hour record last month.

And finally, most people think that just because 180mm is the longest available crank from the big brands, this is the longest they should use, no matter how tall they are. I remind you that Miguel Indurain won a lot of Tours de France on 190mm cranks custom made for him by Campagnolo.
Lennard


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.