Toe Overlap

General cycling advice ( NOT technical ! )
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Mick F
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Re: Toe Overlap

Post by Mick F »

CJ, you are spot on.
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rogerzilla
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Re: Toe Overlap

Post by rogerzilla »

I have a Harry Quinn track bike. It's a wonderful machine and, once you're used to the super-quick steering and the tendency of the rear wheel to skip on bumps, it's great on the road. I did the 2019 Dun Run on it and it was the easiest ride I've had of my eight goes.

BUT...it lives up to the HQ reputation for short wheelbase. The top tube is tiny, the fork offset is a massive 50mm - that's where the twitchy steering comes from - just so the front wheel clears the downtube, and the toe overlap is prodigious. It takes some getting used to. In tight slow turns, pointing my toes at the sky gets me round. At speed it is not an issue. The most embarrassing thing is to start away with a foot on the wrong side of the front wheel!
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531colin
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Re: Toe Overlap

Post by 531colin »

I'm always tempted to drop the BB by 10mm on a bike where I expect the rider to use 165mm cranks compared to a bike I expect to be fitted with 170mm cranks.
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Re: Toe Overlap

Post by rogerzilla »

I set the saddle 5mm higher with 165mm cranks (1/4", really, because I tend to think in inches). Then the maximum leg extension is the same.
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531colin
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Re: Toe Overlap

Post by 531colin »

rogerzilla wrote: 15 Apr 2021, 10:01pm I set the saddle 5mm higher with 165mm cranks (1/4", really, because I tend to think in inches). Then the maximum leg extension is the same.
This approach won't really help somebody get a toe down?
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Re: Toe Overlap

Post by rogerzilla »

I'd prioritise pedalling ergonomics over getting a foot down (in the saddle) but I appreciate, having set bikes up for other people, that the latter is sometimes an overriding concern.
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CJ
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Re: Toe Overlap

Post by CJ »

531colin wrote: 15 Apr 2021, 8:40pm I'm always tempted to drop the BB by 10mm on a bike where I expect the rider to use 165mm cranks compared to a bike I expect to be fitted with 170mm cranks.
Good decision.

One thing to beware of however, is that lowering the bottom-bracket causes the chain to enter the front mech cage at a higher position. If you're using a MTB front mech on a bike with 700C wheels, even a conventional bottom-bracket height produces a smaller 'chainstay angle' (between chainstay and seat-tube) than MTB mechs are designed for (ie 66°-69°, compared to 63°-66° for road mechs). I'd expected Shimano to tweak down their MTB mech chainstay angle specification with the advent of the 29er, but that doesn't seem to have happened. Perhaps they simply expect 29ers to have even higher bottom-brackets!

The frame I designed for my wife has a 'drop' of 88mm, which with 37-622 tyres produces a bottom-bracket height of 260mm, which is just like you say, 10mm less than the 270mm that was standard for touring bikes with 170mm cranks, in the 70s and 80s when these were the norm. (Bottom-brackets have drifted upwards since then, which is a bad thing - except for those who insist on pedalling round corners or don't keep a safe distance from kerbs!) In spite of a reasonably steep seat tube (73.5°) the resulting chainstay angle is only 62° - a bit small even for a road mech! But I'd made some measurements and thought we'd get away with it, which we have - just! If the Deore XT 2×10 mech is positioned to clear the outer ring teeth by 2mm, the chain rubs the top of the cage in big (36T) sprocket, but if positioned 1mm higher, at maximum clearance, it doesn't rub.

Shifting is fine, apart from a tendency for the chain to overshoot the inner ring, which I think must be another consequence of pushing the chainstay-angle envelope. It makes sense that rotating the mech 4° backwards from the furthest back it's supposed to be, it's cage has less guiding influence over the falling chain, which may thus fall further to the left, missing the inner ring's teeth. This can easily be cured by fitting a Jump-Stop, although not quite so easily in this case as it needed a bit of bending to bring its guide plate further forward of the seat-tube, to where the shift actually happens (one of those plastic 'dog-fang chain watchers' would not have worked at all).

All of this is beyond the knowledge of most cyclists - most FLDs for that matter - so I think that for small frames properly to fit small riders AND front shift reliably, they will need a slightly bent seat-tube, so the bottom end can join the bottom-bracket at the correct angle for a standard front mech.

Before anyone says it: one-by is not the solution. It cannot provide the extremely low bottom gear a small rider may need, not without compromising the top gear or the intervals between gears.

Sticking to road mechs seems like a solution - given a customised braze-on positioned wherever may be necessary - but is even less help than one-by in achieving a very low bottom gear.

And I already thought about those MTB mechs that fit via the bottom-bracket - even bought one to try - but they also require a frame fixing and have bits sticking out the back that don't allow them to be rotated very far forward relative to a seat-tube. They solve the chainstay-angle too big problem, but not the chainstay-angle too small problem.
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RickH
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Re: Toe Overlap

Post by RickH »

What is considered a high (or low for that matter) bottom bracket?

I tend not to pedal round corners at speed but you do have to lean the bike quite a way over. A quick test measure on the bike that is in the house is 34° to touch a pedal at bottom dead centre (2016 Kona Sutra, running 40mm tyres rather than the OEM 47s, BB height approx 280mm, 170mm cranks, Shimano M520 SPD pedals. Fitted a shoe & pedal still contacts first).

I find straight line pedalling of a tandem over a speed bump to be a much more likely scenario for pedal strike (as in it happens very occasionally, whereas I think I may have had slight pedal contact on a corner once in over 45 years)
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pete75
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Re: Toe Overlap

Post by pete75 »

CJ wrote: 13 Apr 2021, 6:17pm
My Holdsworth Mystique gravel bike frame, that I chose as the basis for a clubrun/audax style of bike in order to get actual (not just pretend) mudguard and toe clearance in carbon, has 71° & 50mm, so 67mm of trail.
Holdsworth? I thought they went out of business years ago certainly well before the carbon era. Still have fond memories of their Bike Riders Aids catalogue.
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Re: Toe Overlap

Post by Bmblbzzz »

One of many names bought up by Planet X.
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531colin
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Re: Toe Overlap

Post by 531colin »

RickH wrote: 18 Apr 2021, 1:45pm What is considered a high (or low for that matter) bottom bracket? .........
I think "it depends".....

If you pedal like this

Image003 by 531colin, on Flickr

Then any BB height will be "too high" because if you are on tiptoe just to reach the pedal at the bottom, then you are never going to get a toe down while still seated on the saddle.

However, I pedal like this

Imagefoot level on bottom pedal by 531colin, on Flickr

and I easily get a toe down on my Spa Roughstuff bike, which has a bracket height of 290mm (mine should be higher, as I have at least 40mm tyres on)

CJ has 260mm bracket height for his wife's bike; I think she is quite petite, but I'm not tall, I used to be over 5'10" in my socks, but now in my seventies I can't make 5'9" in shoes. :(
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