Wondering if any one has tried taller gearing? Was on a BSA club ride yesterday and there was a few miles where I had no choice but to be on the expressway. Cars going by me at 80 MPH and I could only do 60 MPH at full throttle on level ground. Down to 50-55 MPH if on a hill. Was not fun! Bike is a M21 with solo gearing. I realize I only have 15 HP to work with. Is taller gearing a waste of time as the bike won't pull it? Am considering doing the 720 CC conversion as I have the correct cylinder. How much does that help if I want to try and go 65 or even 70 MPH?
Bruce
55mph going up a hill? I can only dream of going that fast. These old war horses are not really made to go much faster without a LOT of effort on your part. Stay off the freeways?
I did the 720cc conversion on my M21 and it made a big difference in that speed could be maintained going up hills. Not much increase in top speed but it got there a lot quicker.
email (option): petercomley@web.de
Been there done that got the T-Shirt and the box of buggered clutches to prove it.
Mine was geared with a 23 on the box and it would not pull in either 3rd or 4th but it did go a lot faster if you could keep the momentum up.
I did this for the same reason to keep in view of the others riding A65's , B33's etc.
It worked,,,, sort of cause they lost me on hills when I had no option of going back to 2nd and on more than 1 occasion first.
The engine got really really hot and it just was not fun so as the others seem to be addicted to riding on highways , expressways, through shopping centers with traffic lights on every corner and a pedesterian crossing in the middle just for luck, I simply stopped riding with them.
Went down to a 21 so again 4th is still overdrive and allows me to slightly less than expressway speeds on flat or downhill so I don't get run down if I have to ride on one.
Mine runs a 626 or 928 depending upon where I am riding and who I am riding with and I ride with different clubs as a guest.
The BSA club is always bitching that I never ride with them but in reality they don't ride with me.
Those who can recognise these things tell me it also runs a GS touring inlet and std exhaust cam if that makes a difference, I don't know that is what was in there when I got it , 20 odd years ago.
I ride it basically as a 3 speed and only use top on long straits or downhill.
email (option): bsansw1@tpg.com.au
There are so many limitations to the side valve design in terms of extracting more power that there is no 'quick fix' to the problem..
All you can do by altering gearing, with a very limited power output, is to move performance around to improve acceleration for example...However, you will then pay for it somewhere else...
Regarding 'tall' gearing there simply isn't enough power to pull it beyond a certain point and the result is the bike will slow down as soon as there is a hill in the distance...:laughing:
Cams don't really help either as they are not something that are intended to be used in isolation but in tandem with other changes to improve gas flow/speeds across the range...
Compression ratio is limited in practice by the combustion chamber design with a compression ratio of approx. 7-1 being the limit before pre ignition occurs in a standard engine...So, a slight advantage can be found there in the case of the M20 which is only about 4.9-1....
Increasing the cubic capacity is the one change that produces tangible results without any other modifications...At 720cc the engine produces noticeably more power in the midrange and will display much improved acceleration and hill climbing performance...However, top speed only marginally improves and that illustrates how gas flow at the upper end is limited by the design...
I have found my M20 far better for longer runs where the improved mid and lower end performance and its enhanced hill climbing abilities can make a real difference to the average speed over a longer run. It's also nicer to run about on locally...However, sufficient performance to 'keep up' on modern motorways is still out of reach...Ian
email (option): ian@wright52.plus.com
Hi Bruce
I don't think I have ever overtaken a lorry on my M20.
Does filtering through a line of stationary lorries count?
Regards
Pat
ps I tell a lie, I once overtook a septic tank lorry going up a hill.
email (option): sacombsashtrees@hotmail.com
Not looking to be able to do 80 mph although that would be nice. But at 720cc and and a bigger sprocket is running at 65 mph even possible? At that speed I would not feel like I am about to be run over at any moment!
Bruce
Run standard gearing tall gearing would be a pain having to change gears all the time, if you really need to stay with the traffic flow on the freeways buy another bike. I have never had an issue on a freeway as they have multi lanes, I make sure they can see me, I tie a high vis vest on the back of the bike. Country Highways are riskier when I see a Roadtrain coming up behind me I pull off the road and nearly always get a thankyou flash as they pull away. I am planning to build a 720cc outfit trying to source a piston with little luck anyone know what model is suitable? My BSA is a 1948 M21 Sidecar and I cruise at around 70km/h 40-45mph. This is my first post hope it works.
Cheers John
email (option): jbadco@internode.on.net
I am considering having another batch of 90mm pistons made if the price is still within reasonable bounds (I haven't had any made for a few years)and if there is sufficient interest....
I don't want to carry the cost of a batch of pistons myself so a pre order arrangement will be required if I were to go ahead...
The pistons I have had manufactured previously are forged, with a modern ring set and will fit either an M Series engine with an M21 crank for 712cc or B Series engines with an 88mm stroke, a 94mm stroke or an 112mm stroke for 559cc, 598cc or 712cc respectively...Ian
email (option): ian@wright52.plus.com
Hi Ian. John Allison will require a piston for the M21 crank I have already had built up for his spare M20. Let me or John know when you need funds.
I'm quite happy with the 600cc conversion to my own M20, but I do have a B33 piston in stock to take it out to 635cc just for fun, and currently I think my only alteration to gearing is up one tooth to 20T on the engine sprocket. Ron
email (option): ronpier@talk21.com
Hello Ian,
If you are considering making more pistons, I want one for my M21. Let me know when to send funds if it happens.
Bruce
email (option): farrenb@att.net
My m20 has a 19 t on the crank shaft and a 19 t on the gear box.
I believe one of these is 1 t too much and should be 18 t.
Only a little difference, but i still have trouble to reach 55-60 mph.
Hi Ian If you are doing pistons put me down for one, it'll be just right for my B33.
I have a M21 crank & piston along with Gold Star Touring Cams (Ex 65-2450 & In 65-2448) in my '41 M20 which makes it a different bike. Very rideable. Like Ron, I have a 20T engine sprocket ready to fit.
email (option): grcajol@bigpond.com
Also interested in a piston kit.
John
email (option): jbadco@internode.on.net
+1 for a 90mm piston, as well
I'm making some enquiries amongst friends to see if there are more candidates for pistons...I'll make some enquiries next week about the current minimum order level and what will be the likely approximate cost per unit...Ian
email (option): ian@wright52.plus.com
See my concurrent post 'Oversize pistons update' regarding the latest info.....Ian
email (option): ian@wright52.plus.com