[hpv] hpv Digest, Vol 94, Issue 3

[Paul Gracey]

Bushings are great once stiction is overcome, and motion is in progress, but ball or roller bearings are much better at the very start when the motions are reciprocating back and forth as in steering. I note that my very precise German equatorial telescope mount uses huge roller bearings just for a 40 lb. telescope. Its subtle reversals while hunting stars at very tiny angles are probably due to its very low loading and smooth roller bearing.
 Both in railroad applications and others, it is the low starting torgue at high load that is a part of the reason for the common use of such bearings today. As for  Tom Traylor's and the Cruzbike designs I suspect the starting torque is quite limited  to what can be withstood by the arms holding the handlebars as straight as is needed for good balance. Tom has told me he can ride no hands, using differential steering, but I had to back off quite substantially when I tried pushing off on one of his bikes. Normal headsets used in mountain bikes probably see greater stress from rocky downhills than any good rider is likely to impose 
On Aug 13, 2013, at 9:00 AM, hpv-request at hupi.org wrote:

> Date: Tue, 13 Aug 2013 11:37:32 -0400
> From: Garrie Hill <garrie at recumbents.com>
> To: Ben Brown <hbb at cs.cmu.edu>
> Cc: HPV_list List <hpv at hupi.org>
> Subject: Re: [hpv] Steering head bearing loads in front-drive bike
> Message-ID: <8B2BD011-EADE-46AE-AE82-B885AC35812A at recumbents.com>
> Content-Type: text/plain;	charset=iso-8859-1
> 
> One of the things I've pondered over the years is, "Why do we use ball or roller bearings in head sets?"  Nothing is rotating more than a couple degrees ( more than 90% of the time) plus or minus, and a good shock through the wheel into the fork can cause indentation of the balls into the races.  Use a bushing and you spread forces over infinitely larger bearing surfaces than the contact area of balls/races.  In the case of MBB FWD, bushings have the added advantage of extra drag dampening some pedal induced oscillation.
> 

Paul Gracey
pngracey at mac.com