57mm trail for gravel? HTA, trail and handling debate

[gorkypl]

Hello Paceliners,

I'm debating over geometry of my next gravel bike, and still need to decide about the headtube angle. Hope you can help.

My current CX/gravel has 72 degrees HTA paired with 50mm offset fork, which gives ~60mm trail with 38mm tyres.
This is already considered 'racy' geometry I think, but I am still not happy with the handling. The most annoying aspect is 'self-amplification' of the steering input - when I turn the handlebars, the bike feels like it falls to the side, and by this tries to turn even more. I'm not sure if it's called wheel flop or something else, but it's kinda annoying.

I don't have this feeling on my road bike, which has 73 degrees HTA and 45mm offset - these values give 57mm trail with 28mm tyres.
This bike does not amplificate the steering and feels much better in this aspect, but I think it may be too 'twitchy' if used for gravel in this exact configuration. Of course there may be other factors in play - compared to the other bike it has shorter wheelbase and lower BB, but I try to isolate the trail from the equation (if possible).

My thinking is that if I aim for 57mm trail on gravel (by steepening the HTA to 72.5 degrees and pairing it with 50mm fork offset), it should eliminate this self-amplification of turn, but wider tyres (no less than 38mm) will mute the steering a bit, so it will be not as nervous as the road bike is. Does it sound reasonable?

It may be worth noting that both on road and gravel bikes I currently use 120mm stem and 40cm handlebars. For this new bike I would go with 100-110mm stem and 42cm handlebars - not sure if this alone would noticeably change the steering (and into which direction).

I'd appreciate all insights

[rice rocket]

Is your bike on BikeInsights.com? They calculate a wheel flop number, which you can use to compare against other bikes.

The number in itself has value, but needs to be held in context of all the other numbers.


FWIW, my road bike has a 60mm trail and my gravel bike, a 67mm trail and I don't notice a significant difference. I also mostly descend in the drops, and the gravel bike has bars that flare significantly wider than my road bars, which probably negates some of the "self-steer" effect. You should consider the profile of the tires you're using, it can cause a similar feeling.

[robt57]

Generally and IMO.

Loose deeper gravel more rake, woods hard single track and course crud pavement 57 be up my alley [60CM bike].

But I like 51-2 road and 56-7 othewise for rigid on looser generally. Super loose I take the chopper, my 29er...

That looks like CX geom to me. Example and fuel for thought, look at the current Crux. 61mm trail I think...


>If you will use 650b at any point, take the compromise for the 47-650b into consideration maybe..

[Spoker]

The effect of ( much) wider handlebars is large when it comes to control and stability imo.

[David Kirk]

Originally Posted by gorkypl

Hello Paceliners,

I'm debating over geometry of my next gravel bike, and still need to decide about the headtube angle. Hope you can help.

My current CX/gravel has 72 degrees HTA paired with 50mm offset fork, which gives ~60mm trail with 38mm tyres.
This is already considered 'racy' geometry I think, but I am still not happy with the handling. The most annoying aspect is 'self-amplification' of the steering input - when I turn the handlebars, the bike feels like it falls to the side, and by this tries to turn even more. I'm not sure if it's called wheel flop or something else, but it's kinda annoying.

I don't have this feeling on my road bike, which has 73 degrees HTA and 45mm offset - these values give 57mm trail with 28mm tyres.
This bike does not amplificate the steering and feels much better in this aspect, but I think it may be too 'twitchy' if used for gravel in this exact configuration. Of course there may be other factors in play - compared to the other bike it has shorter wheelbase and lower BB, but I try to isolate the trail from the equation (if possible).

My thinking is that if I aim for 57mm trail on gravel (by steepening the HTA to 72.5 degrees and pairing it with 50mm fork offset), it should eliminate this self-amplification of turn, but wider tyres (no less than 38mm) will mute the steering a bit, so it will be not as nervous as the road bike is. Does it sound reasonable?

It may be worth noting that both on road and gravel bikes I currently use 120mm stem and 40cm handlebars. For this new bike I would go with 100-110mm stem and 42cm handlebars - not sure if this alone would noticeably change the steering (and into which direction).

I'd appreciate all insights

When tires get wider and softer they tend to have more distortion of the casing around the contact patch. As the tire rolls along there is a pressure wave (think of it as a gentle shallow fold) of tire built up in front of the contact patch. When the tire is steered to one side the steering wants to pivot around that fold of tire and this gives it the feeling of the bike wanting to add to the steering angle you gave it and to fall to the inside.

The easiest way to feel this is to play with tire pressure on a fatbike. Get the pressure too low and the tire will distort so much that once you steer it even a little bike it wants to crab. They become super hard to ride on dry grippy pavement.

You can experiment with your 38 mm tire but raising the pressure a good bit (not as a matter of course but just to learn something) and I’ll bet that dive to the inside goes away. It does this because the higher pressure reduces the casing distortion and the fold of tire that the steering want to pivot around. If it does you know it’s the “pneumatic trail” that is giving you issues.

As tires get bigger and softer (and the tire distorts more) the frameset can be made with less trail because we know that tire will do its thing and add to it. On soft surfaces you will feel it less but on smooth dry pavement it can be annoying. So back off on the trail and I’ll bet you like it more.


dave

[dustyrider]

When stuff like this comes up, I always think of Tom Cruise and Robert Duval trying to explain to each other how to setup their nascar in Days of Thunder. I have no understanding of the maths surrounding your questions, but I do know there are some very knowledgeable folks in this realm here so hopefully they’ll chime in.

I’m a trial and error person myself and have learned to enjoy the journey as much as the end result. I’d mess around with stem length first, then possibly bars especially if they needed a new wrap, all the while I’d be keeping an eye out for a proof of concept fork that doesn’t cost much. Good luck in the chase for the perfect feel, it’s awesome when it comes together!

[Peter P.]

I recommend gorkypl get a copy of Jan Heine's book, The All-Road Bike Revolution. There's in-depth explanation of bike handling/steering, including head angle, fork rake, trail, pneumatic trail, and how they all affect road bikes, cyclocross bikes, gravel bikes, and so on. There are even recommendations of various combinations for certain bikes.

Sounds like David Kirk already has a handle on it.

Still, get the book. You won't be disappointed.

[spoonrobot]

Originally Posted by gorkypl

My thinking is that if I aim for 57mm trail on gravel (by steepening the HTA to 72.5 degrees and pairing it with 50mm fork offset), it should eliminate this self-amplification of turn, but wider tyres (no less than 38mm) will mute the steering a bit, so it will be not as nervous as the road bike is. Does it sound reasonable?

I don't think .5° head angle change is going to give you what you want. If this is a geometry peculiarity and not tire pressure related*, I would expect you'd need to increase head tube angle by a full degree to 73°, if keeping the fork rake the same.

*or headset related, offset bars related, fork out of plane related - all things I've seen cause floppy feeling on what is generally not floppy geometry.

What tires? Some tires have more self-steer/pneumatic trail/ground trail than others. Schwalbe G-Ones are notorious for turning a non-floppy bike into a mustang.

[NHAero]

My custom Strong has 73 HTA and 51mm offset for 48mm trail with 650Bx38 tires. This combo makes it handle like a road bike. And it is also fine with 700Cx25 at twice the pressure (80 psi). This is what older all rounder bikes often had as front end geo. I copied my ‘72 Bob Jackson, which has been run with skinny tubulars unladen to fully loaded for touring with 32mm tires and handles fine.

If I were riding all gravel and no pavement I'd likely choose to slacken the HTA and increase trail. New England dirt roads aren't like a lot of the chunky gravel I see photos of here from our Western forum members.