Brompton steering geometry

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PT1029
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Brompton steering geometry

Post by PT1029 »

Does anyone have the steering geometry numbers for a Brompton (head angle, fork rake)?
I probably could measure, though head angle might be a bit tricky to measure accurately, so some paper numbers would be helpful. No figures found on an online search, but others might be better at searching than me.

I am comparing steering geometries to help work out why I like one of my frames and less so the other (not Brompton). I have a list of my frames/other bikes fork trail and steering flop. It would be nice to have the Brompton figures for a comparison (as the Brompton could best described as twitchy - ok for what I use it for, but wouldn't want to do touring with that geometry!).
rogerzilla
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Re: Brompton steering geometry

Post by rogerzilla »

I've never seen it quoted. Remember it slackens when you sit on the bike, as the rear end squats. How much, depends on the type of suspension block.
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simonineaston
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Re: Brompton steering geometry

Post by simonineaston »

I wonder what is to be gained by listing the factors such as trail, rake etc.. It's not as if the respective measurements will be readily available for all bikes you are likely to buy and a far better indication as to whether you like the bike would be a test ride. Useful if you are going to design & build your own for sure - but are you?
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S
(on the look out for Armageddon, on board a Brompton nano & ever-changing Moultons)
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531colin
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Re: Brompton steering geometry

Post by 531colin »

Its part of the human condition to look for a "magic number".....in this case, a single number to wholly describe how a bike handles.
If you are going to compare 2 bikes with 700c wheels, similar tyres, similar frame geometry and similar riding position, then yes, "trail" is not a bad place to start.

But comparing Brompton trail to a "big wheeler"? I think thats pointless.

Care to list your existing bikes, with their head angle/offset/anything else relevant?
cycle tramp
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Re: Brompton steering geometry

Post by cycle tramp »

PT1029 wrote: 23 Oct 2021, 4:17pm Does anyone have the steering geometry numbers for a Brompton (head angle, fork rake)?
I probably could measure, though head angle might be a bit tricky to measure accurately, so some paper numbers would be helpful. No figures found on an online search, but others might be better at searching than me.

I am comparing steering geometries to help work out why I like one of my frames and less so the other (not Brompton). I have a list of my frames/other bikes fork trail and steering flop. It would be nice to have the Brompton figures for a comparison (as the Brompton could best described as twitchy - ok for what I use it for, but wouldn't want to do touring with that geometry!).
Bare in mind that its also weight distribution and tyre pressure which can also change the feel of a bicycle. What happens if you drop the front tyre pressure, and put some weight on the front rack mounting block*
(Kinetics do a brompton up grade to either 16 jnch/2 inch tyres or 20 inch wheels, which may also dampen the handling)

(*i set off from the shops the other day - and I thought bloomin'n hileck what's wrong with my steering? The answer was nothing, I'd just loaded my panniers full of food on my rear rack, and not as I normally do, on my front rack)
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simonineaston
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Re: Brompton steering geometry

Post by simonineaston »

The magic number? I'd give my Brompton 87.6, which is the average of the product of its scores, expressed as a fraction of 100... I like the fact that its all orange, and so is the luggage and the D lock - although some other people find this twee, so maybe down to 85... depending on your point of view, I spose. Wouldn't take my hands off the 'bars, mind
S
(on the look out for Armageddon, on board a Brompton nano & ever-changing Moultons)
pwa
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Re: Brompton steering geometry

Post by pwa »

The only thing a newcomer to Bromptons needs to know is that compared to most non-race bikes the steering is sensitive to inputs. One way to adjust for this is to steer by leaning rather than by turning the bar, except at low speeds. Once you get used to it, it ceases to be an issue.
Tangled Metal
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Re: Brompton steering geometry

Post by Tangled Metal »

Our load the front block might help I've read.
Tangled Metal
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Re: Brompton steering geometry

Post by Tangled Metal »

20" upgrade? Nice! Although I doubt it helps much with potholes. I've had experience with the stock Brompton wheels!
PT1029
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Re: Brompton steering geometry

Post by PT1029 »

I am hoping to write something in greater depth at some point in the future. However, for now, in my case, "touring" is often camping/4 panniers/low riders at the front/ small bar bag (2.5l I think, a 7 litre bag gave me cruising speed wheel wobbel).
Some decades ago I had a new tourer built - it felt horribly unstable (unladen) on some bends (* I now put this down to being stable!), unlike my retired Holdworth Mistral tourer which seemed to handle well.
Both 72 deg head angle, tourer 2" for rake, Holdworth 2 1/2" fork rake. A had some 2 1/2" rake forks made for the tourer, problem solved.
For touring on rougher roads (communist east Europe, Iceland), I got a mountain bike for touring also. It road well enough, but on slow steep climbs and stopping/starting, the steering had a tendency to flop one way or the other, some climbs if slow enough became a bit of a wrestling match - with a bit of speed there wasn't a problem. The bike had a 30" or a tad less bottom gear, so I wasn't heaving a big gear.
Although the mountain bike road well enough, it wasn't like the tourer, which rode better fully laden, without fail went where I wanted ot to go and stuck to the road like glue. Just to confirm this possibly rose tinted memory, I recently went camping on the tourer for the fist time in 20 years or more (been using the mountain bike in the interim) and soon found yes, it handled brilliantly (for me), so my memory was not rose tinted.
So, I plan to replace the mountain bike, something with 35 - 38mm 700c (as 26" is on a slow decline), and I am tall enough for 700c, and probably don't need 27.5" (650 B for those who remember the size first time round - nothing is new.....!). Hence my looking at geometry. For camping I want bigger tyres for bridle ways etc which my tourer won't accommodate.
The problem with the mountain bike is what I termed steering flop, so I googled it, and the answer appeared.
https://en.wikipedia.org/wiki/Bicycle_a ... e_geometry
So while fork trail is a good place to start, steering flop is an important contributor to handling.
I have a list of my tourer (original and as now) steering flop and fork trail, along with my mountain bike and a host of other off the peg mechines as a comparison. It seems I am in a minority, I prefer low trail, nearly all the bikes I have listed are higher trail geometries(which might suit others). So I seem to have my answer, something with a trail 45 - 47mm and flop of 13 - 14mm.
The tourer as supplied was 61mm trail and 18mm flop,
the mountain bike is 58mm trail and 17mm flop.
I used this to calculate the figures:- http://www.yojimg.net/bike/web_tools/trailcalc.php
There is also http://www.kreuzotter.de/english/elenk.htm which I have not yet used - this is where Simonineaston's fork geometry diagram comes from.
Why I wanted the Bropmton figures wasn't as a serious comparison, more out of curiosity. The Brompton's handling is so vastly different (twitchy) I was just curious to know what the numbers were, Like Simon in Easton, I take a hand off the handle bars with trepidation! That said, over all the Brompton is good for what I use it for (these days usually to collect a car club car for the few occasions I need to drive).

*Describing handling (to quoate someone in a thread on this forum) is difficult, as different people decribe handling in different ways, and some words mean different things to different people, or in my case, different things to the same person at different times!. Although I thought the original geomerty was unstable, I now think it was stable. The "instability" was, being (what I now call) stable, the bike had a desire to keep going straight ahead, and I had to force it to turn. I now have a less stable geometry, so it is easier to turn the bars away from straight ahead.
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simonineaston
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Re: Brompton steering geometry

Post by simonineaston »

That said, over all the Brompton is good for what I use it for (these days usually to collect a car club car for the few occasions I need to drive).
Perfect! I'd owned a Brompton for ages before one of my chums pointed out that the Brompton fold means the dirty chain is innermost and thus as far away from dirtying the inside of a car as poss.. Classy stuff !!
S
(on the look out for Armageddon, on board a Brompton nano & ever-changing Moultons)
scottg
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Re: Brompton steering geometry

Post by scottg »

simonineaston wrote: 24 Oct 2021, 11:46am I wonder what is to be gained by listing the factors such as trail, rake etc.. It's not as if the respective measurements will be readily available for all bikes you are likely to buy and a far better indication as to whether you like the bike would be a test ride. [snip]
I've a shirt that answers your query.......
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PT1029
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Re: Brompton steering geometry

Post by PT1029 »

The numbers might be useful for some as a pointer to what might be good (for that individual) to ride.
A test ride is short, won't replicate a steep climb -(unless the test ride is in a hilly twn of course), rough track or loaded with luggage, so a test ride may not be the full answer. My mountain bike with its steering flop would probably be ok on a test ride.
The best answer is probably to use (borrow or rent?) the bike for a few days in situations where you are likely to use it.
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fausto99
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Re: Brompton steering geometry

Post by fausto99 »

PT1029 wrote: 23 Oct 2021, 4:17pm Does anyone have the steering geometry numbers for a Brompton (head angle, fork rake)?
I probably could measure, though head angle might be a bit tricky to measure accurately, so some paper numbers would be helpful. No figures found on an online search, but others might be better at searching than me.

I am comparing steering geometries to help work out why I like one of my frames and less so the other (not Brompton). I have a list of my frames/other bikes fork trail and steering flop. It would be nice to have the Brompton figures for a comparison (as the Brompton could best described as twitchy - ok for what I use it for, but wouldn't want to do touring with that geometry!).
I’ve done the same with my motley collection of bikes which include my Dad’s 1930s racer, F frame Moulton’s and a 2021 Pinarello.
I don’t measure angles but concentrate on the distance between contact points in order to get the bikes to feel the same. I am mostly successful but failed with my Dad’s bike and the Moultons because of their lack of trail/castor effect. This is due to a large fork offset probably due to fashion rather than any technical reason. They are both very twitchy.
My current build, a M F frame wreck with many tubes replaced by Carbon fibre will have a a much straighter fork with more trail and be hopefully less twitchy.
rogerzilla
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Re: Brompton steering geometry

Post by rogerzilla »

The Moulton (and Brompton) have little trail because of their small wheels. The distance between the tyre patch and the point the steering axis intersects the road is only about half that with a 700c wheel, if the head angle and offset were equal. You could add trail with less (or zero) offset, as you say, to make the bike less twitchy.

Moulton F frames also suffer from changing steering geometry under braking (the head angle increases as the front suspension dives), something addressed to a greater or lesser extent by the leading-link designs of the AM, TSR, etc.
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