Birdy Monocoque 3 months update:

I give a touring review at the end in case you want to skip all this stuff.

It's been 3 months since getting Birdy Monocoque, during which I almost exclusively rode the Birdy.

Gearing:

I have upgraded the shifter and derailer to 9sp '08 XT after the 8sp Alivio started showing a large amount of hysteresis requiring me to double-shift down and single-shift up in order to do a single down-shift. I have since discovered this is partly due to the cheap chains I was using in rotation with the original chain - the original Shimano one works a lot better. Never mind, I am still wearing out the 3 chains I have in rotation, and the upgrade to XT was intended anyway. Currently I am still using the 8sp drivetrain with the 9sp shifter - it works. Once the cassette is worn I'll install the 9sp one I have on ice.

I originally intended installing a SLX Shadow derailer for its low profile and direct cable routing; but when I got it, I found the Shadow couldn't be fitted due to the cable routing - the cable comes out the rear fork member too close to the RD to allow the cable to go sideways enough to engage the cable stop on the RD. So I did a bit of RD swapping at home and ended up with a medium cage XT for the Birdy. (The SLX Shadow went on SWMBO's Reach Offroad instead.)

As mentioned in an earlier post, the front chainring was swapped for a 58T Vuelta to improve top end gearing. It is adequate now but a wider range for touring is probably the way to go. I may install a second chainring a la pm124 and see how it goes with folding etc.

The chain has self-derailed a number of times when in the lower end of the gearing, falling in between the plastic guard and the chainring. I don't know why this happens. Possibly after swapping the chainring, the guards are no longer firmly held in position and allows the chain to gradually push it aside when touching it, ending up so eccentric that it touches the metal anti-derailing device with every revolution. With the guard out of the way at one position, it could derail. I will get a metal guard from Greenspeed to fix it good.

I still haven't decided to install the Schlumpf Speed Drive.

Tyres:

The original back tyre threatened to blow off the rim several times one day, so I took it to the LBS who gave me a 1.5" Marathon Racer instead for a discount. I have since replaced the back tyre with a 1.35" Marathon Plus for commuting/touring duty. The front still has the Maxxis Birdy on there. The Racer I took on tour as a spare.

Rear suspension:

I have taken to commute with the harder green elastomer; it allows me to spin faster without reaching the bouncing cadence. For more leasurely rides I swap in the red one. I carry the spare in the saddle bag for on-the-go swapping.

Braking:

The front brakes pads were poor from the start, so pretty soon I ditched them and installed better quality units with replacable pads. Big improvement. The rear are still the originals; I hardly ever use the rear brakes so they are still in almost new condition, as are the rims.

Folding:

I have it down to an easy non-rushed sub-30s now. I could do it in less if I rushed but haven't timed myself doing it fast yet. I have to be careful with the chain as the medium cage XT RD gathers less chain and it tends to fall slack. That was unexpected; I thought the RD difference to be small enough so that is not an issue. It may be the lack of cable flexibility that prevents the RD from swinging backwards fully.

The wider rear tyre snags on the front low-rider rack when folding. The narrower Marathon Plus does not have this problem, but the wider Racer and the Maxxis do. The rack has a short section of bar sticking out behind the pivot; I cut this off to solve the problem. I don't now what the extra section is for; you can't use it for hanging a pannier hook on as it will slip off.

Annoying Little Things solved:

- I have found I can remove the rear wheel after all by pulling through some cable from the front to free up the bake arm, allowing the pads to separate far enough to pull the wheel out.

- I have an acceptable solution for bottle cage fitting - a black plastic Cateye cage comes with slots to fit wide cable ties through, and these go around the stempost. For folding, the bottle cage interferes but since it is on cable ties, I just push it aside to the right.

- After fitting the rear folding rack, I found the bike rests on the back of the rack instead of the mudguards, providing a reasonably stable self-standing folded bike.

- The handlebars don't snag on the rear wheel QR when folding down if I push it down past the last safety stop. No biggie - the current lower position is my preferred position anyway. I may drill an extra hole for the safety stop to engage in.

- I have trimmed the cables to a bit better neatness.

Loaded Touring:

I took the Birdy on a 10 day loaded tour of Kangaroo Island. We drove to the island ferry point on the mainland and left the car behind.





As you can see from the photos, I had small panniers on the back; each was loaded just over 5kg. On top of the rack I bungeed the sleeping bag in a waterproof bag. My Crocs were captured under the same bungee.

For water bottles, I had the original Cateye on the stempost, plus another one cable-tied to the top tube. I used white electrical tape to protect the paintwork under the cable ties. Under the rear rack was a third bottle held in place with a big rubber band with a hook on the end. All these worked quite well, but I had to remove the 3rd bottle when folding, which happened only in the car journey to and from the island ferry.

On the front I carried both our Thermarest mattresses rolled up with the tent. Each of those was about 2kg, pannier included. The front "panniers" are home-made vertical racks made from flute-board (Coroflute, Coroplast) with straps for holding the rolls.

On the handlebars I hung my small backpack which was mostly empty; I had it with me for when going out on foot and suchlike. It proved to be very handy for easy storage of extra items like food. It also had a 2l water bladder for insurance, but which I never used.

So, how did the bike handle the load?

Poorly and brilliantly.

By far the biggest effect was from the load on the front. The extra inertia of the load had the effect that the front end of the bike flexed like a belly dancer on steroids. I could shake-rotate the bars from side to side while the frame twisted. This effect led to uncontrollable shimmy at speed. At speeds above 30-40km/h, the front would self-develop a low frequency shimmy which I could not damp out except by slowing down. I could never take one hand off the bars at say 20km/h either, when the same shimmy would set in with one handed riding such as when trying to take a drink. I never lost control, though.

I have put a lot of thought into why the Birdy does this. Other users as well as the Australian manual mentiones that the front suspension pivots have to be tightened to fix this.

I disagree.

The mechanism for the shimmy involves the steering geometry and the suspension. As the wheel turns sideways, the contact patch shifts forwards and sideways. At speed, this has the effect of compressing the suspension spring as well as straightening out the wheel. The stored spring force will tend to exaggerate the straightening of the wheel so it goes in the opposite direction, oscillating to and fro. The frequency of this oscillation is directly dependent on the spring force and the rotational inertia, ie the load. Heavily loaded, the effect becomes strong as the system is poorly damped due to the ratio of load mass and spring force.

Damping can be added by trying to hold the bars straight, by making the steering bearing drag or by making the pivots drag.

Tightening the pivots to make them drag heavily may fix the shimmy, but now the suspension has lots of stiction (which degrades performance) and folds with some effort (which you don't want either). Mine is already as tight as I will allow it; to fold it, I have to drag the front around to the folded position, it won't swing freely. I am definitely not going to make matters worse there. Plus lots of drag will make them wear much faster, and I don't want that either.

So, the shimmy is not caused by "loose suspension pivots" as is alleged; it is caused by the geometry design. A better approach might be to get a highly damped elastomer for the front. This same shimmy is also the cause for me not being able to ride it no-hands while unloaded.

Another approach is not to load the front, or to load it less.

The suspension also made the ride brilliant. The extra load made the ride much more plush again. This was particularly important as a lot of the KI roads are gravel roads which are often heavily corrugated from cars or rough from bad surface wear. The suspension did a huge job to smoothe all that out. SWMBO also agrees that the suspension on our bikes made all the difference between a horrible ride and a tolerable ride.

The only other bike which I have ridden fully loaded, with heavier load, albeit with all the load on the back, was the Raleigh Twenty. My opinion is the R20 is a better touring bike. Much better. I screamed down slopes faster than 60km/h fully loaded, with never a whimper from the R20, absolutely steady as on rails. With the Birdy I would now be nursing sores if I tried that.

Nevertheless, once I got used to the load shortcomings, it was fine.