The Syma S107 is a remarkable little RC heli – despite some possible quality issues in recent times. It is very robust, stable and easy to fly – exceptional value at around AU$30 delivered. Of course, what is really lacking is insanity, so here’s how I went about turning it into an unstable, twitchy and fast monster.
Weight savings is where it’s all at I reckon. Fitting a more powerful tail motor might help, but it is extra weight and power. I instead focussed on stripping as much weight as possible, and shifting weight bias forward for increased forward speed, at the expense of rear speed (or even stable hovering!).
Here’s where I saved the weight:
- Removed undercarriage (1.91g)
- Removed tail brace (1.20g)
- Removed tail rudder (0.44)
- Remove nose cone, weight and led (3.26g)
I also shifted the battery as far forward as possible. One other weight saving was trimming down the balance bar weights. Unfortunately the new S107’s don’t have removable weights like the originals did. Instead, I cut a small section off the end of the weight. Not sure on the exact weight, but since it is rotating mass, it makes a huge difference in rotor acceleration. Stability is affected, however I see this as a positive since it gives more twitchiness.
A total saving of approximately 6.8g. With an approximate stock weight of 38.3g, this gives a saving of around 17.7% – very, very noticable. Hovering can be achieved at around 1/3 throttle (instead of around 2/3 stock), and banked turns are possible due to the increased forward speed. Highly recommended mods once you get the hang of the stock S107 and are looking for a bit of insanity.
As seems to be typical for a bike from the 80s, my 1988 2NX SRX600 has shitty, wooden brakes. This is the result of poor hydraulics, particularly the ratio between the master cylinder size and the total caliper piston area, which dictates the mechanical advantage of the system. The SRX6 has a 15.8mm master cylinder (5/8″, 195.9mm^2 area) and a single four piston caliper acting on a 320 mm floating disc. I wasn’t able to measure the caliper piston sizes at this point. Improving the hydraulic ratio can be done by either using a caliper with a larger piston area, or a master cylinder with a smaller piston bore.
Master cylinder piston size isn’t something that is typically listed, so it took a bit of searching to discover that NSR150 master cylinders are 11mm (piston area is 95mm^2). I got one in decent condition off eBay for around AU$43 delivered. Fitment was fairly straight forward. Due to the increased lever travel I had to rotate the switchblock to give a bit more clearance. Also, there is no mirror mount, so I will use a bar clamp mirror mount. The brakelight switch doesn’t fit, so I will have to find a suitable switch.
Performance is now excellent. Very nice initial bite, and plenty of power for one to two finger use. Lever travel is good – while I could squeeze the lever in enough to get in the way of my fingers during testing, the braking power is good enough that this would only happen if I had some serious brake fade happening, or I was doing a stoppie. I will probably upgrade to braided brake lines at some point, but at the moment it just isn’t really necessary, as there isn’t too much flex apparent.
Overall, A highly recommended and successful upgrade.
If you want to adjust the hydraulic ratio of your brakes to get better feel, then below is a list of possible donor bikes for which I know the master cylinder sizes. Remember, a larger master cylinder will give less power, but require less travel (potentially make the lever feel more wooden), and a smaller master cylinder will give more power, but require more travel (potentially make the lever feel spongy if you go too far). Some master cylinders have the piston size stamped on them (whether it be metric or fractional inch), but a lot don’t.
15.8mm – Very common, especially on 80s bikes, and modern bikes which have twin four piston calipers. Some specific bikes that I know of include: FZR250, FZR400, FZR600, SRX600. It seems to be almost the default size.
14mm – The VFR400 (NC30) uses a 14mm remote reservoir Nissin master cylinder. This is coupled to twin four piston calipers (unsure of the total piston area), and leads to excellent brakes.
12mm – The XT600 (90-2002 at least) uses an integrated reservoir 12mm master cylinder with a two piston sliding type caliper. This gives ok hydraulic performance (although hydraulics can only do so much given the wheel/disc size ratio), but feel is pretty shitty due to the bolt from the lever which acts on the master cylinder piston.
11mm – NSR150 has an integrated reservoir 11mm Nissin master cylinder. The lever is not adjustable for reach. I believe that it is coupled to a two piston sliding type caliper. Unsure of years, but the one I have has piston bore stamped on it.