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Are two bicycles necessary?



 
 
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  #11  
Old December 1st 06, 10:31 AM posted to rec.bicycles.misc,alt.rec.bicycles.recumbent
Peter Clinch
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Posts: 4,852
Default Are two bicycles necessary?

DougC wrote:

The front tire should have /at/ /least/ the same contact patch are as
the rear, /if/ /not/ /more/.


Why?

I've never worried about this on any bike I've ever had, and I don't
recall ever suffering as a result, so it comes over as rather more
grounded in theory than practice.

I don't see why the front tyre needs calibrating to the rear. It
doesn't know what pressure the rear is running at or what the contact
patch is: shirley whether it will skid or not is down to absolutes, not
values relative to the back?

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net http://www.dundee.ac.uk/~pjclinch/
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  #12  
Old December 1st 06, 11:04 AM posted to rec.bicycles.misc,alt.rec.bicycles.recumbent
Edward Dolan
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Posts: 14,212
Default Are two bicycles necessary?


"Peter Clinch" wrote in message
...
DougC wrote:

The front tire should have /at/ /least/ the same contact patch are as the
rear, /if/ /not/ /more/.


Why?

I've never worried about this on any bike I've ever had, and I don't
recall ever suffering as a result, so it comes over as rather more
grounded in theory than practice.

I don't see why the front tyre needs calibrating to the rear. It doesn't
know what pressure the rear is running at or what the contact patch is:
shirley [surely] whether it will skid or not is down to absolutes, not
values relative to the back?

Pete.

Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net http://www.dundee.ac.uk/~pjclinch/


Good thinking Pete! Did you learn how to think like this in medical school I
wonder? Too bad you know as little about bicycle physics as you do about
medical physics - whatever the hell that is!

However, it is always good to hear from the peanut gallery. How else would
we ever learn how a screwball Medical Physics IT Officer thinks. And from
Ninewells Hospital too!

Regards,

Ed Dolan the Great - Minnesota
aka
Saint Edward the Great - Order of the Perpetual Sorrows - Minnesota




  #13  
Old December 1st 06, 04:01 PM posted to rec.bicycles.misc,alt.rec.bicycles.recumbent
Roger Zoul
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Posts: 1,118
Default Are two bicycles necessary?

DougC wrote:
:: Roger Zoul wrote:
:::
::: Really? You're saying that if I'm riding a 26/26 (SXP) with 100
::: PSI (max) tires and I max the rear I should make the front at 45
::: PSI? What about pinch flats and stuff? Would those mainly be
::: limited to the rear tire?
:::
:::
::
:: It depends on what the weight distribution of the particular
:: bike/rider combination is. Put the front tire on a bathroom scale,
:: and sit on the bike with your feet resting on the pedals and hold
:: yourself up with a hand against a wall. Then with the rider and bike
:: total weights you can figure it out.

I think having to figure weight distribution to ride is an rather extreme
thing to do.

::
:: The front tire should have /at/ /least/ the same contact patch are as
:: the rear, /if/ /not/ /more/. So for a bike that carries two-thirds of
:: its weight on the rear tire, then the front should be inflated to no
:: more than about one-half the pressure of the rear. If the weight
:: distribution was 40/60 F/R, then the front tire should be inflated
:: to no more than 2/3 the pressure of the rear. For a 45/55 F/R bike,
:: the front should be no more than about 80% of the rear pressure.
::
:: There will be no greater danger of a pinch flat in the lower-inflated
:: front tire because while the pressure is lower, the load on the tire
:: is lower as well.

What load? Loads can change based on what happens with road conditions, can
they not?

You are not "increasing" the risk of pinch flats
:: as much as you are equalizing it; if both were inflated to the same
:: pressure but the rear was carrying more weight, then the rear would
:: be more at risk for pinching. You are not saving much of anything by
:: over-inflating the front tire; the decrease in rolling resistance is
:: minuscule. All it really gets you is much worse steering response.

How did you arrive at this definition of overinflating? I think of
overinflating as going beyond rated max psi. Can you direct me to a cite
somewhere?

::
:: .....If the thought of a front pinch flat concerns you and you
:: decide to run a wider front tire, then it should be at a pressure
:: that is even lower than the weight proportion. The front needs to be
:: re-figured based on the proportions of the wider front tire to the
:: narrower rear. [-I should probably put up a web page explaining all
:: this-]

I'd love to see that. I don't want to seem as if I'm attacking you for
responding to my post. But I'm curious as to the basis for your statements.

::
:: ---------------
::
:: The problem I think is that people are used to upright bikes, that
:: all tend to hold very-close-to 50/50 weight distribution, so people
:: inflate both [recumbent] tires to the same PSI and forget about it.
:: On a bike with significantly-different front/rear weight
:: distribution, inflating both tires to the same pressure is simply
:: not correct. ~

I must agree with the other poster....why? Is there some reference for most
of these statements?


  #14  
Old December 1st 06, 05:58 PM posted to rec.bicycles.misc,alt.rec.bicycles.recumbent
nash
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Posts: 1,061
Default Are two bicycles necessary?

shirley whether it will skid or not is down to absolutes, not
values relative to the back?




Don't call me shirley


  #15  
Old December 4th 06, 11:03 PM posted to rec.bicycles.misc,alt.rec.bicycles.recumbent
DougC
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Posts: 1,276
Default Are two bicycles necessary?

Peter Clinch wrote:
DougC wrote:

The front tire should have /at/ /least/ the same contact patch are as
the rear, /if/ /not/ /more/.


Why?

I've never worried about this on any bike I've ever had, and I don't
recall ever suffering as a result, so it comes over as rather more
grounded in theory than practice.

I don't see why the front tyre needs calibrating to the rear. It
doesn't know what pressure the rear is running at or what the contact
patch is: shirley whether it will skid or not is down to absolutes, not
values relative to the back?

Pete.


This is mainly a recumbent/"alternate"-bike issue, as these bicycles
tend to have wider differences in weight distribution than upright bikes
do, and riders cannot shift their weight significantly during riding.

The reason that this is important is because if you enter a low-traction
situation and one of your tires begins to slide, you want it to be the
rear to slide first rather than the front. The rear tire will follow the
front tire, even if the rear tire is sliding. On a sandy road surface,
you can slide short distances with the rear wheel locked up and keep the
bike upright fairly easily--but it is damn near impossible to slide more
than a few inches with the front tire locked up (on either road bikes or
recumbents). Because of the front tire starts to slide, you cannot steer
at all and you have a far greater chance of crashing.

So (on a two-wheeled vehicle that is not a Segway) the front tire should
-always- have more traction than the rear. That means since the front is
carrying less weight, you run a lower front pressure so that the contact
patch is at least as large as the rear is.

And if you're running narrow tires, you can see some significant
handling gains by switching to a wider front tire as well. Narrow tires
do not handle loose surfaces well.
~

  #16  
Old December 5th 06, 12:44 AM posted to rec.bicycles.misc,alt.rec.bicycles.recumbent
nash
external usenet poster
 
Posts: 62
Default Are two bicycles necessary?


DougC wrote:
Peter Clinch wrote:
DougC wrote:

The front tire should have /at/ /least/ the same contact patch are as
the rear, /if/ /not/ /more/.


Why?

I've never worried about this on any bike I've ever had, and I don't
recall ever suffering as a result, so it comes over as rather more
grounded in theory than practice.

I don't see why the front tyre needs calibrating to the rear. It
doesn't know what pressure the rear is running at or what the contact
patch is: shirley whether it will skid or not is down to absolutes, not
values relative to the back?

Pete.


This is mainly a recumbent/"alternate"-bike issue, as these bicycles
tend to have wider differences in weight distribution than upright bikes
do, and riders cannot shift their weight significantly during riding.

The reason that this is important is because if you enter a low-traction
situation and one of your tires begins to slide, you want it to be the
rear to slide first rather than the front. The rear tire will follow the
front tire, even if the rear tire is sliding. On a sandy road surface,
you can slide short distances with the rear wheel locked up and keep the
bike upright fairly easily--but it is damn near impossible to slide more
than a few inches with the front tire locked up (on either road bikes or
recumbents). Because of the front tire starts to slide, you cannot steer
at all and you have a far greater chance of crashing.

So (on a two-wheeled vehicle that is not a Segway) the front tire should
-always- have more traction than the rear. That means since the front is
carrying less weight, you run a lower front pressure so that the contact
patch is at least as large as the rear is.

And if you're running narrow tires, you can see some significant
handling gains by switching to a wider front tire as well. Narrow tires
do not handle loose surfaces well.


But since narrow tires are 100 psi and mtb 45-60 your saying just
change tires anyway.
Is that an adequate summary. I changed to narrow because less friction
and lighter tire will give you higher less cumbersome speed.

  #17  
Old December 5th 06, 09:46 AM posted to rec.bicycles.misc,alt.rec.bicycles.recumbent
Peter Clinch
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Posts: 4,852
Default Are two bicycles necessary?

DougC wrote:

This is mainly a recumbent/"alternate"-bike issue, as these bicycles
tend to have wider differences in weight distribution than upright bikes
do, and riders cannot shift their weight significantly during riding.


But it's not a problem on my recumbent tourer either...

The reason that this is important is because if you enter a low-traction
situation and one of your tires begins to slide, you want it to be the
rear to slide first rather than the front.


Why? If it's a rear wheel drive bike (as most are) then you can't get
traction any more if the back is slipping. I've never seen any owners
of freight bikes (I have one for direct experience) fiddle about with
tyre pressures just because they've put 50 Kg load in the front/back
which changes the wheel loading.

The rear tire will follow the
front tire, even if the rear tire is sliding. On a sandy road surface,
you can slide short distances with the rear wheel locked up and keep the
bike upright fairly easily--but it is damn near impossible to slide more
than a few inches with the front tire locked up (on either road bikes or
recumbents). Because of the front tire starts to slide, you cannot steer
at all and you have a far greater chance of crashing.


I spend a fair bit of time on loose surfaces (my house is on an unmade
track), and most times I've ever come off have been when I've lost all
traction at the back, ground to a halt with no effective power and
toppled over sideways. I've also come off with the back sliding out on
me on gravel.

The only time I've locked up the front wheel is under heavy emergency
braking and I've just slid forwards. Since the front suspension dived
it's fair to say far more weight than the usual riding weight was on the
front wheel, and it skidded anyway because of brake lock, not tyre pressure.

So (on a two-wheeled vehicle that is not a Segway) the front tire should
-always- have more traction than the rear. That means since the front is
carrying less weight, you run a lower front pressure so that the contact
patch is at least as large as the rear is.


But I don't, /and it isn't a problem/, and nobody else I know aside from
you does, /and they don't have problems either/. So I look at the
empirical data and it doesn't stack up for this being in any way important.

And if you're running narrow tires, you can see some significant
handling gains by switching to a wider front tire as well. Narrow tires
do not handle loose surfaces well.


That's quite true, but as I run 38mm Marathons it's not something I feel
a need to change.

Pete.
--
Peter Clinch Medical Physics IT Officer
Tel 44 1382 660111 ext. 33637 Univ. of Dundee, Ninewells Hospital
Fax 44 1382 640177 Dundee DD1 9SY Scotland UK
net http://www.dundee.ac.uk/~pjclinch/
  #18  
Old December 7th 06, 04:59 PM posted to rec.bicycles.misc,alt.rec.bicycles.recumbent
Buck
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Posts: 148
Default Are two bicycles necessary?


I mainly stick to riding the one trike for all my needs

Sliding etc.. being less of an issue on three wheels and I find that rough/loose ground is little of an issue, although I get no ejoyment from "hacking" through mud and stuff so generally avoid such conditions anyway.

I do have a folding Dahon hybrid type bike with 26" whhels however for when I travel by train as I can take it with me easier that a trike and still enjoy a cycle around London, it is damned uncomfortable after the hammock seat of the trike however so I use it very little, still it's nice to have the option.
--

Buck

I would rather be out on my Catrike

http://www.catrike.co.uk
 




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