Technical Update

This weekend I set about measuring as accurately as possible the squish clearance of my cylinders, and I learnt some stuff in the process.

I started out using 2mm diameter solder which I bent into "L" shapes with roughly the radius of a piston, I then inserted the solder into the combustion chamber via the spark plug hole, and trying to get the solder pointing along the gudgeon pin axis, and touching the cylinder wall, you turn the engine over and the piston squashes the solder. In principle you remove the solder and measure your squish clearance. You then do it again in the same cylinder but on the opposite side of the piston, and then you have two readings which you add together and take an average.

The problem of this approach is that you can not be sure that your solder is really at the edge of the piston, and therefore it is not as accurate as hoped (good for a quick check though!).

A more accurate method, is to remove the cylinder head, and then "stick" some solder along the axis of the gudgeon pin with some grease, ensuring that as you move the piston the solder is not moved by any ports, once you are convinced that the solder will not move, you position the pistons some where in the middle of their travel ie. not at TDC or BDC, then you replace the cylinder head, and torque it down, and then using the kick start you turn the engine until both pistons have come to TDC (one after the other) and squashed the solder, if you have put too much grease to hold the solder in place, the solder may stick to the cylinder head, so care should be taken to only do one revolution of the engine - no more as the solder will have moved, and the results will be spoilt. You then remove the cylinder head, and remove the solder, and measure the squish clearance. Repeat for the other cylinders!

This method proved far more accurate although the next challenge is where to measure the thickness of your solder! When the solder is pushed up against the cylinder wall, and then squashed, the very tip of the solder is not squashed, because it is in the clearance gap between the piston and cylinder wall, so you end up will a small ridge of solder at the very tip, which should you measure here, will give incorrect results. You need to measure just behind this ridge at the very smallest width - I have tried to represent this in the following diagram.

 

Of course the other problem is the width of your vernier caliper, which induces some error in the measurement although mine has a quite sharp edge which I used to insert just after the ridge of solder.

On a 125cc cylinder the squish clearance is given as an optimum of 0.8mm, mine were all measuring around 0.78 or 0.75 mms, which is just about within tolerance. So I will not be changing them, which is somewhat of a disappointment, as it means there is no additional power to be had there!

Circuit Elevation for Jetting

Le Mans             70m
C-en-T             122m
Magny Cours    222m
Nogaro               96m
Pau-Arnos         202m
Le Vigeant         160m
Carole                 73m
Ledenon            175-201m
Dijon Prenois     483m
Pommiers          589m
Grand Vivier      784m

minus 305m should equal an increase in main jet of 5 points
minus 609m should equal an increase in main jet of 10 points (for Mikuni 042 jets)
And of course assuming  constant temperature and humidity.

Technical Update

So the list of things to do get's bigger. I have ordered some different thickness base gaskets (0.25mms and 0.75mms) in order to get my squish down to 0.8mms. I have been reading all I can about setting squish, and I really need to be working out my compression ratio aswell. This involves measuring the capacity of the head - with a piece of (greased) plastic sealing the bottom, you pour water into the top from a graduated syringe and hence you measure the capacity of the head. You also need to measure the height of the exhaust port, and then you use a complex formula to get the compression ratio and the corrected compression ratio.

Hmmmmm.... I need to brush up on this, although taking the measurements would seem relatively straight forward.

I have ordered a new rear tyre, which will get fitted at the circuit.

I need to alter the final gearing for Magny Cours which is one of the fastest circuits of the year, so I will go down to my smallest sprocket at the back on the RG, and on the RD I will put on the biggest gearbox sprocket!

I can also improve on my cooling system. I made up some plastic fairing pieces which deflect air into the radiator, and these can be modified to fit better, and to seal better to the rest of the fairing.

Croix-en-Ternois Race Report

Arrived at the circuit Friday lunch time, unpacked the bikes and the van, and booked myself 4 practise sessions to really run the RG in. The RG started fine, and although initially only running on 3 cylinders (well spotted Libby!) - I had left a plug cap off, I let it warm up on the stand two full cycles before taking to the track.

As a reminder, the changes since the last race (seized), new pistons (now Technium Honda CR125 items), new small end bearings, gudgeon pins, rings etc... with a rebore. After having checked the squish clearance, I was running at around 0.5mms of squish which is too low. The recommended squish is anywhere between 0.8 and 1.0 mms, so I added another 0.5mm base gasket to bring me up to around 1.0mm. Whilst this is on the big side, it will certainly be less fragile. Also I upped the main jets in the rear carbs, to 230's and added the carb trunks which I consider to be worth at least 5 points of richening.

Nothing to report after 3 sessions of gently bringing the RG up to full revs. We didn't get to complete the fourth session (or even start it) as the heavens opened and we were all awash. The bike was running quite cool at around 65-70 degrees C, which is a good sign.

Saturday morning, I had to qualify with the 350, so despite a damp track I went out to qualify near the back of the grid - I was having problems getting confidence in the drying conditions, and I couldn't get my lap speeds down.
Next out on the now completely dry track, I managed to qualify the RG 500 with a 1:04, which whilst 6 seconds off the leaders pace, it did allow me to qualify for the Superbike final - all be it from the back row. Both bikes were running well, if a little soft, I probably needed to down jet on the 350 a little, and reduce the squish on the 500.

Classic Race1
I got a reasonable start from the rear of the classic grid and managed to pull back some places before the hairpin at turn 1. The organisers had mixed up the slower superbikes with the classics, and although this did provide more bikes, it was a bit of a pain as these bigger bikes are faster on the straights than my 350 so it was a case of lots of overtaking in the bends and then get blasted on the straights. I got my best lap time down to 1:05.8 which is 1 second off what I can do on the 350. I finished 3rd classic bike.

Superbike Race1
Starting from the last row, I was last into the first bend, I had some fun with a VFR750, and a GSXR, on the first few laps. The race was red flagged because of a petrol spill and a multiple pile up involving one rider needing hospitalisation (broken ribs and collar bone). On the restart I was up a grid row, with all the fallers behind me, and whilst they didn't stay there long, it made the rest of the race entertaining. My fastest lap was a 1:04.697 finishing at 22nd position - not last but nearly!

Saturday evening was again stormy from around 17:00 onwards, so much so, we were grateful to have booked a meal in the circuit restaurent!


Classic Race 2
Starting from the second row, I got a reasonable start, and straight away got past my competitors in the classic category, with only the TZ350, and a Ducati in front. I kept roughly the same position, not being able to pick up time on the guys in front, but once again losing out to the bigger power of the Superbikes that were racing with us. The best lap time I could manage was a lowly 1:06.

Superbike Race 2
Once again starting from the rear, the race was consistent with the first i.e. I was fighting to keep off the last place, which once again I managed but only due to the unfortunate fallers or break downs. Still the bike went reasonably well, nothing to report, pulling quite well 'til around 11k rpm.

I will be changing tyres for the next weekend at Magny Cours, and I will need to change the squish on the RG to be a little more agressive. Speak to you soon, and thanks for Chris and Libby for coming to watch.
I will post video soon.

For memory, the gearing on the 350 was too short, being difficult in a number of spots reving to high out of bend 2 and having to short shift into bend 5 (before hairpin). The gearing on the 500 was fine using a 42 tooth sprocket at the back, and standard at the front.