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From: (Jobst Brandt)
Subject: Re: Power transmission aspects of bicyle design
Date: 14 Mar 1997 02:28:47 GMT

David G? writes:

>> Yes, that's the kind I also had in mind when replying, and it is such
>> belts where the Kevlar fibers break in fatigue with the absorption of
>> humidity.  I doubt that anyone makes timing belts with Kevlar these
>> days.  Their frailty must be common knowledge in the business.

> A quick Excite net search revealed quite a few companies with sales
> info for Kevlar reinforced belts. Maybe it's not a major problem for
> certain products or conditions. I'd be curious to learn more.

We don't know where the Kevlar is and how it is used.  It's somwhat like
the sticker "Intel Inside" on a product.

> Jobst - do you have quantitative data, references or experience that
> would be informative for the rest of us ?

I was involved in the development of paper moving plotters at HP
before their ink jet plotters.  These plotters used timing belts to
move the carriage and paper (X&Y).  We tested all sorts of belts and
discovered that, in the humidity chamber, all Kevlar belts failed.  We
used conventional Rayon backed belts after that with no trouble.

No papers were published on these findings, they being insignificant
in that sort of work.

Jobst Brandt      <>

Date: Thu, 24 Sep 1998 21:32:43 CST
From: (Jobst Brandt)
Subject: Re: IRC Mythos tyres

Matthew Noell writes:

> Most Kevlar tires have a Kevlar bead, not belt.  This means that
> instead of a wire bead around the tires edge, there is a flexible
> and lighter Kevlar bead.  This is the difference between folding and
> non-folding tires.

I think you have mixed the definition of the two.  Most tires that are
called Kevlar 'this' or Kevlar 'that' have a Kevlar belt to prevent
punctures.  Folding tires, today, all have Kevlar bead fibers in place
of steel.  Therefore, the appellation "folding" is definitive of that
type where the term "Kevlar" refers to its casing's puncture resistant
makeup.  That is the way Avocet and Specialized use the term and I am
sure others do too.

> Some tires do have Kevlar belts, that is to say, a layer of Kevlar
> like a bulletproof vest where a Tuffy liner would be, or Kevlar
> woven into the entire cloth shell.  This should, in theory prevent
> some punctures but will have no effect on tire wear.  If anyone is
> mixing in Kevlar with the rubber compound then it is news to me.

New tire compounds use silica and Kevlar fibers to replace carbon
black.  The principal reason for this was to make colored tires for the
auto industry that had the same performance as black tires do.  In
that pursuit, it became apparent that these tires also have lower
rolling resistance, but up to no still don't have the durability or
wet traction of a carbon black tread.

Jobst Brandt      <>

From: (Jobst Brandt)
Subject: Re: Tire width and rolling resistance
Date: 4 Nov 1998 17:14:09 GMT

Chas Szenberg writes:

> I recently changed exchanged my Pasela 1.25 Kevlar belt and bead
> tire with a Conti 1.0 on my X01 and "noticed" less rolling
> resistance.  Yet, the Pasela is great tough tire, with nice cush.
> With all the factors identified in the thread, do I just imagine
> this? or is it Memorex?

I'm not familiar with the tires in question, but among tires of
similar cross section and tread profile, no difference is perceptible
although there are measurable differences.  Rolling resistance is not
something you can readily feel, since it affects your progress
minimally in contrast to air drag at speeds above 15mph.  What you may
notice is a difference in acoustic response.  Light resilient tires
have a distinctly higher rushing sound when riding on typical pavement.

> Will an Avocet Kevlar belted 1.25 provide a feeling of more rolling
> resistance because of the belt as many people believe?

Any Kevlar in the tire casing will cause an increase in rolling
resistance over the same tire without that protective band.  Of course
even a non-Kevlar fabric would increase rolling resistance because it
increases the bending stiffness of the casing.  The difference is that
Kevlar does it more so.

> (My all time favorite is the 700x28 Avocet slick K-beaded---the
> fastest and most supple tire I personally have ever rode. Though
> admittedly,have had some squirming problems with fine dirt.)

I don't know what you call supple.  That is one of the old bicycle
terms used to describe immeasurable quantities.  Other words in that
category include "lively", "responsive", "quick"...  If you liked the
performance of that tire, I can assure you that the Avocet Road 20,
the non-K tire version, is measurably lower in RR, cost, and weight.

Jobst Brandt      <>

From: (Jobst Brandt)
Subject: Re: Kevlar vs Steel bead?
Date: 20 Apr 1999 19:06:58 GMT

Shayne (who?) writes:

> Is Kevlar that much better than steel when it comes to tires?

What do you mean by better?  Riding them you cannot measure the

> I know Kevlar is lighter, and you can fold it in a backpack but is it
> really worth the extra $20 bucks a tire?

If you want to carry a spare on a tour, I know of no alternative
unless you have enough space to carry a coiled tire.  I keep a folded
tire in the bottom of my touring bag for "just in case" and have had
occasion to use it.  Other than that, I hate changing a tire with one
of these because they are so much undersize (to make up for their
stretch) that mounting the tire is a chore.

Jobst Brandt      <>

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