TallRider

I know there's no problem clamping steel saddle rails with a seatpost clamp that only contacts two small points along each rail. But is it necessary to be more careful with titanium rails (or carbon fiber with metal coating)?
I'm figuring this is no worry, but want to check and be sure.

Background:
I'm planning to swap seatposts between two bikes because one bike has a really slack seat tube angle and I want to get the saddle in the right spot. The Cannondale Fire seatpost on my Centurion Ironman has no setback, and I can get the saddle in the same spot on that bike using a post with setback.

So I'm planning to move the Cannondale Fire seatpost to my Raleigh (that has the slack seat tube angle).

The only issue I'm unsure about is that the Cannondale C2 seatpost (which I'd be moving to the Centurion Ironman) clamps the seatpost rails with two small contact points on each rail. This would be clamping on the titanium rails of a Specialized Alias saddle.

Those titanium rails are currently clamped by the Cannondale Fire seatpost which has a much larger contact area along the saddle's rails. This spread out the force along a lot more metal, which means much lower force at any given point.

Cannondale C2 seatpost with two contact points along each saddle rail:


Cannondale fire seatpost with large, long contact area along saddle rails:

LarDasse74

As long as the points of contact on the upper and lower points are lined up there is no risk of damaging the rails. I believe there are some seatposts where there are two widely spaced contact points on top and more narrowly spaced contact points on the bottom, which could (theoretically) bend the rails if overtightened.

The only problem I can think of is if the rails are thin walled hollow tubes (which is unlikely) then the possibility exists of crimping the tubes. But with the normal wall thickness of seat rails you would have to intentionally overtighten the bolts.

HillRider

I've been using Ti railed saddles (Avocet O2-40 Ti) for years on a variety of bikes with several different makes and types of seatposts. I've never treated them any differently than a steel saddle rail and have never broken or damaged them. One of my seatposts (an Easton EA50) has two relatively small clamping pads like your Cannondale and has never damaged the Ti rails.

I don't tighten the seatpost clamps with a breaker bar (on any saddle for that matter) but I'm not excessively gentle with them either. Ti rails are solid rods, not tubing, so they should be impervious to all but the most ham-handed abuse.

Carbon rails are another matter. I have no experience with them, and don't plan on having any in the future

TallRider

Thanks guys. that's good to know.

One other question is, now that I've put the setback seatpost on the Centurion Ironman, it's clamped way back on the (titanium) rails of the saddle in order to get the saddle in the same spot as before. I had a fitting appointment for this bike so I'd like to keep the saddle in the same spot.

Is there any danger or problem of the seatpost clamping too far back on the saddle rails here? I'm a reasonably big rider (6'5", 190#) and don't want to risk any danger of bending the rails. Compare this to the picture above, where the saddle's rails are clamped right in the center of the straight section.
If this is problematic, I'll just buy another non-setback seatpost.

Drakonchik

I run a few Specialized Alias saddles -- they feature holow it tubes -- and I would be *seriously* concerned about crushing the hollow tubing with that clamp.

In fact, assuming for argument's sake I wanted to put a series of 8 dimples in a pair of hollow ti tubes, I might build an apparatus that looked exactly like your clamps and keep tightening until -- yup -- it happened.

HillRider

I looked it up and that model Specialized saddle does indeed have hollow Ti rails but I have to believe Specialized knows what saddle clamps are out there and has made them accordingly thick walled. I suppose you can damage anything if you try hard enough. Reasonable torque on the clamp bolts should not harm the saddle.

As Tim's question about how far back the rails are clamped, I have to opposite problem. I have the saddle shoved all the way back on the rails, particularly with one bike that has a Bontrager seatpost with no setback. I'm not that big or heavy (~150) but I've never had any problems with the rails bending or detaching.

LarDasse74

I have seen a lot of people bend saddle rails by putting the saddle all the way back... it happens because the rear of the saddle is where people often put most of their weight. If you have the saddle all the way forward then to bend the rails you have to put a lot of weight on the nose of the saddle, which is very uncomfortable... but very likely if you slide forward to sprint and hit a bump unexpectedly...

Personally, If I were you I would get a seatpost that puts the saddle in the middle of its adjustment range, and get a steel railed saddle... If you and your bike weigh 210 lbs, a saddle that is 30 g more is only a 0.03% increase (3/10000). A 30 g weight penalty will slow you down less than having a broken saddle.

Wanderer

The type of clamp is a non issue - all of the force is always at the outside edges of every clamp - no difference...... MHO

Panthers007

Titanium (Ti) is stronger than steel in many applications. Pressure shouldn't be a concern. Where you run into problems is flexing/bending it. Steel you can bend without problems - within reason - but Ti will gladly snap in half without too much effort. Thus was the fate of my beloved Huret Titanium Duopar RD.

As seat-rails goes, there should not be any problems.

joejack951

Not really. On a wider clamp, the force will be concentrated at the points nearest the clamping bolt(s). If there is only one clamping bolt in the center of a wide clamp, the force will be concentrated in the center with the edges seeing less force (how much depends on the stiffness of the clamp). With that said, metal tubes are extremely difficult to crush with anything wider than a tubing cutter blade. Bending is another matter though. As others have said, for a heavy rider, I'd feel more comfortable with the saddle being centered on the clamp. Then again, back when I started cycling weighing in at around 185-190 lbs., I used a titanium railed saddle (Fizik Pave) with a non-setback seatpost (Specialized suspension seatpost) with the saddle shoved all the way back for at least a few thousand miles (my weight dropped off pretty quickly) with no ill effects. I'm still using that same saddle 4 years later with many more thousands of miles on it but with a setback seatpost.

TallRider

Thanks again for the responses here, everybody.

First, it's good that I put a no-setback post on my Raleigh. Here is what it looks like now, with the clamp is in a reasonably okay spot on the saddle rails, although still toward the back.



I'd never realized until recently that, because of using a fork that was not designed for the Raleigh frame, I'd slackened the seat tube angle quite a bit.
I got fitted on my two Centurion frames awhile back, and the saddles on those bikes were far ahead of where I'd kept my saddle position on the Raleigh, but I didn't realize this because the seatpost looked similar - all of the difference between the Centurions and the Raleigh come from seat tube angle.
Here's a picture (click for larger version) of the Raleigh, which may give seasoned observers a sense of the slack seat tube angle.




Second, on the Specialized Alias saddle with hollow titanium rails, on my Centurion Ironman. I bought another Cannondale seatpost with no setback, on eBay, to nail the clamp position that shows up in picture #2 in this thread. Otherwise, the saddle is clamped nearly all the way back on the rails, as shown in picture #3 in this thread. I'd rather not bend the rails. It most likely wouldn't be a problem, but it looks mechanically tacky, and spending $15 on a post is better than bending the rails of a $90 saddle.

masiman

I would think a fork change would have almost no effect on your seat position as it will not change the geometry of your compartment. I can only think that you are saying you repositioned yourself to compensate for handling differences? Did I miss something?

TallRider

all I know for sure here is that the seat tube angle is notably slacker on my Raleigh than on my two Centurions.

since the Raleigh was designed and marketed as a go-fast road frame, I'm figuring that it wasn't designed with a really slack seat tube angle.
And I built up the frame in a way that would change the geometry. I got the main frame from our Raleigh rep when I worked at a shop in high school, and put a Reynolds 753 steel fork on it, plus a Shimano LX headset.

The fork already has a higher crown height than the fork designed to go with the Raleigh frame, plus the LX headset has a rather high stack height. Both of these factors raise the front end of the bike, which slackens the head and seat tube angles.

There are lots of threads about putting new (generally carbon) forks on older road bikes, that add significant height to the front end of the bike and change the angles.

Anyway, I had to put a seatpost with no setback on my Raleigh to get the saddle in a reasonable position, because the seat tube angle is so slack.

Wanderer

You maybe right with regards to clamping force, which is miniscule, compared to force applied by the weight of the rider on the outside edges of the clamp. Any clamp. The stress concentrator will be wherever the two items meet. Not the clamp itself.....

DannoXYZ 

You contradict yourself. It really depends upon the alloys used. MatWeb.com is your friend.

masiman

Very true. I have only swapped a fork once, decades ago. That fork was very "tour" oriented rather than the club racing I was after. It used to be that aftermarket forks were not very good, and now you can readily get aftermarket forks that are better than OEM.

Without measuring, your fork looks pretty good. Wheel is close to the crown, close to downtube. Not too much rake or trail. It does not look like you could lower the front end much (too steepen your head and seat tube angles) unless you switched to a smaller front wheel and fork.

TallRider

Yeah, I don't know for sure if it's the fork. I never saw the original fork, just got the main frame and had to find a fork to go with it. This was in 1995 and I was just starting to work at a bike shop.
I think the top tube on the frame was intended to be perfectly flat/horizontal, and it has a slight rise heading toward the head tube. I think that is from the combination of the fork and headset. But I'm not sure.
Maybe the frame was just designed with a slack seat-tube angle. Or maybe, just maybe, it's a prototype with weird geometry. It was unpainted when I got it (and a friend who worked at a powdercoat place did the red for free).