Wheel sizes vs Handling

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9494arnold
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Re: Wheel sizes vs Handling

Postby 9494arnold » 25 Jan 2019, 2:03pm

My son prefers his 24" wheel unicycle over the 20" one but I think that's to do with gearing (they are effectively a fixed wheel so one turn of the pedals takes you one wheel diameter distance forward. )
I couldn't comment on that one never managed to master it myself. :oops:

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My 50" High Wheeler is a bit twitchy at slow speed but there are lots of other variables there, effectively a straight fork, spokes provide "cushioning " of a sort , I believe there is a bit of flex in the backbone ,with a solid rubber tyre both ends.

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I am thinking both of your bikes are travelling at the same speed.

I will use 20" and 27" .

So one turn of the 27" wheel will take you 27 "
And roughly one and a third turns of the 20" will take you the same distance.
So the rim is travelling a third faster on the 20" wheel for the same given distance.

Smaller tyre footprint on the smaller wheel :Less friction?

Handling is a really nuanced issue. One mans meat is another's poison.
A time trial / track specific bike would generally be a "quick steerer" (I am thinking about my 37" wheelbase Flying Gate here ( (Which I don't own anymore) . Exhilarating ride but a complete nightmare in Traffic.
Bob Jackson Touring Bike . Ride it all day long.

And even Rim Construction could be an issue.
Most 27" wheels are not box section.
Quite a lot of 700c's are Box Section (like an old school sprint rim) which I perceive makes for a stronger wheel.
And double eyletting will add to the strength I would think , and help the spokes not to deform under cornering inertia.
Butted Spokes, Plain Spokes?

Going for a lie down in a darkened room now. :lol:

(Did I mention Trikes ?) :shock:

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fausto99
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Re: Wheel sizes vs Handling

Postby fausto99 » 25 Jan 2019, 2:27pm

9494arnold wrote:So one turn of the 27" wheel will take you 27 "


!!!Pedant warning!!! ahem ... one turn of a 27" wheel will take you (27 x π) inches. Circumference = πD :lol:

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Mick F
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Re: Wheel sizes vs Handling

Postby Mick F » 25 Jan 2019, 2:34pm

fausto99 wrote:
9494arnold wrote:So one turn of the 27" wheel will take you 27 "
!!!Pedant warning!!! ahem ... one turn of a 27" wheel will take you (27 x π) inches. Circumference = πD :lol:
Yes, of course :wink:
but that isn't taking in the speed of the machine over the ground. Smaller wheels rotate faster.

Does this increased rotation speed make them steer differently with respect to the same speed over the ground?
Mick F. Cornwall

9494arnold
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Re: Wheel sizes vs Handling

Postby 9494arnold » 25 Jan 2019, 2:52pm

I was trying to make it simpler not more complex , honest . (I do have O level maths but it was a long time ago) :oops:
Up to a point I'd say the faster you are going the easier it is to turn a corner : the point where you fall off or hit the kerb or lose your nerve ! :shock:

(Trikes will drift on the wrong surfaces , bikes fall over ) (still have some of the scars) :?

PDQ Mobile
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Re: Wheel sizes vs Handling

Postby PDQ Mobile » 25 Jan 2019, 3:23pm

Mick F wrote:
fausto99 wrote:
9494arnold wrote:So one turn of the 27" wheel will take you 27 "
!!!Pedant warning!!! ahem ... one turn of a 27" wheel will take you (27 x π) inches. Circumference = πD :lol:
Yes, of course :wink:
but that isn't taking in the speed of the machine over the ground. Smaller wheehls rotate faster.

Does this increased rotation speed make them steer differently with respect to the same speed over the ground?

I would say not really.
Axle height is probably a factor. Fork rake/steering geometry too, already discussed

The higher sensiivity to uneven surface already mentioned, ditto.

Larger wheel has more gyroscopic effect but that is partly balanced (no pun intended) by increased rotation speed of a smaller wheel.

Actual rim velocity is the same though, that is only dependent on road speed.

Brucey will correct me maybe?

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Mick F
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Re: Wheel sizes vs Handling

Postby Mick F » 25 Jan 2019, 3:33pm

PDQ Mobile wrote:Actual rim velocity is the same though, that is only dependent on road speed.
Correct.

Bottom of wheel is "stuck" to the road therefore zero speed.
Axle moves forward at the same speed as the whole bike.
Top of wheel moves forward at twice the speed of the bike.
All irrespective of wheel size.
This is why calliper brakes work just as well on big wheels and small wheels, and hub braking is affected by wheel size.

What happens in three dimensions?
What happens when the wheel is steered left or right?
In effect, the wheel is "climbing" up a hill as it moves left or right.

Or that's what I think.
Mick F. Cornwall

Brucey
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Re: Wheel sizes vs Handling

Postby Brucey » 25 Jan 2019, 4:03pm

as explained in a post above the MOI (Moment of Inertia) of a wheel goes roughly as the cube of its diameter (for a fixed rim and tyre cross section). However at a fixed road speed the rates of rotation are inversely proportional to diameter. Thus the actual change in the gyroscopic behaviour (at fixed road speed) is likely to be as the square the wheel size.


Thus a 27" wheel is only x1.35 bigger than a 20" wheel but some aspects of its behaviour are likely to feel over x1.8 (i.e. about double) different.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

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Mick F
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Re: Wheel sizes vs Handling

Postby Mick F » 25 Jan 2019, 4:19pm

Inertia. That must be it.

Not gyroscopic at slow speeds of course, but the inertia is there as soon as it rotates.
Mick F. Cornwall

StephenW
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Re: Wheel sizes vs Handling

Postby StephenW » 25 Jan 2019, 5:17pm

Hello Pete

One of the odder handling differences I've come across is changing the bars on an overseat steering 'bent from aero to tiller style. In either case the bike's geometry is fundamentally the same, and with no weight on the bars and seating position pretty much the same (arms excepted) there shouldn't be any difference in handling. But I can't get really comfy with tillers on any bike I've tried them on, and have always got straight on with aeros (other people find the precise opposite, or don't do underseat steering). It's purely psychology and familiarity.


I have an idea about this. Weight attached to the steering assembly, forward of the steering axis, has a stabilising effect on handling. This is because, if the bike it disturbed, this weight will cause the steering to turn to the side the bike is leaning to. This brings the bike back under the rider. Conversely, weight behind the steering axis has a destabilising effect.

With aero/superman bars, the grips are roughly in line with the steering axis, so the weight of the rider's arms has no effect. With a tiller, the weight of the rider's arms is acting on a point behind the steering axis, giving a destabilising effect. The longer the tiller, the stronger the effect.

PS. I can put this into JBike6 if people are interested (see below).
Last edited by StephenW on 25 Jan 2019, 5:49pm, edited 1 time in total.

StephenW
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Re: Wheel sizes vs Handling

Postby StephenW » 25 Jan 2019, 5:46pm

Hello all

There is a tool called JBike6, which allows the stability of bicycles to be analysed. There are several simplifications, such as no friction in headset or tyres, infinitely narrow tyres etc. Nevertheless, if you only want to compare one result from the program to another, I think that is OK, so long as the factors not included in the model are kept constant.

The model shows the range of speeds at which the bicycle is self-stable - if perturbed, it will right itself, although it may end up travelling in a different direction. Outside the stable range of speeds, the bicycle needs to be actively kept upright by the actions of the rider.

Here are the results from the default bike, a Schwinn Crown:

schwinn crown.png


Now let's see what happens when we change both wheels to 20", without changing the trail, or centre of mass:

schwinn crown smallwheel.png


The weave speed has gone up from 5.3 m/s to 6.3 m/s. So it first becomes stable at a higher speed. The capsize speed has also gone up, from 8 m/s to 10.5 m/s. Capsizing is regarded as less important than weaving, because it is something that happens quite slowly and can easily be corrected by the rider.
The total stability has gone from -6 to -11. (I'm not exactly sure how this is measured).

So it seems that the small-wheel version is less stable at low speeds, but is more stable overall. It's important to note that the trail has been kept constant. I expect that many small-wheeled bikes have less trail than big-wheeled ones.

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531colin
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Re: Wheel sizes vs Handling

Postby 531colin » 25 Jan 2019, 6:41pm

Mick, am I right in thinking your Mercian is more than a little lively, in that you can only just ride no hands?
You are comparing it to a ladies Dawes, dating from the time when they were UK built?
I think the Dawes is probably designed to be on the stable side of neutral handling, and your Mercian is deliberately pretty lively.
I know that you know your Mercian's steering geometry, would you remind us?
What difference do you find in the handling of the 2 bikes (asking for the third time, I think)

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Mick F
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Re: Wheel sizes vs Handling

Postby Mick F » 25 Jan 2019, 9:12pm

Stephen and 531colin.
Thank you both.

Stephen, very interesting and maybe explains a bit regarding my OP question. Maybe I'm right.

Colin, can't remember the figures, but you are correct that Mercian is lively. I designed it that way - or at least asked them to make it that way. 73deg parallel with close clearances. 23.5" frame.
I can ride no hands, but it's not easy - or should I say I find it's not easy due to nervousness of falling off!
I tried riding no hands with Marjorie Dawes but I found I was nervous of it too, though no doubt at all it was much more stable.
I can ride no hands with Moulton too.

I didn't ride Marjorie for long enough maybe. Main issue is the gearing isn't low enough to get up the local hills. I did manage a half-mile there and back, but had to walk the last bit. Bottom gear is 36/28 = maybe 35" .......... as I've not measured the rolling circumference. Had I not been wearing overalls and boots, I could have gone on a longer more energetic ride, but it was just a test to see if I'd got it sorted and able as it's been laid up for months/years.

The ride I did was fine though the main thing I noticed was the size of the wheels. They felt huge, mainly the front looking down at it. At low speeds, the bike handled differently to Mercian at low speed. Can't be gyroscopic as the speed was too low, so must be a combination of geometry and inertia.

At low speed, I favour inertia that makes Marjorie stable.
Same as Moulton at low speed makes it twitchy.

High speed on any of the three .......... Mercian, Marjorie and Moulton are the same I would think.
Mick F. Cornwall

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531colin
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Re: Wheel sizes vs Handling

Postby 531colin » 26 Jan 2019, 10:04am

Mick F wrote:….. They felt huge, mainly the front looking down at it.....


The front wheel of a Dawes Mixte will be well out in front of the rider. ....due to shallow head angle and long offset front fork.
Your Mercian with a 73 degree head angle will have a short offset fork, and the wheelbase as short as possible....do you get toe overlap with mudguards and 23mm tyres?
small wheel bikes you can barely see the front wheel at all.

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Mick F
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Re: Wheel sizes vs Handling

Postby Mick F » 26 Jan 2019, 10:49am

Hi Colin.

Marjorie - 1.75" trail and 41.5" wheelbase. Mixte with 501 main tubes and 19.5" BB to centre of seat lug.
Mercian - same trail, but 40" wheelbase. 23.5" BB to centre of seat lug.
I measured the trails rather quickly, so they could be a slight difference but it's going to be not much.

No, there's nowhere near any toe overlap with Mercian.

If you want more accuracy, I'll get back to it later today. Just in for a cuppa to check in, then I'm back out into the garden.
Mick F. Cornwall

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Mick F
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Re: Wheel sizes vs Handling

Postby Mick F » 26 Jan 2019, 10:51am

531colin wrote:small wheel bikes you can barely see the front wheel at all.
It depends what you mean.
I can see mine on Moulton poking through the front of the front mudguard, though not as much as a big wheeled bike of course.
Mick F. Cornwall