Q: I was recently reading through your technical section and thoroughly enjoyed it. Then while I was watching a re-run of one of last years Moto GP races it induced a question into my head and I was wondering if you could answer it?

What size tyres to Moto GP bikes use and I heard 16.5- inch tyres are used on the front, if so why is this?

Looking forward to your response and keep up the good work.

Ian Houston    Richards Bay

A: The rear tyre is normally a 200-section (sometimes a 190 but not often), running on a 17-inch rim. The profile of the rear tyre will be changed though depending on the track and the grip available.

You are indeed correct though in saying they use a 16.5-inch front, but not all of the time. Basically it’s just down to handling and gyroscopic properties and which set up works the best for each track. 

Don’t forget they’re constantly searching for only a few hundredths of a second so everything is important and necessary in Moto GP.

Even though the rim is only half-inch less in diameter it saves valuable weight and therefore can make the bike ‘turn’ quicker. Also the tyre ‘footprint’ will change slightly offering more grip in certain bends, but might induce a spot of instability at different circuits.

Interestingly though Pirelli no longer produce 16.5-inch front tyres and only make 17-inch tyres now, which might prove the smaller rim is possibly destined for the bin?

Q: I was wondering if you guys could throw some advice my way because this problem is really doing my head in?

I have a Kawasaki ZX-10, which I believe is a 2006 model. Over the past few weeks it has developed a really annoying misfire between 5 and 7,000rpm. It does the same in every gear and every time it passes through this rev range.

I’ve taken it to my local dealer on more than one occasion and they changed the plugs and air filter and ‘stuff’, but the problem still remains.

So please try to shed some light on this before I set fire to thing, or maybe not?

Tinus

A: I’d first get your dealer to check the fuel pressure and make sure both fuel filters are clean because people tend to forget about them. 

One is like a t-bag inside the tank and protects the fuel pump so check that and clean out the entire fuel system.

Are there any fault codes coming up on the instruments? If so find out what they are? 95% of the time the fault code will show, 5% it won’t, so that could be an issue?

There has also been a problem in the past with this bike with the throttle position sensor, which would definitely generate this kind of fault at a specific rpm.

So get your local Kawasaki dealer to run a proper diagnostic test on these parts and stop being so lazy because they should be able to pinpoint the problem.

Q: Hi Noddy, I know you’re heavily involved with racing so I thought you might be able to answer my question.

I own a ZX-10 and always watch the WSB 1000 class. I already understand that they’re heavily modified bikes that cost a fortune. 

But my question is what can they do to the engines in the 600cc Supersport class to make them so fast? Your information will be greatly appreciated and thanks for a cool and free technical section.

Colin Mitchell     Cape Town

A: The Supersport class also involves heavily modified engines that cost a fortune so don’t think it’s a cheap form of racing. In fact they cost just about as much to modify and run as the big 1,000cc bikes do.

As far as I’m aware the engines still have to be 600cc but they run very high compression ratios, heavily modified cylinder heads, and the camshafts have to have the same lift as the originals but the duration the valves stay open is longer.

They also have some severe changes to the electronics including, programmable ECU’S, racing wiring harness and trick ignition systems including data logging features. This will allow the race teams to increase the rpm between 500-1000 rpm. Meaning a Honda CBR600RR for example will rev past 15,000 rpm, which of course can and does make the motor fragile. Of course bigger radiators are used to keep things cool as well.

You’ll also be interested to know that these modifications will push the horsepower past 140 at the rear wheel, which isn’t too far behind a stock 1,000cc engine, amazing eh?

Q: First of all I’d like to say I love your new site and thanks for the free advice in advance.

I have a 2003 Honda CR250 two stroke MX bike. Recently though, when I came to a small river, I was thrown over the handlebars and the bike smashed onto the rocks surrounding me. This bad fall did surprisingly little damage to the bike besides bending the ‘bars and breaking the front mudguard in half. But after I cleaned the bike at home I noticed a big crack around the steering head and hopefully it can be repaired? The forks and triple clamps seem to be OK.

So obviously my question is, can it be repaired or will I need a new frame?

Hans     

A: Having not seen the frame it’s hard to pass judgment of whether to repair or throw it away. If the crack isn’t too bad it may be possible to realign the frame using a jig and then welding it back into its correct position. There is a place in the south of Jo’burg called DOTT engineering that specializes in frame repairs.

But to be honest a MX bike of that age isn’t really worth anything so perhaps try to find another similar bike in a rough shape and make a good one out of both?