So, I’m in the planning stages for a gravel + touring + bikepacking bike, which may also end up doing a few brevets (but most likely will not). It will have road/gravel spacing, 2x, 700c/29” wheels, and most likely 50mm offset (Rodeo Labs Spork v4.0). I expect to be able to use tyres from 30-35mm up to ~52-55mm at the rear, and maybe 60mm at the front. The bike will have relatively narrow drop bars (40-42cm at the hoods), and will not have a suspension fork; a dropper post is unlikely.
The big question is what head angle to use, and I’ve seen bikes from ~69-72° with similar offsets. It seems that if you’re a mountain biker wide bars and slack head angles are the go, but gravel racers tend to be the opposite; I’m somewhere in between, but definitely won’t be doing any gravel racing, or anything very technical. Steep yes, technical no.
Ideally, it’d be possible to test a variety of bikes and see what gels, but that’s not so easy to do.
Suggestions?
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I’d suggest look at 70.5-71. Thats not as steep as some of the grav bikes around, but not as slack as those which start to blend into MTB geo (edit: As in hardtail classic geo of years past).
Easy to over-think this stuff. You will adapt to whatever you end up on, so don’t stress about it would be my advice. If the bike is well designed then the trail figure will be in line with where it should be, which is arguably more important.
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MTB are 66 degrees or slacker, and have 44 mm offset, I don’t see (m)any gravel bikes that blend into that amount of trail.
What is your preference? More or less trail?
The pros and cons are fairly well known:
More trail= more stable at holding a line including cornering , especially when hitting bumps ore in deep sand (both of which reduce effective trail).
Less trail = less wheel flop which means easier to control the balance of the bike at slow speeds.
Personally, if it was me, and especially if paired riding was involved, I’d like to have less fork offset, so that I could have a decent amount of trail, while still minimizing wheel flop (which would be an even bigger issue with a loaded bike).
On thing to consider is that other factors, like wheel base, tire size and BB height, all contribute to a bike feeling more stable at speed or in rough surfaces or sand, so if you increase those, you might be able to get away with a bit less trail.
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FWIW, I definitely don’t like low trail, and IME it doesn’t really help with wheel flop at low speeds, but what it did do (for me) was make it much more difficult to maintain a straight line when drafting, or to corner predictably. I also had serious problems with shimmy/tank slappers. Stable is good; it’s not hard to make the bike turn when needed.
FWIW, normal road geometry (~60mm trail) to older MTB (~70mm or so) feels good to me, and should be achievable by swapping tyres. As for fork offset, 50mm allows a bit slacker HTA and thus a longer front centre, making it a bit easier to avoid toe overlap. While I could just make the top tube longer and use a really short stem, I’d like to use a Shockstop stem, and the flippable ones start at 80mm, plus I have 90, 100 and 110mm “in stock.”
Something like 71-71.5° is looking like a decent compromise.
As many have already said: you need to look at trail, and wheel flop. And while many manufacturers don’t mention trail, you can often find it on dedicated geometry sites.
And, imho, it’s recommended to also take into account front center, to limit toe overlap. Because bikes with the same trail and morevor less the same reach can have enough difference in front center to be either ok or annoying when it comes to toe overlap.
I love MTBs with head angles of 65° or less, but on a gravel bike it does not work for me. I think it is the combination of wheel flop, longer stem and narrow bars that creates an issue. 70°-71° works well for me on a gravel bike. And toe overlap is no issue for me, maybe because every single road bike I ever owned had toe overlap, or because I do not ride anything too technical on a gravel bike.
NB: I do understand trail, wheel flop, etc, but the question is really about what might work the best, and yes, YMMV.
Overlap is something I hate, and the front centre will be long enough that it can’t happen. Juggling the front end geometry, stem length and bars to make everything work is a bit tricky, but I can mock the cockpit stuff up on my old bike, so that’s doable.
The head angle is a bit of a conundrum, where slacker = more stable and less chance of overlap, but steeper = less wheel flop, and maybe nicer handling in road mode…