Touring Frame Geometry

[al_yrpal]

I have been fretting a little about my purchase of a Carerra Subway Commuter bike as a tourer. When I purchased it I was very careful to make absolutely sure that I was comfortable on it, particularly as I find it uncomfortable to assume the crouching position demanded by some bikes.

I have some frame sizings from standard Thorns and I have been measuring other well know tourers. Thinking about it a bit I identified the following dimensions as the important ones: Wheelbase because I have read that that is critical to comfort, albiet at the expense of agility and speed, Chain Stay Length to ensure heel clearance in front of panniers. Fork rake because it affects steering, and Bottom Bracket Height because it affects how far you are off the ground, and thus stability and ground clearance on rough surfaces.

Code: Select all

Dimension  Dawes  Thorn  Kona  Subway Surley LHT
Fork Rake   65      66    45     50     45
Chain Stay  440     445   425    415    460
Wheelbase  1040   1200    1051  1085    1064
BB Height    70     42     45     15     47  (distance below wheel centres)

I measured the various dimensions from photographs, and apart from the Subway, they must be viewed as approximate. I dont think actual steerer length or precise saddle position are important to list because they are fully adjustable by changing stem height and length or sliding the saddle back and forth on its rails. The caveat being that I feel comfortable and effective on the bike by virtue of the positions that I have adjusted things to. My heels clear the panniers with plenty to spare.

I am quite happy now having made these measurements. I find them very reassuring particularly because the important dimensions of my Subway agree closely with the Koga and Dawes. I expect that the Subways high bottom bracket is related to its MTB heritage. I don't think that being all of 30mm higher than on a Koga is any disadvantage. I did always feel a bit high off the ground on my old Dawes.

The surprising thing here is the apparent much longer wheelbase of the Thorn. It is quite a a bit different to the other two classic touring bikes and the Subway, giving 30mm more heel clearance with the frame top tube apparently being the place where the extra length is - more of a Chopper!

Al

[Si]

TBH, you have to take most of it with a pinch of salt. Individual measurements don't really determine bike characteristics by themselves - all the other measurements interact with them (and it's not unknown for some companies to use them as marketing hype). The important thing is, as you've stated, when you are on your bike you are perfectly comfortable and able to use it for its intended purpose - get that sorted and you are onto a winner no matter what the figures say

For instance, on paper the bike sat next to me shouldn't even be on the road if you use conventional thought regarding these figures, but it works great.

[PH]

Sorry, but I think your measurements are way out
There's no way those bikes are going to have that difference on rake for one thing and I don't know what model Thorn you're using But I have a large Raven and the wheelbase is a lot less than that.
As Si says, you're not going to learn much from the exercise, there are too many variables. Chainstay length plays a big part in comfort, sitting between wheels will always be more comfortable than sitting on top of one.
The Surly LHT is classic typical bike geometry and they're good enough to show all dimensions for both the 26" and 700c versions;
http://surlybikes.com/frames/long_haul_trucker_frame/

[al_yrpal]

Sorry, but I think your measurements are way out
There's no way those bikes are going to have that difference on rake for one thing and I don't know what model Thorn you're using But I have a large Raven and the wheelbase is a lot less than that.
As Si says, you're not going to learn much from the exercise, there are too many variables. Chainstay length plays a big part in comfort, sitting between wheels will always be more comfortable than sitting on top of one.
The Surly LHT is classic typical bike geometry and they're good enough to show all dimensions for both the 26" and 700c versions;
http://surlybikes.com/frames/long_haul_trucker_frame/

The rake is in mm, its not an angle, as are all dimensions. I took the measurements from side on shots of several Sherpas by measuring their quoted chainstay length and then adjusting the measured wheelbase using the same scale. They all seemed to have wheelbases in the 1200mm to 1250mm range. The Sherpa BB Height and chainstay length are figures direct from the Thorn brochure. I did check the photos to make sure they had identical vertical and horizontal scales. I note the Ravens also have longish chainstays, but there are no side on photos to calculate wheelbase from.

Surely the only variables that actually matter are the ones I have quoted. On various explanations of frame geometry there is a lot about angles. Downtube angle is almost irrelavent because you can slide saddles back and forth quite a bit, and many steerer angles seem to be very similar.

I am not claiming to know all about this subject but am hoping others will comment on the 'black art' of frame design and enlighten me as to what good touring frame geometry is? There is so much discussion about what makes a good bike, and so many bikes use identical bolt on parts it intrigues me as to what exactly distinguishes good from mediocre, and how much bike design is actually 'me too' , tradition, or folklore? Otherwise all the bike advice that people pass on is just limited experience and not based on any science.

Al

[vernon]

I fear that your paper based exercise is pointless.

Try riding the bikes to determine fitness for purpose. My bikes have widely varying angles and wheelbases, I've toured on them all apart from my MTB and been comfortable on them all.

I've never resorted to measuring anything as I don't think that I could do anything useful with the knowledge other than swap statistics with like-minded cyclists.

It's a bit like determining fitness for purpose of hi-fi systems from their paper specs without resorting to listening to them.

[al_yrpal]

It's a bit like determining fitness for purpose of hi-fi systems from their paper specs without resorting to listening to them.

As with all such devices, groups of products for similar purposes have similar characteristics, that doesn't happen by accident. Having toured on a MTB, dropped bar tourer, and flat bar tourer its easy to remember the different sort of aches that each was capable of producing.

I would agree with your statement if you inserted the word ONLY between 'systems' and 'from'. The numbers are a good starting point. Because I spent a lifetime designing things, and because there are so many with strong opinions about what often seem to be ludicrously expensive machines with some undefinable magic element, I thought I would try to find out what the essential differences are scientifically?


Al

[georgew]

Tony Oliver is the man you're looking for if the ideal touring frame is your goal. Try to get hold of his book which gives good guidance on what is the ideal touring frame.
http://www.amazon.co.uk/Touring-Bikes-P ... 1852233397

That said, I think that the Surly LHT comes very close in my opinion in that it appears to have been designed as a touring machine from the ground up.

[james01]

Wheelbase because I have read that that is critical to comfort, albiet at the expense of agility and speed,

Generally a long bike will feel more stable and comfortable but less agile. However wheelbase shouldn't affect speed, unless you factor in the (tiny) weight of additional metal in a long wheelbase frame.

[al_yrpal]

Thanks George,

Thats extremely interesting, on the Surley LHT frames page they give all the dimensions, and it lines up with what I have measured and deduced quite closely. I have put it in the table on the OP. As for the book (£79) our local library has it - once again thanks.

I can now see clearly that my own bike differs significantly from the norm by having slightly shorter Chainstays, a higher Bottom Bracket and a slightly longer wheelbase. Downhill at great speed it is certainly very stable.

Al

[scottg]

The bikes you have listed are all rear loaders, front loaders with have steeper head angles and more rake.
Tire size also effects front end geometry, fatter tires require less trail.
Measuring frames off the building jigs is notoriously difficult.
Rack stiffness has a large effect on handling of a loaded bike, racks with poor triangulation can make an otherwise
good bike handle poorly when loaded.
Bike setup effects handling.

"The lug is not the frame,
The geometry is not the frame,
The alignment is not the frame,
The material is not the frame,
The frame is the frame"
ATMO

[531colin]

Most of the manufacturers you list actually publish frame data, and others will give you the information if you ask. Thorn would not tell me head angle and fork offset data, they fear industrial espionage, apparently.
Head angle, fork offset and tyre size conspire to determine trail, which you need to know before you can start to consider bike handling. Search these forums for the gospel according to CJ.
Tony Oliver managed to write a book on frame design without mentioning the word trail. (However, having had a traditional 700c tourer built using his numbers, I can confirm that his practical experience gives me the handling I want, and a dishless rear wheel. )
A difference in head angle of 0.5 deg. is enough to alter steering characteristics (at least in the "brighter" handling end of the spectrum). Have a look at the published data. Manufacturers often use a different head angle on different sizes of bike, but use the same fork offset. How can that be right? Surly do the LHT in 26"wheel and 700c wheel versions. In one series they use the same head angle and fork offset throughout, in the other series the head angle varies but the offset does not. How can that be right?
You also need to consider the "front centres" ie. front hub to BB axle to get an idea if toe overlap will be a problem. This is a problem in small bikes with 700c wheels, the handlebars can be too far forward before you get toe clearance, if fork offset and head angle are not chosen carefully.
Good luck with your studies, and keep us posted!

[531colin]

I can now see clearly that my own bike differs significantly from the norm by having slightly shorter Chainstays, a higher Bottom Bracket and a slightly longer wheelbase.

Downhill at great speed it is certainly very stable.

Al

That's the mountain bike "heritage" .

It's probably got shallow head angle for the fork offset too, which gives lots of stability at high speed - what you want in a loaded tourer, but not in a bunch sprint!

[al_yrpal]

I can now see clearly that my own bike differs significantly from the norm by having slightly shorter Chainstays, a higher Bottom Bracket and a slightly longer wheelbase.

Downhill at great speed it is certainly very stable.

Al

That's the mountain bike "heritage" .

It's probably got shallow head angle for the fork offset too, which gives lots of stability at high speed - what you want in a loaded tourer, but not in a bunch sprint!

Thanks Colin,

I had not thought such things were so important. I found my Galaxy to be an extremely irritating bike because the front wheel would flop all over the place and you could never lean the thing against anything at all with confidence, unlike my new bike. If you look at supermarket trolley wheels in operation they flop back in the direction of movement. The kick forward on bike forks seems to be there purely to achieve toe clearance To get stabilty back you then have to rake the stem forward which of course helps regain stability.

Thorn are one of the companies that appear to build a bit of black art in keeping vital details closely guarded. I will run a tape measure over a few at the DA. Surly publish the lot and Surley were in the Times today, so everyone in Sloane Square will have one soon!

Its all fascinating!

Al

You will probably be building some new wheels for me in a couple of years. I plan to 'upgrade' to the Shimano 11 speed hub if it proves to be any good. I used to design sprag clutches and I think their basic design looks sound but needs a little development and 'customer based' testing.

[531colin]

Al
You really have to read up on "trail" but I'll have a quick stab. If you run a straight line through the head tube it will hit ground at some point. Trail is the distance between this point and the point (behind it) where the front wheel touches ground. Determinants of steering characteristics are;-
1 head angle (shallow = stable, steep= lively)
2 trail
But this gets complex because trail is set by head angle, fork offset, and wheel size.
More trail = more stable
everything else being left the same;-
shallow head angle = more trail
small fork offset = more trail
So fork offset is not just to keep your toes out of the wheel, but well-chosen offset and head angle can help in that direction.

Now the business of a parked bike's handlebars turning back on themselves and the bike falling over is simple in concept, but difficult thereafter! The simple bit - the weight of the bike will settle to the lowest position. If that is with the 'bars backwards, and theres nothing holding the 'bars, then round they go! The hard part is working out at what 'bar position the weight is at its lowest, but thats all in these pages somewhere, too. One of the unsung benefits of STI and bar end shifters is that the extra cables tend to hold the steering "midships".

Handlebar stem length governing bike handling is also a myth, particularly when riding no hands.

[531colin]

I can now see clearly that my own bike differs significantly from the norm by having slightly shorter Chainstays, a higher Bottom Bracket and a slightly longer wheelbase.

Downhill at great speed it is certainly very stable.

Al

I'll have another try at this, last time not very good!
High BB = ATB = rock clearance
Short chainstays = ATB = easier to pop the front wheel up over obstacles.
Long wheel base with short chainstays = long front centres = ATB, on a rough descent, its very comforting to have the front wheel out in front, not underneath you like on a race bike.

Stability loaded at speed will come from a shallow head angle, again ATB (although not out of place on a tourer - unless you like "bright" handling)