Ask Josh Anything #15

Some observations on solving high speed wobble (bifurcation) issues. I weigh 134 lbs and ride a Specialized Venge with a Roval CX 50 wheel in front and CLX 50 in the rear. I use 28 mm Continental 5000 TL (set up as tubeless) at 60 lbs, which measure at about 30.5 mm width. I experienced life-threatening wobble events at speeds as low as 42 mph. They were initiated by wind gusts (natural and vehicle-induced), sudden change in pavement condition, and other but unknown factors. I have incrementally elevated the speed of wobble onset to >50 mph by doing the following:

-Installed a new front tire

-Wheel balance: removed solidified sealant and installed golf club weights inside the tire/rim (pressurized) cavity.

-Putting nearly all my weight on the pedals while descending at peak speed.

I’ve learned to sense and react very quickly to wobble onset before the wave propagates through the bike. If I reduce speed before the more than about two front end oscillations have occurred, the wave does not achieve bifurcation stage and is controlled by a small speed reduction. If more than two oscillation cycles have occurred and the wave is propagating, pinching the top tube with knees helps but substantial speed reduction is needed to fully terminate the wobble.

I would like to continue adding stability to my bike. What do you suggest? Would different (bladed) spokes possibly help (or exacerbate) the problem?

Question for a future AJA. Many manufacturers make narrower seatpost and flexible chainstays to give more compliance and comfort. Sometimes they even built compliance technologies in aero bikes like the Isospeed in Trek Madone or the compliance in the seatpost in the Merida Reacto. Do those compliance technologies make you slower on smoother roads?

one thing comes to mind often and I can not do the maths needed to really figure it out: what is the cost/ benefits of light vs heavy rims (tires tubes etc) as circumferential weight on wheel performance. What I am curious about is the inertial issues, ie when the rim (and for the point tire/ tube, sealant) weight is “heavy” it takes more to get up to speed but also takes more to slow it down as well. So in the end how does that weight translate over a normal range for tire rim weights and what does that mean in terms of watts to roll, accelerate and to decelerate? Also as a system it is my understanding that weight at the hub is not inertial and therefore less a concern for rolling watts, more to the climbing watts due to system weight. I would love to hear your thoughts on this topic. So forgetting about aero gains for the moment what are the rim, tire, tube system weight marginal gains?

Two questions on waxing chains:

1. The viscosity of paraffin wax decreases with temperature, so a hotter wax ought to penetrate the nooks and crannies of a chain more easily. But an overheated wax will start to degrade and discolour. Does this adversely affect performance? What is the optimal temperature (even if it’s not as convenient as using a boiling pot of water & a sous vide bag).

2. After a long & wet ride, my waxed chains sometimes show a little rust once they’ve dried out. What should I do to prepare them for re-waxing?

How long does a TT bike change take? I’m imagining three distinct segments:

1. Rider begins to slow, comes to complete stop

2. Car stops at same time as rider—mechanic exits car, removes bike from room, gives bike to rider, rider mounts, mechanic just begins to push

3. Rider accelerates up to full speed

How much time does rider lose in segments 1 (moving at less than top speed), 2 (stopped), and 3 (moving at less than full speed)?