steering geometry angle, offset, trail etc

[Brucey]

http://www.johnforester.com/Articles/BicycleEng/Kvale%20Geometry.pdf

is an interesting article on the topic; usefully it contains a table of trail values for different fork offsets and head angles, valid for 680mm wheels.

cheers

[CJ]

Does a good job of demolishing past nonsense and provides a sound theory of trail, but then the authors spoil it by resorting to speculative mysteries when they find behaviour that is not explained by trail.

It is also very out of date. Kvale & Corbett say that the book 'Bicycling Science' virtually ignores steering geometry, but that was in 1974. Perhaps their comment spurred Whitt & Wilson to expand that topic and include Jones's Un-Rideable Bicycle work in the 2nd edition. Bicycling Science is now in its 3rd edition and has even more about steering. Even that however, cannot take account of some very recent experiments which showed that a bicycle could be stable without any trail at all, provided the mass-centre of the steered part (comprising fork, wheel, handebar etc.) is positioned forward of the steering axis.

We now know that the problem for frame designers in the 1970s was that they'd reached the limit of what you can do with steel. Very light racing frames became either too easily dented or excessively flexible - or a bit of both - so they needed even more trail to counteract shimmy. The move to intrinsically lighter materials answered that problem and with the adoption of ready-made 'carbon' forks control of steering geometry was handed over to fork manufacturers. With that it should have ceased to be an interesting topic of discussion - amongst racing cyclists at least.

[teh]

You said it, "more trail to counteract shimmy"!!!

Which might explain why, large frames with their reduced trail, tend to shimmy!!!

[Ayesha]

I can remember going to Major Nicholls in Smethwick.
He built frames for local raceboys, as well as tourists. He also built frames for local shops who couldn't keep up with demand.

Hanging in the back room were many of frames with varying geometry, all based on the same rider.

There's nothing like a bit of "Trial and error", hey,,,,,

Nowadays, with carbon fibre construction IN A MOLD, the mold costs many times more than the frame, so when the engineer decides the angles ( including forks ) its got to be "Right first time".

Framebuilders are of two varieties. There are 'generalisers' and 'perfectionists'.

The 'generalisers' offer a 'tried and trusted' steering geometry. The 'perfectionists' offer an engineering formulated steering geometry based on the rider's bodily dimensions and weight.

"You pays your money...... "

[teh]

So, why do steering angles get steeper as frames get larger?

[CJ]

So, why do steering angles get steeper as frames get larger?

Because a bigger rider needs a longer top-tube to put his handlebars far enough away from the saddle, but will have read all manner of nonsense about the desirability of a short wheelbase!

[teh]

I see, it's the tall rider's fault for reading nonsense.

I think you'll find that the real reason is that most frame builders/designers haven't a clue - oh, and certain stupid UCI regulations on bike length.

I have had several incidences of speed-wobble, which I think is a different thing from shimmy. I also owned a frame with a 75 degree head angle and large-offset forks for a while which had extremely unpleasant handling. As a result I became a very nervous descender, and almost any bike I rode became very twitchy as a result of my vice-like grip. Strangely, after fitting proportional cranks (thus lowering my saddle by ~3cm), I've had not even a hint of a wobble.

[Brucey]

How restrictive are the UCI regs on frame design really?

The bike's maximum length should be 185 cm, and maximum width 50 cm.
The bike's minimum weight should be 6.8 kg.
The peak of the saddle must be at least 5 cm behind a vertical plane passing through the bottom bracket spindle.
The saddle must be between 24 cm and 30 cm in length.
The distance between the bottom bracket spindle and the ground must be between 24 cm and 30 cm.
The distance between the vertical passing through the lower bracket spindle and the front wheel spindle must be between 54 cm and 65 cm.
The distance between the vertical passing through the bottom bracket spindle and the rear wheel spindle must be between 35 cm and 50 cm.
The maximum internal distance between the front fork ends is 10.5 cm, and of the rear stays 13.5 cm.

I have not done the sums super-accurately, but I think the front centres rule is the sticking point and I think it means that the top tube can be about 60cm long with a sensible fork angle and offset.

Do many people really need a longer top tube than this?

cheers

[531colin]

....................

Framebuilders are of two varieties. There are 'generalisers' and 'perfectionists'.

................. The 'perfectionists' offer an engineering formulated steering geometry based on the rider's bodily dimensions and weight..........................

To me "steering geometry" means head angle and fork offset. How should these vary with rider dimensions/weight?

[531colin]

So, why do steering angles get steeper as frames get larger?

I'm going to echo that. Why do head angles get steeper on larger frames? And also when did it start ?
Now lots of touring frames have steeper head angles in the larger sizes.....can't blame UCI regs for that!
In point of fact, when you are descending a steep, rough, bendy road (or track) with a touring load on board, its very reassuring to have the front wheel well out in front, rather than underneath you.

..................

I have had several incidences of speed-wobble, which I think is a different thing from shimmy. I also owned a frame with a 75 degree head angle and large-offset forks for a while which had extremely unpleasant handling. As a result I became a very nervous descender, and almost any bike I rode became very twitchy as a result of my vice-like grip. Strangely, after fitting proportional cranks (thus lowering my saddle by ~3cm), I've had not even a hint of a wobble.

I think it works like this. The wheels aren't perfectly balanced. At a particular speed, the frequency of the wheel rotations chimes with the resonant frequency of "something" and it all starts shimmying, that is, laterally flexing at its resonant frequency. The amplitude of the flexing can build up alarmingly to a speed wobble, or the motor cyclists "tank sl@pper" where the handlebar grips (feel like they might) slap the petrol tank. You can kill the build up by changing the characteristics of the structure...pressing your knee on the top tube, or even just shifting your weight, can be enough.
I'm surprised a bike with 75deg head and long fork offset was actually rideable for any length of time. Surely you didn't tame that by lowering the saddle a bit?

[robc02]

Do many people really need a longer top tube than this?

No, as long as they are using drop bars and currently accepted stem lengths - as those directly affected by these UCI rules will be.

I understand the purpose of those rules was to preserve the conventional drop bar racing bike for UCI governed events and to prevent the intrusion of aero/prone riding positions (except for time trial events). I seem to remember that the 5cm saddle setback (combined with saddle length) was a bit of an issue - designed to restrict the use of "excessively" prone positions, but criticised because some shorter riders would be unable to use their normal riding position.

It is interesting to see these rules quoted like this - I have only seen selected bits in the past.

a bigger rider needs a longer top-tube to put his handlebars far enough away from the saddle, but will have read all manner of nonsense about the desirability of a short wheelbase!

Aided and abetted by some frame builders (frequently without any scientific/engineering background - as evidenced by clear confusion between strength and stiffness of different tube types - but I digress). Having said that, I have some sympathy with them as the doubtful nature of our understanding has been highlighted by the studies referred to in CJ's first post (bikes with zero trail etc.). Who could blame them for going along with the current perceived wisdom, even if it later turns out to be incomplete and/or inaccurate?

To me "steering geometry" means head angle and fork offset. How should these vary with rider dimensions/weight?

One would think they shouldn't, as long as weight distribution stays more or less the same.

[Brucey]

It is interesting to see these rules quoted like this - I have only seen selected bits in the past.

I found them on this webpage

http://www.ineed2know.org/transportation/racing-bikes.htm

I assume they are correct; there are others too...

e.g. you are not allowed front-wheel drive..

cheers

[CJ]

I see, it's the tall rider's fault for reading nonsense.

No, it's the fault of the journo's who write it, who stand in awe of pro racers and swallow every superstition they they hear from that quarter without question (because they don't know enough science) and gold-plate it by publication in print.

I think you'll find that the real reason is that most frame builders/designers haven't a clue - oh, and certain stupid UCI regulations on bike length.

Quite so, all part of the same problem. Myth and superstition are the rule in cycle design. And whilst the UCI limits should not be a problem for most people, it is for some and nevertheless inhibits cycle design beyond the field of racing. For it UCI thinks it's unsafe to make the front of a bike any longer than 65cm, it takes a little courage (or the engineering knowledge which is sadly lacking in so many cycle designers) to make ANY type of bike longer than that. There is also a tendency to suppose that if 65cm is the absolute limit, surely it's safer to keep well within it?

Then there are the racers who prattle about short stems making a bike handle worse and denigrating the riding ability of anyone who has a problem with toe overlap... It all adds up to pressure to keep the front centres as close as possible to a figure halfway between the UCI's two magic numbers.

There is on the other hand, a far more robust safety argument for extending the wheelbase of a bicycle in direct proportion to the height of the rider's centre of gravity, since that is the ONLY way to ensure that a tall rider can stop in the same distance as an average rider. An average pedal cyclist will already topple over the handlebars should he attempt to decellerate as quickly as most other vehicles can stop, so it's clearly irresponsible to make that even worse!

[Brucey]

whilst I absolutely agree the UCI rules are daft and that they could (and should) be revised, I think we should take what is said in CJ's link by Zinn with a healthy pinch of salt.

1) 'body size manadation' is obviously wrong in principle but it will happen anyway in practice. Very small people are similarly in effect excluded from many forms of competitive cycle sport and not just by UCI frame design rules.

2) Zinn's sample bike that 'must' be outside the UCI rules could easily have been made within them had he so desired. He's chosen an extra inch-and a bit front centres for 'added stability'. By the time you need this extra length on a CX bike (e.g. when descending) you should be moving your CoG by many inches, not just one. What you really need perhaps are longer chainstays when climbing, which he has hardly bothered with by comparison. He's got a 60mm trail figure. This might be just what you want on a large road frame that is built a bit light, but I've ridden a CX bike with a 60mm trail and it was perfectly horrid on rough surfaces (because of the camber thrusts) so IMHO it is not a good design for that purpose. He also has a 12cm stem AFAICT (as drawn) which is hardly adventurous on a frame that size. He could have built with longer chainstays, a 73 degree head angle (again 'perfectly normal') a 2cm shorter top tube, matched fork offset, and a 2cm longer stem and he would have been fine. Arguably the bike would be lighter, stiffer, and handle better, too.

I don't think you can really blame the UCI for framebuilders be daft/sheeplike enough to build their touring bikes to racing dimensions, either.

The 'UCI sticker' rule and the costs of design approval are frankly ridiculous though. Since there are many rules (not all of which are set in the frame design) which might be broken, the commissaires of any event must have the ability to exclude any rider who is using non-conforming equipment. It is effectively up to the rider to ensure that his bike isn't set-up to infringe the rules; I say let them ride what they like in the knowledge that if (by design or adjustment) their bike doesn't conform, they can be excluded, fined, or put on 'three strikes' notice. People would pretty soon make sure their bikes conformed if the rules were enforced.

Already the intent of the rules (that you can't 'buy performance' or gain advantage from using stupidly lightweight/fragile equipment) is obstructed because bike weights are not routinely checked in amateur races. How hard would this be to do?

I'll clamber down off my soapbox now...

cheers

[CJ]

Zinn's sample bike that 'must' be outside the UCI rules could easily have been made within them had he so desired. He's chosen an extra inch-and a bit front centres for 'added stability'. By the time you need this extra length on a CX bike (e.g. when descending) you should be moving your CoG by many inches, not just one.

Surely ALL riders in that situation will be moving their CoG backwards as far as they can. But they all have to keep hold of the handlebars. So if a line drawn from the handlebars to the point where the front wheel contacts the road is made steeper on a tall rider's bike (either through a steeper head angle or a disproportionally longer extension or both), his forward stability will be compromised relative to an average rider.