Best gearing for speed

With 6 speeds close ratio, if you are in the right gear all the time, the gears are spinning faster, so the internal loads are less than with a 5 speed or 4 speed box. I got nervous using the 4 speed in clutch starts - that is probably the worst case scenario. I notice my 6-speed box is about an inch longer than the 4 speed , so the internal gears are probably wide enough to take the loads.

I spent about 12 years racing a very uncompetitive, but still fast bike which had a close ratio 4-speed box. I could choose where I wanted to lose a race. I usually geared high, so it still had legs at the ends of the straights. I once purposely geared it low and blitzed a whole field full of modern superbikes and two-strokes for almost a lap. They got me towards the end of the straight on the old (shorter) Winton Circuit and I had to pull the bike up and run off the next corner to avoid hitting a couple of them, after they had got into the corner too fast and chickened out. With 6 speeds close ratio, that sort of thing should be much less of a problem. If you race-change a Commando with a close box, it accelerates like buggery, especially if you are using methanol fuel. Getting out of corners extremely fast is much more important that still having legs at the ends of the straights. But if you race like that, it is more dangerous, when the other guys are on faster bikes and less competent. With a solo, the most important thing I have ever found, is to get the bike to tighten it's line when coming out of corners. With the Commando engine, if you tend to run wide naturally, you are done. Always go under the others, never over, unless you have no choice. When I used to look at race bikes, the steering on all of them always looked the same - it isn't.

With my bike, I usually brake while cranked over, about a third of the way into even the slowest corner, then get straight back onto the gas from a very long way back, so I always have the run on the other guys. Doing that, you end up quicker towards the ends of the straights. The bike steers itself in corners and you have to anticipate where it will end up as you come out of the corner. I always ride very relaxed - the Seeley 850 is an easy ride.
With a sidecar, you don't have so many options.

I had a mate - excellent sidecar racer - who had various Hillman Imp engines, 998, 1040 and a 1175, all 100+ BHP. He had a 6 speed from Hemmings (Quaife), and whilst blowing engines with regular monotony, (well, it was an Imp!) he never had an issue with the gearbox.
Mick Hemmings, as with Bruce, has a reputation to maintain, and he sells loads of 5 and 6 speed boxes, and has done so for many, many years.
When I had my Imp unit with a 100 BHP 998 engine, the Quaife original 5 speed box (not a Mick Hemmings unit) did explode on me, leaving nothing inside or outside repairable!
When we race, we do so at our own expense!!!

I am a bit lost with your post really. The design of the Norton/Quaife/TTi boxes prive a 1:1 top gear ratio. Changing the box won't change your top gear ratio. So really, if it already revs to 6500 or 7000, it still will! If it won't, you gearing is already too high. Fit a smaller front or larger rear!

Whilst you say you have a belt drive, you don't mention the primary ratio it delivers.

I have a spreadsheet calculator I use based on one I downloaded some years ago and set up for a TTi 5 and 6 speed box, but I am not sure if it works with 10" wheels. Provided with all the relevant data I could try it.

My 5 speed HD TTi box ran in a sidecar for a season and I had to replace the mainshaft before I used it. TTi main shafts and sleeve bearings are much larger than the Quaife ones but it had suffered.

Sidecars load the box a whole lot more than solos!

If your overall gearing is too high, the motor will usually reach peak revs if you wait long enough. Wind resistance becomes the limiting factor. It is when you reach peak revs halfway down the straight and you get passed, that you have a problem. If the overall gearing is to high and you don't have close internal ratios, the acceleration rate is too slow. The trick is to have the gearing high enough that you still have some legs at the end of the longest straight, while having enough go to get there fast. It is the slowest corner on the circuit which is a major determining factor. If you can get around that really fast and come out of it like a blur, if you reach peak revs just before the end of the following straight - all is good. With a close ratio box, the race- change usually simply involves easing the throttle slightly, then standing on the gear-change lever. That way the revs are always way up there. If you let the revs drop below 5000 RPM with the 850 motor, you will wait forever to get them back again. With my motor, most of the torque seems to be well up in the rev range. So you ride the top of the torque curve and climb with the gears.

There is one thing about the steering geometry. On even the slowest corner my motor is still revving very high in a low gear, you cannot do that if your bike is on extreme lean. The steering geometry reduces the lean so the bike turns more under itself, so the bike stays more upright - thus safer when you gas it hard and early.

acotrel said:

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.

Click to expand...

Ha, I had an early GSXR750 and second gear didn't last long in that.

Champy72 said:

I had a mate - excellent sidecar racer - who had various Hillman Imp engines, 998, 1040 and a 1175, all 100+ BHP. He had a 6 speed from Hemmings (Quaife), and whilst blowing engines with regular monotony, (well, it was an Imp!)

Click to expand...

I would say that sets the precedent.

SteveA said:

I am a bit lost with your post really. The design of the Norton/Quaife/TTi boxes prive a 1:1 top gear ratio. Changing the box won't change your top gear ratio. So really, if it already revs to 6500 or 7000, it still will! If it won't, you gearing is already too high. Fit a smaller front or larger rear!

Whilst you say you have a belt drive, you don't mention the primary ratio it delivers.

I have a spreadsheet calculator I use based on one I downloaded some years ago and set up for a TTi 5 and 6 speed box, but I am not sure if it works with 10" wheels. Provided with all the relevant data I could try it.

My 5 speed HD TTi box ran in a sidecar for a season and I had to replace the mainshaft before I used it. TTi main shafts and sleeve bearings are much larger than the Quaife ones but it had suffered.

Sidecars load the box a whole lot more than solos!

Click to expand...

All gearboxes final gear is 1:1 as far as I know. The object of this exercise is to be able to a) get through the gears more quickly and more flexibly (I have run a 4 speed, and whilst very robust, never had the gearing options to get out of bends and accelerate quickly), and b) due to it's close ratio, it will be possible to change the gear/rear wheel sprockets to increase the top speed. A 23 gear sprocket with a 46 rear wheel gives 2 engine revs for one rear wheel rev. If you can change this to 21 gear sprocket and 36 rear wheel, the engine rev is 1.7 for 1 rev of the wheel, hence a faster top speed. Examples only used here for clarity. Of course, you need to be careful that you don't sacrifice the torque too much just to gain some extra speed, or vice versa.
I also have 'Gearing Commander' calculator, and just playing with this shows the results in speed and torque.
I ran a 5 speed Quaife for many years in a 100 BHP Imp engined unit before it blew, and the 4 speed Norton just seems to keep going (13 years hill climb and 2 years road racing) with a slick up change without clutch.
Appreciate your comments.

There are two things - getting out of the corners quicker and still having legs at the ends of the straights. I usually run out of puff towards the ends of the straights, but getting on the gas earlier in corners means you have a better run because you start accelerating sooner. So your speed further down the straight is higher. That makes what happens in the last few metres before the next corner less relevant. With a close box, your acceleration rate is higher than with wider ratios and with six speeds you will have sufficient gears to keep you accelerating down to the ends of the straights. With the close box, the distance between the gears is less, and with six gears you will probably end up with the same spacing as in your 4 speed box as you raise the overall gearing. If you continue accelerating out of corners from the same points, you won't come out any faster, but further down the straights you might have two more gears. So your motor will keep pulling instead of going above max torque. A bit depends on how high your first gear is in the 6 speed box - the gears are usually evenly spaced between first and top.

It is probably better to run a larger engine sprocket to raise the overall gearing. That means the gearbox turns faster, so the internal loads are less.

Champy72 said:

All gearboxes final gear is 1:1 as far as I know. The object of this exercise is to be able to a) get through the gears more quickly and more flexibly (I have run a 4 speed, and whilst very robust, never had the gearing options to get out of bends and accelerate quickly), and b) due to it's close ratio, it will be possible to change the gear/rear wheel sprockets to increase the top speed. A 23 gear sprocket with a 46 rear wheel gives 2 engine revs for one rear wheel rev. If you can change this to 21 gear sprocket and 36 rear wheel, the engine rev is 1.7 for 1 rev of the wheel, hence a faster top speed. Examples only used here for clarity. Of course, you need to be careful that you don't sacrifice the torque too much just to gain some extra speed, or vice versa.
I also have 'Gearing Commander' calculator, and just playing with this shows the results in speed and torque.
I ran a 5 speed Quaife for many years in a 100 BHP Imp engined unit before it blew, and the 4 speed Norton just seems to keep going (13 years hill climb and 2 years road racing) with a slick up change without clutch.
Appreciate your comments.

Click to expand...

Well, most British style gearboxes are 1:1 top gear...most of the Japanese ones I have dealt with are not! With top gear typically designed as an overdrive.

A Manx style 4 speed is robust if it has the kickstart mechanism and layshaft bush thrown out and replaced by a ball bearing at the timing side, and if it is spun quicker. I did that in the '70s and didn't have to open the box.

The standard Commando primary ratio is not suitable for racing. Most belt drives raise it sufficently.

Since typically a sidecar will benefit from a flatter torque curve, which the 850 should have as compared to the 750, logic says a 5 speed is a good compromise. It will be interesting to see how you get on with the 6 speed.

I considered a 6 speed for my 750 short stroke, but I compromised when a decent used 5 speed TTi became available. Works well. Most people seem to feel the only benefit of the 6 speed would be not having to change gearing for different circuits!

In terms of lap times there is a trade off between suitability of the ratios for each corner and the time taken to complete up and down gear changes in each sector.

So the tracks you ride will also be a factor.

I usually gear for the slowest corner and the following straight. If you get the slowest corner right, the rest is usually a soda. At Winton Raceway, the slowest corner is a hairpin at turn 6. I get around there in second gear and wheel-spin coming out. If I have trouble getting around the corner at the end of the following straight, I am happy. I usually brake about a third of the way into corners while cranked over, then get straight back onto the gas. If the other guys have got 100 BHP, they will usually pass me down near the end of the longest straight. But with a Seeley, that is not really a problem. The rest of the circuit is fairly tight. With the 6 speed box, things might be a bit different. The main problem with the 4 speed box, is the high first gear and clutch starts. 5 gears is probably enough.

The last thing you want to do at a sidecar race meeting is change the gearbox sprocket! Removing the fuel and oil tank to get to and remove the belt clutch, and refitting and re-tensioning in between races is almost guaranteed to create tears, especially if you don't get the alignment/tension of the belt fairly accurate.
Changing the sprocket on the rear wheel is just as bad, as the whole wheel needs to be removed! (Running on 10" wheels...).
So, yes I hear what everyone is saying, but to get maximum flexibility at every track without the necessity to change sprockets for each meeting is the aim. In fairness, I never changed the gearing on the Imp engined unit...
I am very close to confirming the sprocket sizes - just need to check a few figures and calculations.

Thanks to everyone for their input!

I agree totally changing gearing at a circuit is a pain......I don't change the gearbox sprocket anyway....but I change the rear sprocket at home. A Rickman with swinging arm plates is no easier than a sidecar for setting chain tension.

With the aid of a gearing calculator and some circuit reference data I haven't changed gearing in the paddock since probably '77, maybe '78!

Good luck.

Thanks for that. I'll see what happens at Chimay, as the gearing will all be new to me! If necessary, I will change gearbox sprocket and/or rear wheel sprocket but only if it really needs it! Otherwise I'll fine tune when I get home.

At chimay you need the tallest gearing you can fit on the bike!

The straight is very long ( depending on which chicanes they use). It’s faster than Snetterton.

Look at YouTube videos of Chimay from recent years. If they don’t use the first chicane after turn one, you will be flat in top for ages.

Yeah. Rode my GSXR 750 there 2 years ago - speedo went way over top indicated speed!
Went again last year with sidecar and 4 speed standard box. The long straight where you bury your head and hold it at max revs praying for the end is only interrupted by the awful bus stops they sprinkle around the track.
I will use this as a good try out. I've upped the ratios and can up them more when there.

Actually prefer Gedinne - a much better road circuit with some interesting natural bends!
Can't get there this year, but definitely will for 2020 - visa's, carnets and certification dependant...