Tuesday, March 21, 2006

BB pivot

Most four bar linkage full suspension bike have their main pivot just above the bottom bracket. Since pedaling feedback come from a variation in the chain lengh, this pivot should ideally be placed on the BB instead of near it. This would equal a constant chain lengh no matter where the bike is in its travel, without the use of a crazy pivot system (like the Giant Maestro). Probably that the design constrains were to have bearing small enough to get around the bb while being still strong. Several manufacturer have produced such a design. Jamis with the carbon Diablo, Rotec, Dean and then this custom frame builder. The bike shown here succeded well on this task. The bearings are german needle bearing, both small in diameter and strong (the rear-end looks a lot like a Dean). The design are is a faux-bar, with the secondary pivot on the rearstay instead of on the chainstay. This a maybe more prone to brake-lock than the patented Specialized horst-link, but it ensure that the chain is always at the same lengh. This would allow this bike to run a single-speed cog, are more logically for a 2000$ frame, a Rohloff hub. The downfall of this design is that a little bit a chaingrow might be good for traction, but again, at the price of pedaling feedback. This bike is made by a small american frame builder out of steel. I lost the manufacturer website, but I will had it to this post soon.

Thursday, March 16, 2006

Titus Exogrid

Good design in not just about mechanical properties. It's also making things beautiful. You you at the Titus Exogrid tubing. It is made out of machined titanium with an underlayer of carbon. They could have made the titanium holes industrial looking with FEA. Instead, they push the design up to put their brand on the tube. Beautiful! This is the kind of raffinent I like. Carved lugs for steel bike is great, but it is to past inclined for me. I prefer futuristic things like the Exogrid.


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Tuesday, March 14, 2006

Single-speed drop-out

Single-speeds are cool, but they have a big downside. The rear hub can slide in the drop-out under heavy torque. Many solution exist. First, Paul Component designed a drop-out with a screw to ajust after the wheel is in the proper position. The design is simple and neat. More interesting are the Paragon Machine work slinding drop-out. More complicated but they have the major advantage of allowing the use of disk-brake without crazy solution (like the one from Paul Component). Another solution that allow disk-brake is the eccentric bottom bracket. It allow regular vertical drop-out for the price of weight, complexity and money. It is a paradox to see that a bike designed to be simpler become quite a headache!

White industrie Eccentric hub

There are many solutions for transforming a regularvertical drop-outs equipped mountain bike to a single-speed. First, you can be lucky so your chainstay/chainring/sprocket combinaison is exactly want you need to obtain the right chain tension. Or you can toy with differents gears until it fits. For this, you have to be quite flexible on the gear size you can propel. A popular solution is to add a chain tensioner. It works like a derailleur, but at my opinion, itdisturb the single-speed harmony and a bit of the efficiency (the Paul component version is pictured here). A better looking alternative is the White Industrie Eccentric hub. It's axle can move reporting to the frame, so a perfect chain tension can be achived. It cost 159 US$ Finally, a more radical alternative for steel frames is to change the drop-outs for horizontal ones. This may cost around 40$ in material and 200$ in labor, plus a new paint job. At my opinion, all these alternative are far from perfect. A better idea would be to sale the frame and to buy a single-speed specific one. Why not? Maybe it is time for a new baby!