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Lowered gearing on Africa Twin
Hello out there! Does anyone have any experience of lowering the gearing on an AT? I've searched the forum but can't find anything specific to the bike. I know of a suplier who sells rear sprockets with one tooth extra. I'm hoping that by down gearing I can to some extent smooth the jerky "on-off" effect when I close the throttle at low revs - not a good feature to have at low speed; does anyone else have this on their AT, or is it just mine?
Anyway, another benefit (apart from slightly sharper acceleration) I'm hoping to get is that it might actually IMPROVE fuel economy. This might seem mad, but a reply I received from someone on another thread (Rich Lees??) mentioned that raising gearing is not always the way to improve economy. My thinking (flawed as it may be) is this. The AT is big and heavy, and uses much more fuel when it's used on motorways, especially above 80 mph (attested to by others on this forum). If constant, flat motorway work does this it seems that wind resistance may be the major factor in fuel useage (as it is for most vehicles). In head winds and on gradual hills we all tend to unconciously wind the throttle open, waisting fuel that a lower cog would have saved - ever used low ratio on a mountain bike up a particularly steep gradient? Based on this, lowered gearing should allow the same speed to be held using smaller throttle openings. Also, it should allow easier riding at in top, and maybe take a little strain of the engine when it's loaded. If anyone has tried this I'd be grateful to hear how they got on, because the non standard sprocket and extra O-ring chain length required will make the experiment expensive. Cheers, Stig |
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When I first got my AT I found that it would rev all the way to the redline in top gear, which I didn't feel comfortable with. So, given that the engine has oodles of torque low down I changed the gearing. I chose a sprocket for the back with 42 teeth rather than 45 and found the bike a lot more relaxed at higher speeds without losing any of its 'get up and go'. When I last replaced the chain and sprockets the supplier told me that there was no 42 tooth rear sprocket available and I had to go with a 41 instead. The bike still works great and the engine never feels stressed not even when it is being abused.
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electric_monk, can you let us all know what sort of revs you're pulling at certain speeds with the 41t rear sprocket? 60 and 80mph in top gear seem to be my main cruising speeds.
The main effect on fuel consumption of the AT in my experience is wind drag. If I remember my physics classes, drag increases with speed squared. Empirically this seems to be the case, with fuel consumption at 80mph much higher than at 50mph. LordStig your logic is indeed flawed. Whilst a lower gearing requires less effort, and thus less throttle, more revolutions are required to achieve a desired speed. However if you go the opposite way and dramatically increase the gearing you'll have a situation where the engine is labouring, and you're pumping in more fuel than can be effeciently burnt. So in effect your speed (and hence drag), engine revs, and throttle position all come into play at the same time. Something I've always wandered about: is it more economical to maintain a set speed (by varying the throttle) up and down hills, or to maintain a set throttle position (and let the speed rise and fall) up and down hills? I suspect it works out the same! Iain '02 Africa Twin '00 XR400 |
Iain
Just back from a quick blast around the M50 and can tell you that at 60mph the revs are 4200(approx) and at 70 mph they are at 5000. Now, as you probably know our speed limits here in Ireland are the same as those in England so I have no way of knowing what the revs are at 80 mph, but I am reliably informed they are 5600rpm. I don't know for sure what oter factors might influence these readings, but for the record my rear tyre is about to be replaced and my left hand headlamp bulb is not working. ------------------ |
Hi Iain,
Thanks for the reply; it's great to hear a scientific perspective on this! I was curious to hear whether anyone had tried the mod to see what actually happens. I follow your logic entirely (I already knew this), but in the real world logic is usually also affected by other factors. What happens on a flat motorway in neutral wind conditions, or on a gradient in the same wind conditions will significantly affect theoretical predictions. You are certainly right that overgearing would result in inneficient fuel burn when the engine is loaded, and that an overly lowered ratio would increase the number of detonations of a given fuel charge - both resulting in non-optimal fuel consumption. Honda must have taken this into account when they designed the machine, providing it with a 'compromise' set up. I very much doubt that they were particularly worried about economy. Electric_monk, did you notice an improvement in economy when you raised the gearing and, if so, what kind of riding do you mainly do? That would be interesting too. I'm not in the slightest bit interested in top speed. If I was, I'd buy a sports bike....... Nevertheless, I suspect that lowering the final drive gearing by a single tooth on the back should not alter the gearing to point where the engine is wizzing away doing very little except keeping its own mass moving. The reason I suspect that there may be an improvement in economy is that as overlanders we generally carry a great deal of weight, and our luggage often has to be mounted in such a way that it must cause a considerable increase in drag over that of the standard machine. The standard gearing of the AT might be _slightly_ too high for the conditions encountered by the average overlander (I would think most of us are keen to avoid motorways, which are after all the habitat Honda probably had in mind for the AT). It seems odd to me that most who have overlanded with their AT have changed their suspension, modified the frame, increased the fuel capacity (etc. etc.), yet apparently no one has tried lowering the final gearing by a tooth. Well, I'll have to give it a go and report back! But that's the fun, isn't it? Cheers! Stig. |
I still haven't got round to testing the idea - the chain I have just keeps on going - but I did find the reply from RichLees that gave me the idea in the first place. I might do a bit of further research into this.
Originally posted by RichLees: "a guy in the tuning department at Rover once told me that you get the best mpg by operating at the revs that would give peak torque on full throttle. so, gearing to run at lower rpm means less efficiency from slower turning engine and, hence, potentially worse mpg. ie I don't agree with gearing up for lower fuel burn and I agree it can put more load and hence wear on the chain. my xrl chugs at below 100kmh in top and so I usually change down to cruise at 90kmh. even then, I only get 62mpg and I used to get 65mpg when being gentle with my ZX9" I've conducted a few long term 'experiments' with my AT on standard gearing, and these confirm that a) throttle opening has a major affect on fuel economy; b) riding in fourth rather than fifth gear at a constant low throttle opening has a greater affect on fuel economy than throttle opening in fifth alone; c) head winds (around 5-10 mph, sometimes oblique rather than directly head on)decrease economy by around 10% with a constant throttle opening. It's possible that Iain is right about set throttle/variable speed versus set speed/variable throttle are equal, but my data are not detailed enough to be able to tell (piss poor for a scientist, but there you are!). Riding in fourth rather than fifth (only for a third of my morning journey) suggested that the economy drop would be greater than 10%, but fourth is a lot lower than fifth (much more than the minor gearing revision I have in mind), and the distance I tested this along with the relative lack of wind (and hills) on the day I tried it mean these data are inconclusive. In reality I suspect the change with only a single tooth added to the back will not make enough of a difference, and it's all probably down to individual preferences. I just like to fiddle.... Stig |
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