Team 3UpRacing Blogs

After summer full of interesting bike projects, we finally found some time to give the latest engine specification a good run out. For this occasion, we were very fortunate in having Toby Markham offer his time to evaluate our little project.

We’ve been talking to Toby for several months and everyone’s diaries finally lined up for October 12th. Toby has been riding a privateer Aprilia in the European rounds of the World 250cc MotoGP Championship. Toby was by far the most capable rider we’ve had on the bike to date. And, the first one to bring the much desired 250cc 2-stroke perspective. Afterall, our 95Racer was designed to compete with the 250cc class.

The engine now makes slightly less peak power at 92 bhp at the rear wheel but a significantly improved torque curve. It also hangs onto power longer. The exhaust system still needs more work and this will happen over the winter months. However, we felt it was worth testing before the end of the year, especially given Toby’s availability now.

So, we set off to Cadwell Park in Lincolnshire, England for a day of testing. We even managed to get a beautifully sunny day, which is not the norm at Cadwell Park at the best of times!

Well, now that the 95Racer is finished, which is hard to believe in itself, I’ve decided that I want to see what the antidote would be like. So, having thought about this for some time, this next project will be a road bike. A 2-stroke no less!

In brief, it will consist of:

• Yamaha TZR 3MA frame
• Yamaha Powervalve engine (somewhat special)
• Honda NC35 single sided swinging arm
• Supersport front-end
• Yamaha TZR125 tank
• Self supporting seat unit

How did I arrive at this combination? A good friend had this RGV250 with a bit special YPVS engine sitting in his workshop for some time. Everytime, I went there I asked him about it. As he is too busy and would probably never get it finished, I offered to help and take it off his hands. This is what it looked like gathering dust in his workshop.

Now, I was just going to finish it off as it really didn’t need that much. But, as it does, one thing led to another, and the goal posts kept moving. I really liked the thought of doing something with that engine. It has TZ750 pistons, a TZ250 crank, a close ratio gear box with dry clutch. Ideally, I’d like to put a Cheetah top end on it and take it out to just below 500cc. However, that’s for later. I really want to ride it this summer!

So why move the goal posts? It all started after seeing this TZR 3XV with a Stan Stephens 535cc power valve engine in it. It was really lovely looking and far more modern than the RGV250. So, after some measuring and chassis geometry CAD work, I decided to use a TZR frame. Is easily fits the powervalve engine, has a much sportier geometry than the RGV, looks bang up-to-date, and also happens to easily accept a NC30/35 single sided swinging arm. Of course, it needed a modern front end as well, which came from a Kawasaki ZX6-R, but that was more due to availability at a reasonable cost. Most modern radial brake front-ends would work.

So, after a bit of chassis geometry analysis using the ever-so-useful “whole bike software” from Tony Foale, and a bit of alteration, I am now at this stage. The TZR frame is completely unmodified, although it will be later to fit the engine and rear suspension mounts.

The swinging arm was a very easy fit. All it needed was a different size bearing in one end, a couple of spacers, and new inner spacer. The 916 wheel needed the centre hole boring out slightly.

The front was very similar, just needed a new set of bearings and the stem needed cutting down and rethreading.

The main chassis specifications (at static sag) are pretty much ideal with this combination:

• Wheelbase = 1368 mm
• Headstock angle = 23.6 degrees
• Trail = 100 mm

Well after some delays, the bottom end of the newest spec engine is ready to go together. The cylinder head is getting some additional work done including further porting work and dual valve springs fitted. It should be ready in another week or so.

In the mean time, I’ll be busy assemblying the bottom with the improved parts.

Improved Crank:
The balancer shaft gear has been removed and the crank lightened again and balanced to a new balance factor to move the vibrations to a more appropriate part of the rev range. As you can see from in the picture below, it’s a far cry from the stock part. It also weighs about 1.5 kg less!

This picture shows the shortened end of the crank with a slot cut into it for the external starter drive. Removal of the internal starter gears allowed us to shorten the crank and move the flywheel mass closer to the centre. As you can read further down, the flywheel is a fair bit lighter as well, as is the racing generator is now incorporates.

Reworked Stock Conrods:
The standard rods have so far proved strong enough. Plus we couldn’t find any lighter ones (although maybe stronger) on the aftermarket. Of course, there are always titanium rods. We concentrated on perfectly balancing them and strengthening the neck area with polishing and shot peening afterwards. It’s amazing how much hand work is just in rods alone.

We also found out that while originally Kawasaki had two types of rods available, J and K rods, they now have obsoleted the J rods. It turns out the the K rods are slightly thicker in the neck and small end area. It seems that the K rods were chosen maybe for slightly greater strength. They weigh roughly 8 grams more than the J rods. We now only use K rods.

During a track day at Cadwell Park last September, Gary Inman of Performance Bike Magazine arranged for their photographer to take a few pictures, which Gary turned into a 3 page article now published in their February issue. It’s at your newstand now! I especially like the 3/4 frontal view.

After acquiring another spare engine with very few miles on it, a new winter project is taking shape. We knew from this year’s track and dyno testing that the valve train wasn’t performing at its best. The springs are likely not ideal probably resulting in some valve bounce and float during the closing cycle of the intake and exhaust valve. With the lightened conrods and pistons together with a re-balanced crank, raising the rev limit to 12,000 RPM is another goal. This will only exacerbate any valve train issues.

We felt the best way forward was to perform some CAE analysis to determine what an ideal valve train would look like, and then go back to the dyno for more testing.

The first round of analysis showed clearly the amount of valve bounce on the intake side. The following sequence (one inlet cam rotation) is at 10000 RPM with our race cams. All other valve train components are standard, including the valve springs.

You can clearly see the valve tappet (beige rectangle) separating from the cam and the valve head bouncing off the seat at the closing point of the sequence. To quantify this further, the following graph shows valve bounce versus RPM and you can see that there is almost 0.8 mm of bounce! All is fine until about 8500 RPM. It’s not so good thereafter.

Needless to say, this doesn’t help! It also explains some of the wear issues we’ve seen on the race engine. So, the next step is to use the simulation software to design an appropriate valve spring to eliminate the bounce and float. Given these results, we feel that the 95 RWHP goal is realistic for next season. Afterall, we’re only 3 HP short now!

What else do we have planned for this engine? We considered increasing the displacement to just under 700cc, but decided against it as it would somehow go against the grain. It would feel like cheating somehow.

We also considered using aftermarket rods. The only ones available off-the-shelf are from Falicon. However, these are considerably heavier than stock. Therefore, we’ll carry on with the standard rod albeit lightened, polished, and shot peened. So far, we have had no issues with the standard, so modified rods.

We’ve moved to an electric water pump. This will allow us to remove the balancer shaft completely. Up to now, we had run a plain shaft with the balancer weights machined off. This plain shaft’s only function, thus, was to drive the standard mechanical water pump. After weighing all the components, the electric waterpump weighs roughly the same as the mechanical pump it replaces. This creates a further weight saving from removal of the balancer shaft in excess of 1 kg.