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DT 125 gearing and RPM

  • srsnow
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DT 125 gearing and RPM was created by srsnow

Question on gearing and RPM.  was reading another tread with someone restoring a DT125 and was asking about going up a tooth in the front to increase the top speed. A couple people mentioned going down a tooth instead. My question is what is a safe RPM to run these at? Curently on mine with the stock 14/45 at about 55 it is at the redline on the tach. What is a safe RPM to run these at, and is that different than the 175's?
21 Jun 2022 21:17 #1

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Replied by Tom P on topic DT 125 gearing and RPM

My CT-1 175cc came with 16/45. I put one more tooth on the front sprocket so it's now 17/45, and at 5000 RPM I'm doing 50 mph, at 5,500 RPM I'm doing 55 mph, and at 6,000 RPM I'm doing 60. It seems like it's kind of a balance, because if you go too high in gearing, you may lose top speed. The listed top speed of the CT-1 is 65 +, and if I'm on a level road with no wind, I can still reach 65. I mainly like the 17 tooth front sprocket because around town I'm normally doing 45 to 50, and it cuts down the revs.

If you're primarily riding on the road, you might want to try one more tooth on the front sprocket.
21 Jun 2022 21:44 #2

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Replied by MarkT on topic DT 125 gearing and RPM

What year?  14/45 does not ring a bell as stock gearing?  15/45 maybe?  If so 14/45 would probably redline at 55 in 5th (early model) at 7,000 rpm.

DT125 generally does not have the power to pull higher gearing.  175 is different in that respect.

What is redline on tach?  You might have wrong tach installed.   Early model had 7,000 rpm redline I think...  later was 8,000...  I have a couple modified engine that will run 10,000 rpm + all day with stock bottom end...  crank was same on MX and YZ models so bottom end holds up well.  I wouldn't worry until around 12,000 rpm.  But of course stock porting won't rev that high and revving higher wears things out faster.

Speedo could be reading slow too.  I've had that happen a few times.  GPS app on phone can verify. 

 
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
21 Jun 2022 22:44 #3

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Replied by srsnow on topic DT 125 gearing and RPM

Redline on he tach is 7500, 1st and second seem very low so I was thinking of going to a 15, I have the 175 cylinder and head I just need to get them sent out and machined. I was hoping te one tooth would at least let m do 60, right now I get ran off the road even on city streets
22 Jun 2022 15:28 #4

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Replied by Tom P on topic DT 125 gearing and RPM

When I search for a standard DT125 sprocket set on Niche, it says 15/45. Maybe the previous owner went down a tooth on the front? Back in the 70s I put a larger rear sprocket on my CT-1, when I was primarily riding off road.
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Last edit: 22 Jun 2022 15:44 by Tom P.
22 Jun 2022 15:40 #5

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Replied by MarkT on topic DT 125 gearing and RPM

I agree with Tom, 15/45 was probably stock gearing.
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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22 Jun 2022 16:05 #6

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Replied by Pete-RT1 on topic DT 125 gearing and RPM

I disagree with you both, only in this instance

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Manual for the DT125A (I think that's the bike we're talking about) says 14/45.
I'm sure going up to 15/45 would be fine.
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23 Jun 2022 02:28 #7

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Replied by MarkT on topic DT 125 gearing and RPM

I'm sure you're correct Pete.  I didn't even bother to look.  It could be a 1978 DT125 or a 1972 AT2 model as well. 

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Sometimes I look up past questions by user to see if bike was previously identified but even that's a crap shoot...  so many people have multiple bikes and sometimes new additions. Or they are going by what they were told it is rather than verifying with VIN tables.

I gave a very generic guess because if someone can't take the time to at least look up VIN and present a positive ID, then all we can do is guess.  I even asked the year and didn't get a response to that part of my question.  If a year would have been given I would have looked for the specs.  Otherwise it's garbage in, garbage out.  

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P.S. Pete, you are correct.  US model specs sheet for 1974 was 14/45 with 1975 at 15/47.  In previous questions a 1974 year was mentioned.  I just didn't remember smaller than a 15 being used but I also tend towards the 175 models. 
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
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23 Jun 2022 03:19 #8

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Replied by Tom P on topic DT 125 gearing and RPM

I was just going by what Niche sells as a "DT125 sprocket and chain kit" In any case, if it was originally 14, it seems to me that going up one tooth more shouldn't be a problem. Especially if later models went to 15, though that could have been because they developed a little bit more power. However it's not that expensive to just buy a 15 and see what happens.

www.amazon.com/NICHE-Drive-Sprocket-Chai...087YBYZ26?th=1&psc=1
23 Jun 2022 07:07 #9

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