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11
Douglas Motorcycles - General Discussion / Re: 80/90 Plus Gearing
« Last post by ccmman on 20 May 2019 at 21:31  »
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.
12
Douglas Motorcycles and Parts Wanted / Re: Clutch Bridge Wanted
« Last post by GlennMckenzie on 20 May 2019 at 11:48  »
Bump
13
Douglas Motorcycles - General Discussion / Re: 80/90 Plus Gearing
« Last post by ccmman on 20 May 2019 at 10:20  »
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.
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Douglas Motorcycles - General Discussion / Re: 80/90 Plus Gearing
« Last post by eddie on 20 May 2019 at 09:40  »
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.
15
Douglas Motorcycles - General Discussion / Re: 80/90 Plus Gearing
« Last post by ccmman on 20 May 2019 at 09:02  »
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.
16
Douglas Motorcycles - General Discussion / Re: 80/90 Plus Gearing
« Last post by eddie on 20 May 2019 at 08:05  »
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.
17
Douglas Motorcycles - General Discussion / Re: 80/90 Plus Gearing
« Last post by ccmman on 19 May 2019 at 23:07  »
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.
 
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Douglas Motorcycles - General Discussion / Re: Mk4 sports
« Last post by eddie on 19 May 2019 at 12:08  »
tck,
       The postwar series started with the T35 (it never assumed the title 'Mk1' until later versions appeared). Concurrent with the T35, Mr Douglas was experimenting with other models, including one with shaft drive - but nothing became of them. Somehow, a Mark 2 never reached the marketplace - the T35 being replaced by the Mark 3 and it's sportier sibling - the Mark 3 Sports, this being given another 3 BHP by fitting the sports cam (stamped with an 'S')(the standard cam was stamped 'T' for touring). Both the touring and sports models continued into 1950 being labelled Mark 4 and Mark 4 Sports. Later in 1950, the sports version was dropped when the bikes developed into the Mark 5's. In the period spanning late Mark 3 production to the early Mark 5's, Douglas were also producing the even sportier 80 & 90Plus models. Early development of the Plus series was not without it's problems and on a Mark 3 Sports, Don Chapman beat the 'Pluses' to win the 1950 350 Clubman's race at Silverstone.

  Hope this helps,
                             Regards,
                                           Eddie.
19
Douglas Motorcycles - General Discussion / Re: 80/90 Plus Gearing
« Last post by eddie on 19 May 2019 at 11:48  »
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.
20
Douglas Motorcycles - General Discussion / Re: 80/90 Plus Gearing
« Last post by ccmman on 19 May 2019 at 11:11  »
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
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