Tech FAQ: Seat posts, tubulars, spider tabs

Lennard Zinn addresses a variety of questions for this week's edition of Technical FAQ.

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Altering a seat post

Dear Lennard,
I’ve got a couple of questions for you:

I have an older Giant TCR advanced frame/bike that I purchased used from Sellwood Cycles in Portland approximately one year ago. I love the frame and the bike, but unfortunately the maximum seat post height is a little too low for me. Initially, we cut it down to allow swapping of two different seat post mast heads (is that the correct terminology?), including the shorter (stock size?) and a longer one which we are using now (is this the longest one available?). We recently had to cut it down a bit further as it had developed a crack near the top edge of the integrated aero post. I bought it for my son to use when he’s in town, and it still fits him, but I also like to use it in my pain cave on a Wahoo trainer for indoor training. Now it’s now approximately 1-2cm too short/low.

1. I’m wondering what might be the best way to get more seat post/saddle height without breaking the bank (or the frame). I’d love to be able to cut it down just below the aero section where the integrated seat post section becomes round and retrofit a shim and a clamp in order to use a 27.2 (or other) seat post. Would that be at all possible?

2. Or would it be possible to fashion a carbon extension that could be inserted into the existing aero post section, reinforcing that section down a bit, and extending up another few centimeters (4-5cm more, perhaps) in order to use the OE seat post mast?

Obviously, I’m looking for the best, simplest, most durable solution here and one that won’t break the bank. I’d like to continue to use this frame but I don’t want to spend a ridiculous amount in order to do so when buying something else (new or used) would make better sense.
— George

Dear George,
Here’s the answer from Giant:

“Hello George, It appears you own a 2011 TCR Advanced SL frameset … Giant’s second year of offering our now-famous ISP (Integrated Seatpost) design. While the frameset came with a stock seatmast clamp that offered an additional 20mm of height increase (over the cut seatmast), it also appears you have the extra-long option seatmast clamp — which offers 40mm of additional height over the cut seatmast.

It’s a bit hard to see in your attached photos, but if you’ve added the requisite spacers to bring the clamp’s limit-line equal to the top of the cut seatmast, then that is the maximum extension you’ll be able to get out of the Advanced SL frameset.

Cutting the seatmast off and trying to retrofit a round seatpost is absolutely not advised and potentially quite dangerous — please do not do this.

If all you need is an additional 3-5mm of height adjustment (on your maxed-out frameset), you might want to look into a “taller” saddle that features additional rail-to-saddle height over your Fi’zi:k saddle.
— Andrew Juskaitis
Giant Bicycles Senior Product Marketing Manager-Global

Tubular wheelsets

Dear Lennard,
I am almost sure that you once wrote about the possible evolution of tubular wheelsets, while disc brakes were entering the road bike market, but now I cannot find this post. That was several years ago, and it seems that nowadays, tubular disc-specific wheels are a rarity, except of course from WorldTour teams.

I am a big fan of tubular tires, as I enjoy riding them and that is what worries me. Therefore, I would like to ask you for your insight and prediction on the future of tubular tires.

Now, only few manufacturers (like Vision/Metron, Lightweight) offer tubular disc wheelsets to a public market. I remember in that post of yours you wrote, that introduction of disc may result in redesigning of rims as braking track will no longer be needed, which should result in lowering the weight of rims (rotational weight goes down which kind of compensates general increase of disc wheelset weight).

Has it happened? Which companies produce disc-specific carbon tubular rims/wheels? Are those wheels used by pros (from Zipp, Mavic, Roval, Bontrager, and others) using standard, traditional tubular rims (with brake tracks), or are specific, modern U-shaped ones only for pro teams? If the latter is true, what about the UCI rule insisting that pro riders must use equipment that is available to the public? Will carbon tubular disc rims be continued at all (apart from cyclocross wheels)? Or should we just get used to and switch to clinchers and tubeless wheels? I am asking, as I plan to switch to a disc road bike and would love to have tubular wheels on it. Can you share your point of view and advise, please?
— Witold

Dear Witold,
Yes, the current tubular disc-brake wheelsets on the market generally do not have a brake track on the rim. Those are definitely tubular-specific rims.

I wonder where you are looking when you say that there are no carbon tubular disc-brake wheelsets on the market. My impression is that there are lots of them. I personally have a number of sets. They are much slower sellers than clincher wheels, so availability may be a bit more limited.

Shimano, Enve, Hed, Zipp, Mavic, Roval, Bontrager, etc. all use tubular rim designs specifically for disc-brake wheels that have no rim-brake track, if for no other reason than that they all redesigned them in order to make the wide rims that the market is demanding. Those wheels are available to the public, just as they are to ProTour teams.

As for cyclocross wheels, in general, there is no such thing. Cyclocross riders generally ride road wheels, and as you can see at any ’cross race, there are plenty of them out there. The carbon disc-brake tubular wheels being completely annihilated by cyclocross racers any time there is a rough and super-muddy course are road wheels. They get mushy around the edges because they are being ridden hard with 33mm tubular tires pumped up to around 1-1.5 bar (14-22psi); this tends to destroy the cotton sidewalls of their tubulars as well. I believe cyclocross-specific tubular carbon rims could be designed to counteract this, but the demand is too small for that kind of investment to be made in them. Also, amateur racers do not destroy the rims as quickly, because they do not run as low a pressures or ride as hard. If people actually buying the wheels were to be ruining them in one race like pro ’cross riders often do, there might be more interest from rim manufacturers in making cyclocross-specific rims.
― Lennard

More on crank-spider tab thickness

Dear Lennard,
I was very interested in your recent post regarding crank-spider tab thickness.

I’ve found that if the tab is too thick, when changing to the inner ring the chain can balance on the teeth of the inner ring instead of seating properly. This is more likely if one changes to the inner ring while using one of the smaller sprockets. And if the tab is too thin, then one finds that one can’t use the inner chainring with the smallest one or two sprockets.

And I usually assume that I’ll encounter one of those problems with any double chainset. But not both problems, of course.

However, when I fitted a Stronglight 34-50 chainset to one of my bikes some years ago, I encountered both problems. The tab was so thick that the chain balanced on the teeth of the inner ring, while simultaneously being so narrow that the chain rubbed on the outer ring while using the 34 and smallest two sprockets. The solution that I’ve used in the past to solve the chain-rubbing-on-the-outer-ring problem is to make spacers that fit between the crank tab and the outer ring. Not just washers, but spacers that run a short distance up the inside of the outer ring. That cured the chain-rub, but of course the chain-balancing-on-the-inner-ring-teeth problem still persisted. So I made the top of the spacers bulge slightly toward the inner ring, and it’s been working perfectly for years.

So I was most surprised to find that the FSA 34-50 chainset on a Specialized that I bought four years ago exhibited neither problem. It always changes perfectly onto the inner ring, yet never rubs on the outer ring.

Of course, the problem of the chain rubbing on the outer ring could be avoided by careful design of the outer chainring, but the FSA chainring doesn’t appear to be in any way special.

So I’m somewhat bemused by those two chainsets — one of them has crank tabs that are simultaneously too thick and too thin, while the other has crank tabs that avoid both problems.
— David