In part 1 I explained why I didn’t like the steering characteristics of one of my bikes.
In summary, on one bike I like the steering, on one bike I didn’t like the steering but changed the forks, so I now like the steering. On the 3rd bike I don’t like the steering at low speed.
Part 1 also described the factors that affect steering geometry: -
• Outer wheel diameter.
• Head tube angle.
• Fork rake/fork offset.
Other factors include;
• Tyre - ground contact area
• Angle of lean.
• Gyroscopic effect of the rotating front wheel.
Now I will try to put some numbers to these and other bikes, and (briefly) what conflicting considerations need to be considered in designing a frame. Hopefully all this remains easily accessible and not lost in the mists of time.
A look at my bikes,:-
PT – original Geometry as originally supplied
Custom ATB Geometry as supplied/is currently. I have ignored 2” off road tyre use.
PT – current Geometry as now
Mistral - original/27” Holdsworth Mistral whose steering I liked (now 700 X 32 tyres, but basically the same).
Wheel flop is how much the front end of the frame lowers at its lowest point when the steering is turned from straight ahead.
“u – stability factor”. Jones “stability criteria” in Bicycling Science. Experience says stability criteria is best when u is between -1 and -3. See Bicycling Science for more details.
In the tables below,
Re: Fork trail values, colour denotes steering I liked, and steering I did not like.
What I did not like: - approx 60mm fork trail, approx 17 – 18mm wheel flop
What I did like: - approx 46 mm fork trail, approx 13 – 14mm wheel flop
Thus it looks like I prefer a low(er) fork trail and low(er) wheel flop values.
This gives rise to 2 points:-
• If I got a bike with fatter 700c tyres, what acceptable (to me) steering geometry could I use whilst still avoiding pedal (shoe) overlap with the front mud guard? Retaining a 72 deg head tube/steering axis angle may well make shoe/mudguard clearance difficult with wider 700c tyres and mudguards.
- There is also the question of the bigger tyre size. This will have more pneumatic trail, so the geometric trail might need to be a little lower to allow for the higher pneumatic trail. How this is worked out/calculated I don't know. I would hazard a guess the the pneumatic trail (a product of tyre size (diameter and fwidth) and tyre pressure would be measured by the contact surface area. Just a guess.
What acceptable steering geometry could I use whilst still avoiding shoe overlap with the front mud guard?
We need to take a step back and look at what factors you need to take into account for a bike frame (as distinct from the narrowed point of steering geometry).
What factors you need to take into account for a bike frame (as distinct from the narrower point of steering geometry).
As well as the previously mentioned points of: -
• Outer wheel diameter.
• Head tube angle.
• Fork rake/fork offset.
• Tyre - ground contact area (“pneumatic trail”)
• Angle of lean.
• Gyroscopic effect of the rotating front wheel.
We must also consider overall frame design, thus we also need to consider: -
• Trying to avoid front wheel/mud guard – foot/shoe overlap.
• Handlebar reach from the saddle.
If the rider is fairly tall, there is likely to be plenty of space, the top tube/cross bar can be long enough to allow the front wheel/mudguard to be far enough forward to clear the rider’s feet when pedalling while the steering is turned.
The main design issues appear when the bike is being designed for a short rider. Due to the short length of the bike (short riders can’t lean/reach so far forwards), design compromises need to be made to fit the wheel front wheel in the space available whilst trying to maintain shoe/mudguard clearance (no overlap). They will need a shorter forward reach, so needing a shorted top tube. If the rider is short enough, their shoe/toe clips will overlap the front mud guard. During low speed manoeuvring (which allows for a reasonable amount of steering turn), the rider’s feet may well clash with the mudguard. For some riders this is a problem they can live with, for some it spells disaster as their pedals stop turning and the rider come to an unexpected – and possibly undignified - stop.
How to overcome this. The fore – aft position of the saddle should be set with respect to the bottom bracket, so you don’t want to move the saddle forwards (or use a steeper seat tube angle) to allow for more reach.
The options for extra foot – mudguard clearance are: -
• A shallower head tube/steering axis angle.
• A larger fork rake/fork offset.
• A smaller wheel size.
In reality, it is likely to be a combination of these.
A shallower head tube/steering axis angle.
Making the head tube angle shallower puts the front wheel further forward, so increasing shoe/mudguard clearance (good), but also results in increasing wheel flop (potentially detrimental to sheering characteristics).
A larger fork rake/fork offset.
Increasing the fork rake/fork offset will also increasing shoe/mudguard clearance (good). It will also however reduce fork trail, making the steering less stable.
A smaller wheel size.
Using a smaller wheel size for small sized frames/riders is probably the best way forwards. You can maintain a fairly normal steering geometry while increasing shoe/mudguard clearance.
Some makes use 700c wheels on the medium and larger frames, and use 26” wheels on their smaller sized frames.
Chris Juden sums it up well: -
Thus, the small rider will generally have to suffer with one of the following:
1. long top tube—in order to avoid toe overlap while using a stock fork and a standard head angle.
2. super stable bike—due to a very shallow head angle coupled with short fork rake. This is done to avoid toe overlap with a stock fork while offering the rider a short top tube. The bike becomes so stable that it is hard to get it to turn, and the front wheel snakes out ahead of the rider when standing out of the saddle.
3. toe overlap—in order to offer a short top tube, stock fork, and standard head angle. Any of these situations can be uncomfortable or dangerous. To fit small riders with a stable bike and no toe overlap, either use a custom fork with more rake, or smaller wheels and geometry adjusted to offer large enough fork trail for stability.
This effect of this can be seen with the 2022 Trek FX3. I have chosen this just as an example: -
It might be why some women (statistically more likely to be short) appear not so confident in their bike handling. They have to contend with possibly too stable a steering (“The bike becomes so stable that it is hard to get it to turn” above), plus there is more wheel flop to contend with.
On the other hand, Sonder seem to have the same geometry for all frame sizes on some of their models, OK if you like a big fork trail and big steering flop.
So what is “normal” geometry?
What is available out there? I seem to prefer lower fork trail/smaller wheel flop geometry. I looked to see if what was generally on sale was lower or higher fork trail. It seems I am in the minority, with most bikes being higher fork trail/wheel flop geometry.
Looking at the bike geometries tables below: -
blue highlighting is a geometry close to the low fork trail/wheel flop geometry I prefer.
Yellow highlighting is higher trail/higher wheel flop geometry I don't prefer.
There is a lot more yellow than blue, which puts my preferred steering geometry in the minority.
Pink highlighting – my guess would be more in a class of its own!(?) Some may well like this geometry, I suspect I would not.
Some of the off road bikes have quite big fork trail sizes. I guess is this is to help keep going in a straight line and preventing side deflections when hitting obsticles with the front wheel while riding off road.
Ultimately, there is no "right" or "wrong" geometry, only right and wrong geometries that do or do not work for you.
Happy to correct any incorrect numbers in the table.
Bikes chosen that interest me, or just as a reference. Figures come variously from this forum, the internet, manufactures supplied data on their websites and "Cycle" magazine.
PT – original – my tourer as originally supplied
Custom ATB My mountain bike.
PT – current My tourer as is currently, with the replacement forks.
-----------------------------------------------------------------------------------------------------------------
