Fork rake/head angle/trail

[fossala]

I'm guessing this is a mistake but always willing to learn.

If the head angle and fork rake is the same on a geometry table wouldn't that lead to the same trail?

Stanforth do their "skyelander" bike in mid/low trail and low trail. But in their 22", 23" and 24" models the geometry appears the same but the trail varies.

https://www.stanforthbikes.co.uk/skyela ... b-geometry

[Brucey]

the mid-trail numbers make sense (*), the low trail numbers must be wrong; needless to say if you want 10mm less trail, you need about 10mm more fork offset. So the rake values for low trail ought to be ~73mm (for 71 degree head angle) and ~68mm (for 72 degree head angle), rather than 66 and 58mm respectively.

(*) compared with the table here http://johnforester.com/Articles/BicycleEng/Kvale%20Geometry.pdf NB the wheel diameter may be slightly different.

cheers

[fossala]

the mid-trail numbers make sense (*), the low trail numbers must be wrong; needless to say if you want 10mm less trail, you need about 10mm more fork offset. So the rake values for low trail ought to be ~73mm (for 71 degree head angle) and ~68mm (for 72 degree head angle), rather than 66 and 58mm respectively.

(*) compared with the table here http://johnforester.com/Articles/BicycleEng/Kvale%20Geometry.pdf NB the wheel diameter may be slightly different.

cheers

Cheers Brucey, it's pondering rather than possible purchase. I was more curious if there was any way to have the same head angle and offset but different trail. In my head it was obvious you couldn't but I never underestimate my ignorance.

[iandusud]

It is possible to have differing trail with a given head angle and fork offset depending on the section of the tyre.

Brucey, you say that the low trail numbers must be wrong but they would appear to calculate correctly using this tool: http://yojimg.net/bike/web_tools/trailcalc.php

Cheers, Ian

[Brucey]

....... Brucey, you say that the low trail numbers must be wrong but they would appear to calculate correctly using this tool: http://yojimg.net/bike/web_tools/trailcalc.php

Cheers, Ian

the mid trail values do, but the low trail values (in the second table in the link) don't. If you want 39mm trail at 72 degree head angle you need more offset than 58mm.

cheers

[iandusud]

Hi Brucey, I was looking at the quoted figure of 66mm offset for the smaller frame sizes. I've now seen that they quote 58mm for the larger frame sizes even though both use a 72° head angle. Clearly something is amiss!

Ian

[Brucey]

they say they will make any geometry you like but the easy way to make a 'low trail' version is to keep the same head angles etc as in the mid trail version, So I wonder if the quoted head angles are right too, although having said that the front centres are fairly constant in the smaller sizes with the quoted dimensions.

cheers

[Jamesh]

Am I mistaken - for many years I had thought head angle excluded rake?
I.e. 73/73 frame would have parallel head and head tubes irrespective of fork rake?

Thanks in advance

Cheers James

[kylecycler]

They say they will make any geometry you like but the easy way to make a 'low trail' version is to keep the same head angles etc as in the mid trail version, So I wonder if the quoted head angles are right too, although having said that the front centres are fairly constant in the smaller sizes with the quoted dimensions.

cheers

Brucey will know this, of course, but the great thing about 'old style' curved steel forks is that they can be raked to any offset - they come straight and are bent ('cold set') on a former. One of my heroes is the American framebuilder Dave Kirk, one of the most highly respected and experienced framebuilders in America - this is his fork bender:

If you look closely you can see that there is an adjustable 'stop' which allows for any rake, so rake - and trail - are infinitely adjustable, more or less. Presumably Stanforth rake their fork legs in a similar manner.

To do the job properly, mind you, I think a true 'low trail' bicycle needs to have a steeper head angle as well as a larger fork offset, although that might just apply to 'porteur' bicycles, designed specifically to carry a heavy front load, and that's not what the Stanforth is trying to be. Lower trail geometry tends only to work with wider tyres, which have 'pneumatic trail', to work in conjunction with the steeper head angle / larger fork rake. This is an early '60s René Herse porteur. I've drawn in the 'trail lines'. It not only has a fair amount of fork rake but also a fairly steep head angle, likely 73-74 degrees.

(To my eyes it's also one of the most elegant bicycles ever built (of its kind) - the bars, the brakes, the chainguard, the valanced mudguards...)

[kylecycler]

Am I mistaken - for many years I had thought head angle excluded rake?
I.e. 73/73 frame would have parallel head and head tubes irrespective of fork rake?

Thanks in advance

Cheers James

Head angles would be independent of fork rake, but there would be an optimum fork rake for a given head angle to give what would be considered to be 'optimum' trail.* Optimum trail for a road bike is - supposedly - somewhere around 60mm. More trail, i.e. less fork rake, or a shallower head angle with the same fork rake, might be more stable at higher speeds, but then you apparently (eventually) run into problems with the bike moving around too much under you when climbing out of the saddle. Also, a bicycle with higher trail tends to 'lock in' to its line in a corner and it gets harder to vary your position, such as to avoid a pothole. These are just extremes, though, and the rider will compensate up to a point, usually quite unconsciously.

It also depends on tyre width. A 650B/584mm wheel with 47mm tyres will have the same overall diameter as a 700C/622mm wheel with 28mm tyres, but for it to handle properly it should ideally have lower trail, i.e. more fork rake, to factor in the pneumatic trail of the wider tyres.

On a bicycle with a 73 degree head angle, using the trail calculator posted above by iandusud, 700C wheels with 28mm tyres and 650B wheels with 47mm tyres would both need a fork rake of 45mm to give 58mm of trail. But the wider 650B/47mm setup would have more pneumatic trail and would handle better with, say, 55mm fork rake and therefore 47mm of trail. For the 700C/28mm setup, some might prefer less fork rake, say 40mm, to give more stability on descents, the downside being that it would tend to move around more when climbing.

*To help you to understand that, trail is the distance between where the two lines intersect the 'ground line' in the porteur illustration in the post above - you can see they're very close together - the trail on that bicycle would probably be less than 30mm. This is to avoid 'wheel flop', which would mean you'd have to wrestle with the bars when carrying a heavy load on the front rack.

Here's the trail lines drawn on to a Carrera Subway (with 650B/584mm (aka 27.5") wheels and 48mm tyres):

It has a straight fork, of course, but the rake is achieved by angling the fork legs forward rather than bending them. You can also see how more rake would give less trail, and vice versa. You'll see the trail is a lot higher than the porteur (but then a porteur is an extreme example of low trail geometry) - too high unladen - I use a Subway identical to that and it handles better with two full rear panniers - the front lightens up and although it's obviously a fair bit harder to climb laden it feels nicer and just about as easy, especially when out of the saddle. That applies on-road; off-road it helps to have more trail to help you to turn tighter on looser surfaces, but I don't tend to go off-road very often so I wouldn't know about that!

Trail isn't the be all and end all concerning handling - there's head angle, front centre, wheelbase, bottom bracket drop... and they're interdependent - but it's considered to be one of the most important factors. Others with far more knowledge and practical experience than me - the above is all theory - like Brucey, 531colin & co will no doubt explain further, but I'm just trying to help since you asked the question.