Best gearing for speed

Hi all.
I have an 850 Mk 2 Commando engine with Maney stage 2 head and a belt drive, running in a classic kneeler sidecar racing outfit.
Just about to install a 6 speed gearbox (Hemmings), and need to work out the best gearing (sprockets) to get highest speed at 6,500 to 7,000rpm.
Don't want to watch the rev counter hover on 6,000 because the engine won't pull anymore.

Any ideas, experience, calculations out there???

Thanks in advance.

may depend on circuit, obviously. CRMC forum?

jan nelder said:

may depend on circuit, obviously. CRMC forum?

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CRMC, BHR and CRMB! The 4 speed box has amazing torque off the line, but suffers in top speed, (Snetterton, Chimay, Cadwell etc.,) and also badly at smaller, tighter circuits like Lydden Hill, where the correct gear is just not available to power out of some of the bends.
Maybe it is Utopia, but I thought I'd like to just try to get it as best I can!
Changing gearbox sprockets for each circuit is a real bind, and changing rear wheel sprockets almost as bad. I thought that with a 6 speed box, it would offer more options...

If you're using the standard Commando drive-wheel setup one option may be to use different wheels for various circuits.

I have 16-, 18- and 19-inch rear wheels that with assorted tires probably alter the final ratio by 15% or so


Tim Kraakevik
kraakevik@voyager.net
630 940-2420

kraakevik said:

If you're using the standard Commando drive-wheel setup one option may be to use different wheels for various circuits.

I have 16-, 18- and 19-inch rear wheels that with assorted tires probably alter the final ratio by 15% or so


Tim Kraakevik
kraakevik@voyager.net
630 940-2420

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Thanks. The design of the sidecar allows only for 10" mini wheels - not possible to increase the size...

Go to https://www.gearingcommander.com/ and load the preloads for the commando. Then you can modify the transmission gearings to what your Hemmings box will run and see the changes on the different primary sprockets.

gortnipper said:

Go to https://www.gearingcommander.com/ and load the preloads for the commando. Then you can modify the transmission gearings to what your Hemmings box will run and see the changes on the different primary sprockets.

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Thanks for that! Looks like a great place to start...

If you want to avoid changing ratios for different circuits then logically you need to gear it for the fastest speed you think you will achieve. Let's say you think it would reach 120mph, then you need to calculate a ratio that will give you that speed at maximum revs.

Of course it's all theoretical until you get out on a circuit and try it. We used to race a sidecar outfit with a BSA A65 engine and rarely changed the gearing, only changed it for twisty slow circuits. We changed the rear wheel sprocket.

Ian

An old mate of mine raced an A65
said he used to change up when he saw three tachos !

Ha! Currently racing with a standard 4 speed box with 23/41 sprocket ratio. Have not altered this at all.
With a 6 speed box and 'best' sprocket ratio, hoping to do the same but faster!!!

If you are using a non-restrictive 2 into 1 exhaust system and have adjusted the valve timing to suit, your 850 motor should pull right through the top regardless of what gearing you use. I use methanol fuel, so my experience is probably slightly different to most. I use a single row primary chain with a manx clutch. I have just fitted a six speed box, but with the four speed CR box, my gears were - engine sprocket - 24 teeth, clutch - 43 teeth, countershaft - 18 teeth , rear - 38 teeth . To me that seems ridiculously high for the tight circuit at Winton Raceway. Winton has 12 corners and three moderately long straights in 3 kilometres. The bike was not fast enough towards the ends of the straights, and first gear was too high for clutch starts. But everywhere else the four speed CR box was perfect. When I fitted the six-speed, I raised the engine sprocket to 25 teeth, and the countershaft to 19 - too much. I will be pulling the box apart to re-seal it in the near future, so the countershaft will go back to 18 teeth. I am working towards going to two practice days in August.
For a sidecar you will be pulling two people, but the 850 should do it well, as long as you have close internal ratios. Every time I have raised the gearing on my Seeley 850, it has accelerated faster.
The gearbox as supplied with the normal Commando is a bummer. It bogs down in every gear coming up through the box and needs a heap of revs coming down. If you've got close ratios, the overall gearing is not so critical for different circuits - especially with six gears.

The difference between 4 speeds close ratio and the standard Commando box when racing, is stupendous - even if you have to risk destroying the box during a clutch start by revving the tits off the motor. Once you are moving - zoom. And six speeds will be even better. Friends of mine told me that because a Commando engine has loads of torque I should not need a close ratio box - BULLSHIT !
I am not bragging - just stating a fact - with the six speed CR box and with my steering geometry, my Seeley 850 would be extremely difficult to beat in Australian Period 4 historic races. And there are some bloody fast bikes in that class. The last time I raced, I turned under the first three guys on turn two, and was up beside the leader when the fuel line came off - that was with the four speed CR box. They were not hanging about when I did that. Those 1100cc methanol-fuelled CB750 Hondas probably turn out around 100 BHP.

Fascinating information - many thanks. I have 2 into 1 large bore exhaust system (unrestricted), a belt drive to the clutch, and currently looking at a 21 sprocket at the box, and a 36 at the rear wheel. This offers higher top speed without compromising the torque too much.
I agree the std 4 speed box is hard work - can't get it into first on the way down unless engine revs are zero it would seem!
I run on 100% AvGas with big 400 jets on the Amal Mk2 carbs, and regularly take out 3-4 racers off the grid!

Are there any precedents for using a Hemmings 6 speed in a sidecar?, I wanted a 6 in my solo, but as soon as mentioned Chimay and Geddine the advice was a 5 speed extra heavy duty box (TTI). I cannot see a sidecar giving the gearbox an easier time than a solo. Were you at Lydden last week?

Changing the rear wheel sprocket is just about your only option, you can use a split sprocket for a quick change which some people use in the solo class.

storm42 said:

Are there any precedents for using a Hemmings 6 speed in a sidecar?, I wanted a 6 in my solo, but as soon as mentioned Chimay and Geddine the advice was a 5 speed extra heavy duty box (TTI). I cannot see a sidecar giving the gearbox an easier time than a solo. Were you at Lydden last week?

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Originally wanted to go 5 speed, but decided on 6 speed Magnesium box (stronger) as the gear change is much slicker, and for Chimay and other long straight circuits, it offers a higher top speed. Plus, Lydden is a tight short circuit and the standard 4 speed couldn't give me the right gears to pull away out of a bend. Naturally, it isn't necessary to use all the gears available, as it is a CR box, you just use the ones most applicable.
Yes, I was at Lydden last week! Had a 750 lump in the outfit, but the circuit proved that the box was just impeding any progress I'm making with the performance!

Bernhard said:

Changing the rear wheel sprocket is just about your only option, you can use a split sprocket for a quick change which some people use in the solo class.

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This is true. Taking belt and clutch off to change gearbox sprocket is a definite no-no! However, the 850 has excellent torque, and with the 6 speed CR box installed, I could probably get the sprocket ratio as close as needed so no changes really needed.

Champy72 said:

Yes, I was at Lydden last week! Had a 750 lump in the outfit, but the circuit proved that the box was just impeding any progress I'm making with the performance!

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Ha, it was the circuit impeding my performance, couldn't get going.

Are you doing Chimay and have you got an entry, I cannot seem to find how to enter.

storm42 said:

Ha, it was the circuit impeding my performance, couldn't get going.

Are you doing Chimay and have you got an entry, I cannot seem to find how to enter.

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Yup. Chimay is booked . It can be a difficult site, but the web address is info@circuit.be You need patience and keep trying. It does work eventually and look for Pilote Registrations. You only have until 13 July to register!

storm42 said:

Are there any precedents for using a Hemmings 6 speed in a sidecar?, I wanted a 6 in my solo, but as soon as mentioned Chimay and Geddine the advice was a 5 speed extra heavy duty box (TTI). I cannot see a sidecar giving the gearbox an easier time than a solo. Were you at Lydden last week?

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When I bought the 6 - speed from Bruce, I asked him would it cop 80 BHP. He said it would easily. But he sells them. If they fail in service, that might destroy hi business. So there are two ways about thinking of this.
If you think about it, the gearbox in a Japanese superbike cops a lot of stick, - but I once asked one of the car guys whether he'd ever used the close ratio 6 speed from a GSXR750L in conjunction with his GSXR1100 motor in his race car. He said it was great, but did not last very long. That would have been more than 100 BHP and pulling a lot more weight.