Author Topic: 80/90 Plus Gearing (Read 6004 times)

ccmman · « **on:** 19 May 2019 at 08:04 »

Have now got the engine running really well, though still running in, which is tricky on such a cammy engine!

But the gearing has me completely flummoxed. LJKS talks about a close ratio box on his 90, my box is the complete opposite. The first three gears seem ok, then top is crazily high.

At 2000rpm in top the bike is doing 55mph, which equates to 192mph at 7000 rpm!

My gearbox sprocket/rear wheel sprocket ratio is 17/53 or 3.12:1.

Any advice please, I am confused!

Clive · « **Reply #1 on:** 19 May 2019 at 08:13 »

Dont forget the bevel gears in the gearbox which must be at least 2to1 may be more
Regards Clive

ccmman · « **Reply #2 on:** 19 May 2019 at 10:06 »

This is the problem Clive, I'm not good at motorcycle gearing. I'm not clear where the gearing is measured.
To me, it makes most sense to refer to the engine revs/back wheel revs, because that gives you actual rea-life gearing.
In that case, my bike does just under 3 engine revs for every rear wheel rev in top. That means, say, 2.8:1.
Now take the figures I found on a factory spec sheet for the Plus 80:

1st 17.2:1, 2nd 10.6:1, 3rd 7.78:1, and top 6.17:1.

Clearly nothing like 2.8:1, so how are these figures arrived at? Is it engine revs to gearbox output sprocket maybe?

ccmman · « **Reply #3 on:** 19 May 2019 at 10:11 »

I meant to add, the factory figures I quoted show the gap between 3rd and top being 7.78:1 and 6.17:1, which looks like a nice close jump, whereas on my bike there is a massive gulf between 3rd and top.

The sun is shining here, so Im going to go for a blast, meet my mate for a Sunday lunchtime beer, and will check the speed at 2k revs in 3rd and top and report back.

Thanks

eddie · « **Reply #4 on:** 19 May 2019 at 10:48 »

ccmman,
You cannot possibly have a 3.12:1 ratio top gear! On post war Douglas machines, the gearbox main shaft is driven directly off the end of the crankshaft - then the drive goes through a pair of bevel gears to turn the drive through 90 degrees - the ratio of that drive is 14/26. This has to be multiplied by the ratio of the sprockets which, in your case, gives you an overall ratio of 5.79:1 (still too high for a 'Plus'). Mr Douglas listed various gearbox and rear wheel sprockets for the '90 Plus' - depending on what the bike was going to be used for (fast road machine, clubman racer or outright road racing machine). Also listed were several sets of gears giving normal, close ratio, or ultra close ratios - but none of these would affect the top gear ratio as, in all cases, the drive in top is direct through the box to the bevel gears and rear chain.

Regards,
Eddie.

ccmman · « **Reply #5 on:** 19 May 2019 at 22:07 »

Eddie
if you read my post, 3.12:1 is the ratio of gearbox sprocket to back wheel sprocket. I included that as information in case.
Can you simply relate how the given gear ratios are calculated? Which 2 rotating member of the drive train are the two parts of the ratio referring to?
Main issue for me is to source some gears that work with my set-up. There is no way it will pull the current top.
Thanks, Tom.

eddie · « **Reply #6 on:** 20 May 2019 at 07:05 »

Tom,
As I said in my previous posting, initially, there is a direct drive through the gearbox in top gear(i.e. 1:1 ratio) - the only reduction is through the bevel gears on the final drive at 14:26 (1.86:1). The overall gear ratios are then adjustable by changing the gearbox/rear wheel sprockets - in your case 1.86 x 53/17 = 5.79:1. What further reduction you get in the intermediate gears depends on the gear cluster within your gearbox. So, as I said previously, you cannot obtain a top gear ratio of 3.12:1 (or even 2.8:1) - the lowest ratio possible with your sprockets is 5.79:1!! This now begs the question -"How are you counting the engine revs?" - if it is by the number of times a valve opens, if so, then you need to multiply that number by 2 (as the valves only open on every other revolution of the crank)(doubling 2.8 would give you almost 5.79!).
All the postwar gearboxes (other than the close ratio boxes on the Plus models) had the same internal ratio for 3rd gear (1.266:1) and that would give you a 3rd gear ratio of 7.33:1. Any other combination of internal gears would give you closer ratios - in the case of the ultra close 'box, the internal ratio for 3rd is only 1.099:1 - the resultant 1st and 2nd ratios are so high, it is almost impossible to get the bike off the start line!

Regards,
Eddie.

ccmman · « **Reply #7 on:** 20 May 2019 at 08:02 »

Thank you Eddie. I have two problems, one is I'm thick, and the other is my gearing!
You are absolutely right. I have undercounted my engine revs by a factor of two.
I also have a rev counter which is reading half engine revs which doesnt help.

I will go back to the drawing board and work out all my ratios, hopefully correctly this time, so that I can plan a way forward.

One last thing, what is the smallest sprocket I can buy to fit to my gearbox output shaft?

Thanks for your patience.

Tom.

eddie · « **Reply #8 on:** 20 May 2019 at 08:40 »

Tom,
The smallest sprocket that the LDMCC spares stocked was a 16 tooth for a Competition (trials) Douglas - that is, assuming you have the 5/8 x 1/4 rear chain. Originally, Douglas also listed rear sprockets up to 57 teeth. I doubt that the spares scheme will be able to help with the larger rear sprocket, but they can be obtained from bearing stockists (known as 'plate wheels') - then you will need to get the centre machined out to suit your brakedrum.
Regarding gear ratios - most British road machines, as supplied, were overgeared. My Dragonfly was the same, so I lowered the gearing by fitting a 16 tooth gearbox sprocket (instead of 17T). Even with the lower gearing, when fully run in, the engine was capable of almost 60mph in 2nd gear, about 75 in 3rd, and would cruise at about 65 - 70 in top. Having said that, it was probably at it's happiest at about 65, but Douglas engines don't lend themselves to slogging - they are happier if you play tunes on the gearbox!

Regards,
Eddie.

ccmman · « **Reply #9 on:** 20 May 2019 at 09:20 »

Thanks Eddie, your comment on Douglas long legs is helpful, because maybe I just have to put up with it.

I will work out my gearing, create a gearing chart, and get back to you.

Tom.

ccmman · « **Reply #10 on:** 20 May 2019 at 20:31 »

Having done the gearing by counting wheel revs/engine revs., I have got pretty well what it says on the factory (?) chart.
That is: 1st 16:1, 2nd 10:1, 3rd 7.92:1 and top 5.96:1.

But the problem is that unless I am on a straight with the wind behind, or on a long downhill, it wont pull top at all. This may be partly because I am still running in, so am worried about running at high revs.
If I change into top at 6500rpm in third, the revs drop to around 5200rpm, which is not really in the power band, believe it or not.
When I am a bit braver, and can rev it to 7500 in third (or 8 maybe??!!), that drops revs to 5900, which is better power band wise.
I am thinking that a 16 or 15 tooth gearbox sprocket will help simply in terms of top being too tall to pull.
Eddie, is my max revs 8k when she's run in?

Thanks Tom.

eddie · « **Reply #11 on:** 21 May 2019 at 06:53 »

Tom,
The 'Plus' machines were produced for Clubman racing, and were subject to continual development during their production. During this time, 3 different camshafts were used - the original cams and the 'number 2' cam were for racing use - at least one type was designed to run with 25 thou tappet clearance. Later engines had a milder cam (number 3) that I believe ran with 10 thou clearance. The only way of telling which cam you have in your motor is by removing a barrel and looking at the camshaft - it is usually stamped with the number between the 2 cam lobes.
As for the revs a 'Plus' engine will go to - 7,500 seems to be where maximum power is developed - later engines went even faster - the final development 'black iron motors' were reputed to be good for 11,000 RPM!! (but they were not known for their longevity). At those revs, very little oil got to the back bigend - Eddy Withers did a lot of modifications to try to cure the problem, but by that time the Pluses were outclassed by the BSA Gold Stars.
With regard to gearbox sprockets, 16 tooth is the smallest available - any further lowering of the gear ratios has to be done by increasing the size of the rear sprocket.

Regards,
Eddie.

ccmman · « **Reply #12 on:** 21 May 2019 at 07:42 »

According to my gearing chart, with the current 17/53 sprocket set, top speed would be 97mph at 7500 rpm!
I cant see much point in having an unattainable top speed (unless its for coming down the Mountain..), and, just changing the gearbox sprocket, 16 teeth would give 92mph, 15 teeth 86mph.
So I will try a 16 tooth if there is one available, or make my own 15 toother.

Its all very interesting!

eddie · « **Reply #13 on:** 21 May 2019 at 08:35 »

The biggest problem with making a smaller gearbox sprocket is the fact that it incorporates the shock absorber cam - this being awkward to make and it is bulky enough to foul the chain side plates when you get down to less than 16 teeth.
Because of the complication involving the shock absorber cam, when we were involved with Club Spares,we found getting rear sprockets made was the cheaper option!

Regards,
Eddie.

ccmman · « **Reply #14 on:** 21 May 2019 at 08:48 »

Is there any problem going more than 53 teeth on the rear sprocket? Other than my sporty bike looking like a trials weapon!

Neville Heath · « **Reply #15 on:** 22 May 2019 at 14:41 »

ccmman
I have been following your letters with considerable interest. It may be an idea to remove the foot change cover (drain oil first) and see if you have a standard set of gears. The 90 close range is 30,25,23 21 on the main shaft and 22,27 29,31 on the layshaft. I spent all last Sunday and again last evening, on V.M.C.C. runs on my 90 with these ratios. It needs a bit of help when pulling away with 16/49 sprockets but once away the close ratios are a dream on the winding roads of Suffolk. Great fun.
Neville

ccmman · « **Reply #16 on:** 22 May 2019 at 22:05 »

Thank you for taking an interest Neville. I am away from home at the moment but will certainly check my gears as soon as I return. I the meantime... your sprocket ratio is 3.06, mine is 3.11, so if top is always the same in the box, yo have a slightly higher gear there than me.
Eddie has said that top is straight through, so only the intermediate gears are variable.
Can you give me an idea of your revs to mph ratio in top? I am doing 50mph at 4000rpm...

Neville Heath · « **Reply #17 on:** 13 Jun 2019 at 14:32 »

Having in mind your question I compared my speedo and rev counter during a run on Tuesday and found in top gear they corresponded. i.e. 40mph = 4000 r.p.m.: 60mph =6000r.p.m. etc. As I said the gearing is 16/49. Back tyre is 350 not 300. Speedo read 30 when I got a grin and "30" from a local speed camera.
Neville

ccmman · « **Reply #18 on:** 15 Jun 2019 at 13:37 »

Thanks Neville, very helpful. I am going to buy a 16 tooth sprocket from the club, and swap it with my 17 tooth. I should be nearer what youve got then. The good thing about going one tooth different on a smaller (as opposed, say to a rear wheel sprocket) sprocket is the percentage change is greater.
Only downside is they are bloomin expensive!

Neville Heath · « **Reply #19 on:** 19 Jun 2019 at 18:55 »

I have just looked up my copy of the factory Maintenance Instructions booklet issued with new Pluses. I quote:-
Gear Ratios 90 Plus.
1st 11.48 : 1
2nd 7.78 :1 Special 7.17 : 1
3rd 6.68 1 Special 6.17 : 1
4th 5.70 1
Range of sprockets available G/B 16,17, 18. Rear between 49 and 57. These provide a variation to top between 5.06 :1 and 6.62:1
Maximum Safe Crankshaft Speed 7.500 r.p.m.
Neville

Neville Heath · « **Reply #20 on:** 21 Jun 2019 at 15:43 »

ccmman
What do you call "Blooming expensive?" I have a gearbox sprocket available which needs new teeth welded on if that would help.
Just out of interest I also have in my bits box a 19 toothed sprocket assembly. Should be good for 200 m.p.h. with a tail wind!
Neville

Neville Heath · « **Reply #21 on:** 04 Jul 2019 at 19:02 »

Photo of 19T Plus sprocket. Bet you thought I had got confused with a Mark 19T one. Both available if anyone interested.
Neville

Attachments converted to linked images - Dave, 07Feb2021