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which rear sprocket for Scottish

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  • which rear sprocket for Scottish

    Hi all,

    I have a bit of a head scratcher re. the gearing of my new-ish TDS Scottish. After struggling a bit with slow speeds on a recent trial, it was suggested I look at something like a 12 tooth drive sprocket and 60 tooth rear wheel sprocket. I bought the 12 toother; clutch, primary chain and casing off only to find....it already has a 12 tooth drive sprocket fitted. Looking a little deeper and counting revolutions of clutch against drive sprocket whilst in each gear, it would appear I have the 'road' gear cluster fitted rather than the trials. I gather this wasn't uncommon as back in the day these bikes may have been used for daily transport through the week and trials at the weekend.

    So what I now have is the road gearbox ratios (which are obviously too high for trials), a 12 tooth front sprocket (which lowers the gearing so far), and a 56tooth rear sprocket. My question is, how much bigger to go on the rear sprocket. Will a 60 tooth rear lower the gearing sufficiently or should I go bigger. (Villiers services offer up to a 66 tooth sprocket). I suspect this may be a try it and see issue, ( or there may be a complicated calculation that would make my head hurt) but wondered if anyone had a similar problem in the past.

    cheers

    Paul

  • #2
    Paul, the situation depends on your intended use. If you want to do trials locally, or with little road mileage, use of the road box can be viable, but if you want to do longer events with sections and road mileage, then it will have to be a compromise. 12 to 56 is a reduction of 1 to 4.666, 12 to 60 is 1: 5 and 12 to 66 is 1: 5.5. That means your speeds at any given RPM will be (assuming the present gearing at 100%) , 60T will be 93.33% and 66T will be 84.8%. So it can be seen that adding each successive tooth to the back sprocket will lower the speed by 1.5%.
    Comparing Road and Trial boxes we can use 3.0 as bottom for the road box and 3.6 for the Trials box. That equates to a difference of 83% Ish...., so adding your 66 T back sprocket will give you almost identical bottom gear as the Trials box......BUT it will mean your practical top speed will be down to about 35 mph.

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    • #3
      A brilliant explanation Brian, many thanks. I totally take your point that to get slow speed I will need to sacrifice higher speed, but for the local Peak Trials speed isn't a factor, so I'm happy with that. I will order a 66 tooth sprocket and let you know how I get on. At the end of the day changing the rear sprocket & chain depending on the nature of the event is no big problem. And in some ways, now I know I have the road gearbox on the bike it is a bonus, as I can increase front sprocket size with the existing 56 tooth rear if required, making the bike super flexible.

      On the subject of gearing, any news on the Griffon enduro project?

      Paul

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      • #4
        Hello SW. Just for future reference perhaps; on my '58 TA/Scottish I use a 13t front sprocket with a 60t rear wheel sprocket. But I do have the trials ratios inside. Happy trialling!

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        • #5
          Hi SW,
          My 1960, TCS has a 12T on the front and 60 (VS one) on the back. wide ratio gearbox, with STD primary set up. Find it perfect for our Australian Trials.
          I did make a cover strip for my swing arm, made from an old white chopping board. The front 12t runs it on the swing arm, when extended down.

          whitehillbillies

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          • #6
            got similar type of issue with my TFS. I use a 12 tooth final drice with 56 tooth rear sprocket for trialling. I can use 2nd gear for a lot of sections to great effect grip wise. when green laning ,and the long distance trial with my local club (eastbourne & district), I use an 18 tooth final drive. I can change the final drive over in about 1.5 hours, but when it comes to replacing chain and sprockets I am going for a final drive sprocket which will suit 2 rear sprockets which will give me the gearing I get at the moment with the 2 final drive sprocket I use. I will get another sprocket carrier so it will be a lot quicker changing from one to the other, without having to strip the clutch and primary drive each time. There is a useful chart on Wemoto's website for gear ratios and sprocket sizes

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            • #7
              Parkwood do a new 60T rear sprocket for full width alloy hubs for £37 to original design. https://www.ebay.co.uk/itm/new-greev...8AAOSw2s1Z5RM1
              They also do a 58T one for £35. Just a couple of the high quality BRITISH made Greeves parts from this firm. https://www.ebay.co.uk/sch/parkwood2...p2047675.l2562 check them out.
              Last edited by John Wakefield; 11/12/2017, 10:48 PM.

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              • #8
                Paul,
                no real progress on the Enduro project at present. I'm just back from a major car event and not been "on 2 wheels duty" for a while. I'll be contacting those interested after Christmas and we'll see where we go from there. A lot will depend on what gears we can get from VS. The early sleeve gears are known to be weak, not ideal for our intended purpose, so we may need to upgrade to the later ones. Obviously that means later boxes etc. We'll see what's about.

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                • #9
                  All good info gents and interesting that the 12 and 60 combination now seems to be the default. I believe the smallest drive sprocket back in the day was a 15 tooth....I guess the move to lower gearing has become a necessity with the change in trials over the years.

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                  • #10
                    Just thought I would feed back results of rear sprocket change. I went for the 66 tooth sprocket in the end which looks worryingly like a dustbin lid when fitted to the bike. One downside is chain guide no longer fits so slight risk of chain jumping off but this is easily sorted. it was pointed out that the larger diameter obviously puts chain and sprocket closer to the ground which could catch on rocks but I've attended another club meet and had a couple of practices on a local very rocky BOAT (byway open to all traffic) with no problems.

                    Oh, and the overall gearing is now much better. Was out today on the BOAT practicing ( I need it! ) and all seemed well. However, was practicing a few low speed, tight turns before loading onto the pick-up, toppled bike over and snapped clutch lever...what a clumsy oaf..

                    Thanks to all for their expert contributions on this thread and across the forum, and best wishes for the new year.

                    SW

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                    • #11
                      Yes, broken levers......bring the perch more to the centre of the bars so the end of the lever is more protected by the bar end. Some people use some insulating tape under the perch and allow the lever to rotate in a fall.

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