Any way to assess toe(clip) overlap on x frame?

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Freddie
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Any way to assess toe(clip) overlap on x frame?

Postby Freddie » 10 Jun 2020, 12:21pm

One frame with a 57cm top tube & 73 degree headtube may have overlap, but an otherwise identical frame with a 57cm top tube & 71 degree headtube may not.

Is there any easy way in which you can find out by having frame measurements and angles, whether you can expect toe(clip) overlap from X frame?

rotavator
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Re: Any way to assess toe(clip) overlap on x frame?

Postby rotavator » 10 Jun 2020, 1:12pm

What I have done is to measure the "front centre" distance i.e. front axle to centre of BB, on bikes with and without toe overlap so I have a good idea of the minimum FC that I require.

If that is too low tech for you, there is special bike geometry software available.

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CJ
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Re: Any way to assess toe(clip) overlap on x frame?

Postby CJ » 10 Jun 2020, 3:32pm

It's not easy but there are ways.

The key frame geometry dimension is Front centres Fc, distance between bottom bracket and fork end. But as you've no doubt observed, a lot of brands omit that dimension from their geometry tables. Usually they'll give the Wheelbase Wb and Chainstays, what I call Rear centres Rc, plus the bottom-bracket Drop D below the wheel centres.

If you've got those last three dimensions Fc = { [ Wb – (Rc^2 – D^2)^0.5 ]^2 + D^2 }^0.5

(^2 means squared, ^0.5 means square root)

If it's a complete bike geometry you're looking at however, rather than just the frame & fork, they'll sometimes omit the Drop and just give the Bottom-bracket height Bh. In that case the first thing you've got to do is calculate the Wheel Radius Wr for the particular tyre size supplied on that bike. If the bike comes with 700×28C tyres, they'll also have an ISO marking like this: 28-622, which is the Tyre section Ts and the Rim diameter Rd. Given those dimensions the Drop D = Rd/2 + Ts – Bh. So if that 28-622 wheel is in a bike with Bh of 275mm... D = 622/2+28–275 = 311+28-275 = 64mm.

But before that, you've got to calculate how much front centres you need. That varies depending on Crank length C, Left pedal tread Lp, Shoe projection Sp, Wheel radius Wr (see above) and Mudguard clearance Mc. The ones you'll have to measure are done like this. Left pedal tread Lp is the distance sideways from the middle of the frame to the middle of the left pedal. No need for great precision, simply align the left crank on any similarly equipped bike you already have, with the down-tube or seat tube and measure to the nearest 5mm. Shoe projection Sp is measured by turning the shoe upside down on the floor, toe against a wall and measuring with shoe sole and tape both horizontal, from the wall to the centres of the SPD cleat screws. Or if you have some other kind of pedal, to a point on the sole that is above the pedal axle. For mudguard clearance Mc measure from the surface of the tyre to the outer surface of the front mudguard, preferably at it's tail end near where your shoe might hit it.

Then: minimum Fc = [(Wr + Mc + Sp)^2 – Lp^2]^0.5 + C

When I do that sum for my Touring bike, on which I have 37-622 tyres, a generous 20mm of mudguard clearance and 172.5mm cranks that put the left pedal 130mm from the bike's centre line, I calculate that my size 41 touring shoes, that project 100mm forward of the pedal, need front centres of at least... [(622/2+37+20+100)^2 – 130^2]^0.5 + 172.5 = 622mm. The frame has 623mm front centres and my shoes just clear - so long as I don't pedal toe-down!

Of course, rather than all that maths you could simply measure similar bikes where your toes do or do not clear by a known amount.
Chris Juden
One lady owner, never raced or jumped.

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CJ
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Re: Any way to assess toe(clip) overlap on x frame?

Postby CJ » 11 Jun 2020, 11:53am

I've put those formulae (plus another that uses Reach, Stack, Head angle and Fork offset) into an Excel spreadsheet you can download from here.
Chris Juden
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531colin
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Re: Any way to assess toe(clip) overlap on x frame?

Postby 531colin » 11 Jun 2020, 1:09pm

I have always found "wheelbase minus chainstay" to be near enough for practical purposes.

Freddie
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Re: Any way to assess toe(clip) overlap on x frame?

Postby Freddie » 12 Jun 2020, 5:58pm

CJ wrote:I've put those formulae (plus another that uses Reach, Stack, Head angle and Fork offset) into an Excel spreadsheet you can download from here.
Thank you for that.

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CJ
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Re: Any way to assess toe(clip) overlap on x frame?

Postby CJ » 14 Jun 2020, 10:43pm

531colin wrote:I have always found "wheelbase minus chainstay" to be near enough for practical purposes.

Gives a result about 1cm less than the actual dimension. As a lot of bikes (Spa tourers included) give me less than 5mm, that's not near enough to be useful. Unless, of course, you're comparing two bicycles on the same 'false front centres' basis.
Chris Juden
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531colin
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Re: Any way to assess toe(clip) overlap on x frame?

Postby 531colin » 15 Jun 2020, 4:41pm

Serves to reinforce my prejudice against manufacturers who publish incomplete geometry data. :wink:
To be in fashion, a bike must be supplied with a long handlebar stem; of course this puts the bars out of reach of a sizeable proportion of the population unless the front centre dimension is made short. If the bike has a sloping top tube, you can then "size up" in order to get toe clearance, with the added bonus of a sensible handlebar height. (and possibly a less steep seat tube angle as well)
In a rational world, all you need is to find some carbon forks with about 60mm offset, then you can use a shallow head angle and get a short top tube, toe clearance, and stable steering.
Now that sensible width tyres are becoming mainstream, perhaps the industry will see the light and do it......or perhaps not.

mikeymo
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Re: Any way to assess toe(clip) overlap on x frame?

Postby mikeymo » 18 Jun 2020, 10:52am

531colin wrote:Serves to reinforce my prejudice against manufacturers who publish incomplete geometry data. :wink:


They may be slightly preferable to manufacturers who publish incorrect geometry ;-) ;-)
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fastpedaller
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Re: Any way to assess toe(clip) overlap on x frame?

Postby fastpedaller » 18 Jun 2020, 11:27am

531colin wrote:In a rational world, all you need is to find some carbon forks with about 60mm offset, then you can use a shallow head angle and get a short top tube, toe clearance, and stable steering.
Now that sensible width tyres are becoming mainstream, perhaps the industry will see the light and do it......or perhaps not.


Won't look 'cool' in the marketing man's eyes, far better to have a straight fork and the rider fallen onto the road :shock:

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Paul Smith SRCC
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Re: Any way to assess toe(clip) overlap on x frame?

Postby Paul Smith SRCC » 22 Jun 2020, 2:28pm

As 531Colin has mentioned manufacturers geometry data is often incomplete; it's occasionally wrong as well! But, if each of the bikes you are considering publishes enough accurate data you may find value in composing each in BikeCAD free, the 'toe overlap' feature is not available like it is on the pro version but the 'front center' (622below) can be displayed. In addition you will need to factor in tyre size, crank length, shoe size, cleat positioning and even shoe type.

You have referenced that both the frames are identical except for the head angle, pay careful attention to just how similar they actually are, there may be more changes to the geometry than you think, composing the drawings will sometimes help flag those differences; these may also influence your conclusion over and above 'toe overlap'.

CTC Gravel bike.jpg
BikeCAD Free drawing example with a 57cm top tube with a 71 degree head angle

If that appeals before you do anything create your free account so that you can save your drawing when finished. You can start with either their quick start drawing, or any from their design archive including mine above, in each case you can edit and save to your free account as your own drawing and keep it 'private' or make 'public' as desired. When you have found the drawing you want to start with simply click on the green "open in BikeCAD' tab, note as they correctly state "it can take several minutes to load"; it does.

The free version works best if you do not display the chain and rear derailleur, plus even though technically you can I would not try and upload any brand logos as it is inclined to crash; adding text to frame and components as I have done in the example above is not a problem.

As useful as data can be you need you need to be confident it's correct; I'd definitely echo what Chris said "rather than all that maths you could simply measure similar bikes where your toes do or do not clear by a known amount"; but if you find one that works and you have drawn up the two bikes then you could always draw that up as well; as you then have a reference point to start from.