Correcting a frame mistake.

[stewartpratt]

When the front wheel hits a bump, the axle goes upwards and forwards......doesn't it?

Oh, yes, sorry - I misunderstood your previous post. Yes, upwards and forwards, so when the wheel hits a bump the axle moves forwards in relation to the frame - I thought you were implying it moved backwards, though on reflection I'm not entirely sure why I thought that.

When I mentioned "vertical" in the context of the bracing of the head/steerer tubes I meant the longitudinal vertical plane (ie fore/aft and up/down) as opposed to laterally (where flex occurs by twisting the frame, eg sending the headset to the left and the saddle to the right)... I appreciate I may have started a chain of misunderstanding there - sorry

[Brucey]

Don't forget the crown and steerer tube when considering overall fork stiffness.

Surely the steerer is firmly braced by the headset bearings, which sit in the head tube, which is braced (vertically at least) by the top and down tubes. I wouldn't have thought that it contributes anything to the fork's vertical compliance...?

well, it varies. Note that what Colin says is correct IMHO, that the fore-and aft movements of the fork give the apparent vertical compliance.

However, some headsets are stiffer than others. Some are intrinsically less easy to deflect than others by virtue of their geometry. (And a 1" steerer is kind of twangy or even very twangy by comparison with a 1 1/8" one...)

So anyhoo...the geometry of a traditional cup and cone style headset lower race allows the lower part of the fork to rotate around a point near the crown quite easily. If it didn't do this the headset design would be intolerant to imperfect fitment, like some others are. This movement happens all the time as you go down the road; the highest bending loads are in the steerer just above the crown. Large displacements are discouraged by the net tensile load that starts to appear as the deflection increases

By contrast some headsets with cartridge bearings don't work at all well with flexible steerers; the bearings get badly loaded up by flexing, just like they would be if they were installed wrongly.

Going the other way, does anyone else remember the the 'red s' headset? Basically a stronglight A9 with delrin fitted parts. I didn't like the roller bearings anyway but the plastic parts are not very stiff. A chum who had one of these heasets bent his 531ST steerer simply by putting the brakes on.I am sure that this wouldn't have happened with a stiffer headset.

To summarise though; most headsets do not significantly restrain the small movements that occur as a flexible fork tracks over road irregularities.

cheers

[stewartpratt]

Note that what Colin says is correct IMHO, that the fore-and aft movements of the fork give the apparent vertical compliance.

Surely in terms of reducing the transmission of bumps to the rider, the fact that a fork necessarily deflects more forwards than upwards is circumstantial (since the head angle is generally quite upright). The actual factor in question is one of compliance in the axis of the bump's force vector. And that is near-vertical...

If you had a fork which enabled the hub to move only horizontally then the hub would by definition move vertically with any vertical variation in the surface; the vertical deflection at the head tube would then be solely dependent on the relative distances of it and the hub from the rear wheel's contact point - again I'm not doing the back-of-the-envelope trigonometry to work it out but I'm suspecting 1cm or so of forward deflection doesn't account for any significant change in the vertical deflection of the head tube. I am willing to be proved wrong by someone with a handy envelope

[Brucey]

72 degree head angle gives 18 degrees included angle. My envelope says...

tan 18 is 0.325. So a 1cm horizontal deflection gives a 3.25mm vertical deflection, even if the fork blades don't flex per se.

cheers

[stewartpratt]

72 degree head angle gives 18 degrees included angle. My envelope says... tan 18 is 0.325. So a 1cm horizontal deflection gives a 3.25mm vertical deflection, even if the fork blades don't flex per se.

Yes, but that's demonstrating my point about the head angle requiring the fork to move more horizontally than vertically for any given deflection, and isn't related to the point about compliance being negligible if vertical deflection is eliminated. I contend that it's [near-]vertical deflection that gives actual compliance, not "that the fore-and aft movements of the fork give the apparent vertical compliance". In other words, the horizontal deflection is purely a prerequisite for vertical deflection given the basic form of a fork, and itself contributes nothing to compliance.

(Note that suspension forks try to minimise horizontal deflection and provide significant [near-]vertical deflection.)

[Brucey]

you are appearing to ignore the fact that, on most bikes bike that are comfortable, the forks do move that amount or more.

You are also mistaking the compromises inherent in any telescopic suspension system as in inherent or necessary feature of that, or any other, suspension design. There are plenty of leading link, trailing arm etc designs where any vertical movement of a wheel is accompanied, deliberately, by a significant horizontal displacement.

Only inertial forces of the unpsrung mass resist a horizontal deflection component in response to a vertical bump load in such suspension systems. And if the unsprung mass is small, this matter little.

cheers

[stewartpratt]

you are appearing to ignore the fact that, on most bikes bike that are comfortable, the forks do move that amount or more.

Er, I don't think I am. I broadly agree with that point. What I'm saying is that in order for a fork to be comfortable (taking the definition of that as something like "reducing the forces and displacements transmitted to the frame from surface irregularities encountered by the front wheel"), it needs to displace vertically; and that in order for a fork to displace vertically it has to undergo a rather larger displacement horizontally. But the horizontal(ish) vector is not itself beneficial in terms of comfort; only the vertical(ish) is.

You are also mistaking the compromises inherent in any telescopic suspension system as in inherent or necessary feature of that, or any other, suspension design. There are plenty of leading link, trailing arm etc designs where any vertical movement of a wheel is accompanied, deliberately, by a significant horizontal displacement.

Yes, fair point, though in fairness (a) those designs haven't survived well and (b) forks designed for off-road are a different bag of beans because unlike a road the terrain has lumps of significant size which can strike the wheel at a rather more significantly off-vertical angle, and it's that which gave rise to the J-shaped travel of some link forks. With road bumps, that curve isn't beneficial.

[Brucey]

oh fair enough I think I see what you are driving at a bit more now... mind you you can't see the vertical movement from the saddle, only the horizontal easily. I look for the movement on a new bike, and try and judge to what extent the fork, the tyre, the frame, the bars etc affect the feel.

Incidentally I agree about suspensions with long travel on steered, braked, or driven wheels; these have all kinds of interesting compromises to them. But a good example of a very well proven suspension that defies conventional logic is the short leading link design, used on MZs, Honda Cubs, and Moultons. At some points in the travel the small bump wheel movement is very similar to that of a nice springy 'rigid' fork...

cheers

[bikepacker]

Just to update everyone who contributed to this thread.

Following some of the advice and comments plus a long conversation with Colin531 at the York Rally, I decided to give the frame another try. I have therefore made some adjustments that appear to make the bike more comfortable for me to ride, although up to now I have only taken it on day rides. Here is what I have done so far:

Changed the chainset for a Stronglight Impact, fitting 3mm longer bottom bracket axle than recommended. This has given me an extra 14mm width on my pedal centres allowing me to re-position my cleats to avoid the chainstay but still ride with a pain free leg.

Changed the 60mm stem for a 70mm one and compensated by raising the bars by 5mm. Also put a softer walled tyre on the front and kept the Marathon + on the back. Thinking is, if I puncture the front it is less hassle to repair on the road. Both of these together (tried them separately and it didn’t make that much difference) feel they are giving me a comfortable riding position and made the front end less harsh.

Once we get some half decent weather I will be off with a full camping load to give it a good try.

[steeleagle]

Steel is stiffer than titanium. Titanium is noted for is flexibility.

Your frame probably feels painfully stiff for other reasons such as oversize tubing and compact geometry. This is important to bear in mind if you opt for a new steel frame, as if you get the same modern features like oversize and compact geo, you may end up with even worse problems.

[531colin]

Steel is stiffer than titanium. Titanium is noted for is flexibility.

Your frame probably feels painfully stiff for other reasons such as oversize tubing and compact geometry. This is important to bear in mind if you opt for a new steel frame, as if you get the same modern features like oversize and compact geo, you may end up with even worse problems.

Gram for gram, the stiffness of Titanium and Steel are not too different.
See page 2 of this thread.

[bikepacker]

Further update.

I have decided to finally give up on this frame (my wife thinks I am mad for persevering with it). I have tried very combination, even to completely swapping parts with my other bikes, and still find it harsh to ride. Extra bits recommend by others, have been purchase but made little or no difference. Where CJ gets his comfortable smooth ride from I don’t know but it does not happen with my frame, I can assure him of that.

It will either be sold or hung on the wall to remind me of one of my not so good purchases.

[531colin]

Thats a real shame.
It does point out that its a good idea to "waste" a day and take out one or two test bikes from the shop.
I don't think you will have much trouble selling it on, particularly if its a 57, as there is no stock!

If you decide to get a new steel frame rather than recondition your existing 531, I would recommend that you take a test ride on a similar new steel frame.....many steel touring frames are now stiffer than the old 531 was.