A low bottom bracket allows the rider / bike to shift weight quicker from one side to the other. Darren Baum reported (The Darren by Baum: An adjustable bike for finding the ideal gravel geometry - Velo) that lots of (pneumatic) trail combined with a low bottom bracket can be tricky/dangerous because the strong uprighting effect of the high trail will initiate a forceful movement after a sudden and strong steering movement at high speed (like when you try to avoid an object on the road that you see very late) and the low bottom bracket does not slow this down much so that you are catapulted quickly to the side opposite of where you were steering towards. The article is a bit vague on the mechanism at play here so the above description is, to a degree, my interpretation of what is causing the phenomenon that Darren experiences.
The article does not state what trail Darren’s test bike had but based on his Orbis bikes, which are the result of this tests, I assume the trail is ~65mm with 35mm tires, 66.5mm with 40mm tires and ~68mm with 45mm tires. The bottom bracket drop also isn’t mentioned but I assume it was ~78mm (again based on his Orbis+ bike).
With 35mm tires he didn’t experience the “forceful self righting” problem described above.
With 40mm tires the problem was noticable but manageable with wider handlebars.
With 45mm tires wider handlebars alone were not enough. The problem was eased by increasing bottom bracket height by ~4mm to a 75mm bb drop (the 4mm is my guess, based on a bb drop comparison between the Orbis+ and the Orbis X bike because the former is designed for 32 to 40 mm wide tires and the latter is designed for 40 to 45 mm tires).
I was planning with a 88mm bb drop (feasible with 160mm cranks), 63mm trail and 34mm wide (measured width) tires mostly ridden on tarmac (and a bit of gravel).
Compared to the 35 mm wide tire case in which Darren didn’t experience any problems my bike’s trail is 3mm lower which is advantageous, my tires are 1mm or 2mm less wide (depending on the rim he used) which is also advantageous, but my bb is 10mm lower.
I have not ridden a bike with an unusually big bottom bracket drop (>82mm) and somewhat high trail (>62mm), at least not avoiding an obstacle at high speed. So I’m curious whether you have experienced something similar as Darren or whether you have ridden a high trail + low bb combination on the road and did not experience this effect?
I made a drop bar bike with 85mm trail and 85mm BB drop. It was absolutely the best descending gravel bike I’ve ever ridden, but you had to handle it like a mountain bike. I don’t think I’d want it as a pure road bike though. Like you said, as trail goes up, the responsiveness of your steering decreases.
This may be a complete non issue on loose surfaces because the pneumatic trail which Darren associates this issue with is very small there (less friction between tire and ground).
There is also a theoretical issue/question related to this: pneumatic trail, i.e. the friction between tire and road, should not increase the uprighting effect of fork trail.
Of course a bigger tire radius creates more trail but I am not sure whether that is the effect at play here also because 5mm of tire difference only creates about a 1.8mm trail difference.
So it’s an open question to me what caused the phenomenon Darren described. Maybe I should give him a call or invite him onto here
Perhaps I missed it, but I didn’t see any mention of tire pressure – which I think would impart a big effect on the tire’s contact patch, and thus pneumatic trail…?
I second this.
One of the models we offer at my place of work is an offroad bikepacking/rough gravel bike with a low BB (87.5mm) and what would be considered high trail (88mm) for chunky tires (29 x 2.4"-3.0").
It’s an amazing descending bike yet handles smooth flat sealed roads respectably as well even with the chunky tyres if you don’t mind the extra resistance. I’ve been riding one for over 2 years and I’ve never felt that the geo is dangerous and I’ve ridden a bit of everything on it. Quite the opposite actually. If anything I feel the handling is confidence-inspiring.
There was no specific tire pressure mentioned. But what was mentioned was that he experimented a lot with the right tire pressure. So I guess he ended up with a tire pressure appropriate for the tire size. So wider tire = lower tire pressure = bigger contact patch/higher pneumatic trail.
In fact a change in tire pressure (besides the weight resting on the tire) is the only thing that changes the size of the contact patch, not the width/shape of the tire.
Edit: I just read the article again and actually specific tire pressure will mentioned: “Baum eventually settled on a front tyre pressure of 41psi for a 34mm tyre, 30psi for a 40mm tyre, and 25psi for a 45mm tyre (to suit a total weight, rider and bike, of 90kg).” Those precious are quite low!
Mechanical trail is ~ trail independent of tire size. The two are correlated. I guess I should have said “mechanical trail” instead of “trail” in my OP.
Edit: Mechanical trail isn’t independent of tire size. I had thought of it as being independent of tire size. Learned something new today:) But they are pretty titely correlated so it doesn’t make that much difference which one you look at.
Yep. Mechanical trail does get mentioned in the article, but it gets dismissed as equivalent to head tube angle which isn’t right. Angle does change trail but they do different things. It seems like what Darren’s describing is the experience of aligning torque rather than pneumatic trail. Changing traction by changing contact patch size will change aligning torque as described independent of pneumatic trail. Weight and leverage on the front tire also changes aligning torque, and adjusting that is ultimately what this experiment is doing. Riders already do that when a significant tire change does undesirable things, they’ll move around bars and stem and saddle location with varying degrees of success.
