12 May 2015

12 May 2015 Probing!

Following the recent discomfort from further saddles I thought I'd better do some proper research into "real life" saddle flexibility under a rider's load.

So I whipped up a slotted angle iron device to lower the heads of two coach screws onto a test saddle. The saddle was set up in a normal saddle pin clamp on a folding workbench using a water pipe extension for stability. A saddle camp is vital to test "real world" saddle rail flexibility.

I arranged the coach screw heads to be 100mm apart to match my own sit bone centres. The coach screws have typically domed heads to simulate the small surface area of my unprotected sit bones. I have used smooth headed "climate" finished screws to avoid marring the test saddles. The long screws were simply to allow free examination of the saddle under test without a rider's bod obscuring the view.

I am not really convinced that much of the rider's weight is carried anywhere but at the sit bone resting place at the rear of the saddle. The soft tissues forward of the sit bones aren't designed to carry loads and soon complain if they are forced to. The buttocks overhang the saddle so contribute nothing to the rider's support. So that just leaves the sit bones.

The 2m [6'6"] long slotted angle-iron arms were intended to allow me to load the saddle [via the domed screw heads] without having to lift huge weights to simulate a normal cycling bodyweight resting on the saddle.

The weight of the arms themselves also pre-load the saddle testing screws. I can check the pre-load at the screws with a large spring balance. The loads added at the far end of the levers can then be reduced by this amount.

The arm length ratio is arranged to be approximately 2:1 [~65:130cm] allowing load weights to be halved at the far end of the levers. I intend to load the saddle itself with up to 12 stones or 76kg. This is my present riding weight in light cycling clothing.

Extending the load arms to achieve much higher leverages is a matter of moments to bolt further angle sections together. However, the greater length would put me much further from observing the action taking place at the saddle. It also increases the unloaded weight on the saddle while I am making fine adjustments.

Note how close 100mm sit bone spacing is to the edge of the saddle! I chose the widest spot for the pressure screws.

I had the apparatus all set up on a test saddle... And then, it poured with rain for the first time today. The forecast was rain all day but it remained dry until after 4pm. I shall continue working between showers. As soon as I have something to show I shall post images. Or possibly make a video to share on YT.

Experience suggests that the whole thing wants to tip as I load the far end of the arms. So I used a batten brace under the seat pin clamp down to the ground, which helped a bit. Then I added a concrete fence post but it was still trying to tip. Finally I added a pipe to pull down the end of the arms via a large spring balance without my having to perform acrobatics while still needing to watch the screw heads denting the saddle.  I managed to get up to 20kg [Its maximum reading] Plus the 8kg  pre-load at the saddle from the weight of the arms. Quite a bit short of the 37kg load required at the end of the arms.

The screws really were digging deep. With a surprising depth of foam compressed compared with what the fingers can achieve working alone. I wonder why the saddle is so uncomfortable when so much padding is hiding under the innocent vinyl cover? Does the thin layer of flesh around the sit bones resent the pressures applied? Since every force is matched by an equal and opposite force the sit bones must "see" the full weight of the rider. The padding helps to spread the load to a wider area but cannot undo the full force pushing back up.  Plus the jolts, from uneven road surfaces as upward shocks, are transferred to the rider's sit bone area working against the rider's considerable mass. Peak loads on the sit bones must be quite dramatic. The "hammering" must be reduced somehow or comfort will be very short lived!  

Which brings me right back to the share taken by the sensitive tissues of the crutch supporting the rider's weight. When I removed the load the saddle still showed marks where the screw heads had pressed. I really don't think this is true of the saddle when I climb off. The marks were [fortunately] quite short lived. I had deliberately chosen an inexpensive vinyl-covered sports seat for my first tests. I shall have to find a way to measure the rider's weight on the non-sit bone areas. A saddle shaped membrane with lots of pressure sensors would be useful.

Many TT riders pull themselves well forward on the saddle so that the crutch is carry all of the rider's weight on the nose of the saddle. However, they are usually taking some of their weight on the elbow pads. Plus, the downward force on the pedals lifts the rider bodily. I can easily rise right off the saddle myself when pedalling hard and am down on the tri-bars.

The whole arrangement of my saddle test apparatus needs to be considerably beefed up and properly braced against the ground to avoid flexure and tipping. The loads applied by the rider's weight alone are really quite remarkable. Add the constantly changing pressures applied by pedalling and pulling on the handlebars and it is a wonder a bike [or trike] holds together!


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