KTM Forums banner

Varying Ratios with Sprocket Changes

56K views 160 replies 33 participants last post by  KTMCol 
#1 · (Edited)
At my recent trackie, the 2008 DukeR which has been remapped, but is identical to my set up otherwise, cleanly pulled past me on the straight. Don't worry, I have a therapist.

Also, his dyno run gave him no more than 1/2 a hp over me.

He is a smaller and lighter rider than I am, but that shouldn't alter terminal speed. He might be more aerodynamic too?

I was topping out at 7'500 (500 short of redline which should be raised a little by the Akra map, so maybe 900 short of the rev limiter) and 194 km/h.

He is running stock final ratio, 16/40. I was geared tall 16/38.

Today I have taken Kev's (O2 controller Kev) advice and geared down by re-fitting a 15 on the front and an orange 42 on the rear. 2/40 + 1/16 = 11.25% reduction from stock. 7'500 x 11.25 = 8'437 rpm, which should be 500 short of the rev limiter at 194 km/h: theoretically that gives me 205 km/h at 8'900. Fingers crossed; I may see that 200. I still won't be able to test terminal speed until I get to the track - it's not worth it.

Geared down and about a 10mm shorter wheelbase. Wheelie time.

I've had too many beers to test it, and it is dark. Stay tuned.

 
See less See more
1
#3 ·
I won't deny, RR, this thing is like an ultimate Meccano set for grown men who are meant to behave better.

Total cost for this mod: less than $150. Well worth a try. It think I am just addicted to trying different set ups on my Meccano bike. But for $150 (Orange 42 sprocket $109); this mod gives you more "settings" by mixing and matching the front and the rear; the changes in feel of the bike are as big, at least, as changing the ignition map. 60km/h is best in 4th gear now, but still with a "theoretical" top speed of over 200. I am going to leave it at the most lowly geared set up for a while - until my excitement calms down, I get used to it and I can give an non-emotional report.

After the "first squirt" down to the shops for milk this morning: I am excited.
 
#5 ·
S'not going to happen with the 15/42 set up 4'500 rpm sees 95km/h in sixth gear. So I am reckoning it'll have Buckley's of getting much near 180 km/h.

But Fcuk it's fun!

It feels like a dirt bike with road capabilities. Not as harsh and raw as a true Motard, but a lot closer than the stock set up.

Kev was onto something with the suggestion to gear down. At 15/42 I think I have overdone it in a practical sense. 110-120km/h is more vibey now, but the engine feels happier. Wiwwee, I am guessing you'd do your load over my bike at present.

I have always been better at power wheelies exiting corners; I don't have the balls for point-of-balance stuff. Corner exit power mingas rock my sox. My Kato now does them in third gear with me hanging forwards on the tank.

If you don't spend much time above 120km/h, this mod is awesome.
 
#8 ·
It's six and two threes whether you change the front down a tooth or the rear up two. 1/16 approx = to 2/40; actually the front is the bigger change.

So you are accusing me of what you want to do yourself Orange Man.

I think your option is a good one though 'cause it'll reduce your wheelbase too.
 
#7 · (Edited)
Been running 15/42 on my Duke III and love it in the mountains, but it gets a little tiresome during highway droning there and back. Still, the trade-off is worth it, once the corners arrive. I'll do a 520 conversion eventually - might go down one tooth in the rear then... or not. Will definitely go for the trick orange kit shown above!
 
#10 ·
Actually there IS a difference whether you change the front or the back but it isn't in the ratios. A change to the front sprocket has a larger effect on the angle and offset of the chain to the swingarm rear sprocket to swingarm pivot. On a road bike this a noticeable as rise/squat differences in accelleration and at high loads ( Duke riders NEVER apply high loads of throttle......er....) will change the traction and suspension height during cornering .
Also if you are using steel sprockets, the bigger sprockets have a much larger effect on acc. and unsprung weight because even a few teeth is a lot of extra steel.
By the same token, a larger rear sprocket will reduce the tooth load and wear.
Remember not to have simple ratios between sprockets as that increases chance of asymmetric wear. Avoid things like 14/42 ( 3:1) or 15/45. Even 14/49 ( 2.5:1) is a bit iffy.
 
#13 · (Edited)
At my recent trackie, the 2008 DukeR which has been remapped, but is identical to my set up otherwise, cleanly pulled past me on the straight. Don't worry, I have a therapist.

Also, his dyno run gave him no more than 1/2 a hp over me.

He is a smaller and lighter rider than I am, but that shouldn't alter terminal speed. He might be more aerodynamic...
Have you changed bikes and had a go on his to be sure "the bike" is faster ?
 
#16 ·
Cop a load of this cheeky Fella. (said pointing at the tall man)

All I can do Wiwwee is set up my final ratio to be exactly the same as Phil's at our next trackie. Phil is kinda anal retentive about his Baby. He doesn't care for Bolder's history of binning it. I am not allowed "a go" on his Lass.

I was concerned about your suggestion until Phil just motored past me half way down the straight.

But leaving lap times alone for a mo.

I have ridden the new 15/42 set up for a while now, approx 300 km. It is staying!! It'll need to be geared up for the track, but on the streets is marries up the gearbox and the motor just beautifully. It makes the Duke the bike I have been looking for since I bought it. If the option of bolting on more power comes to me, I'll take it, but for now I am very happy. I was trying to make 1st useful and leave 6th for the track. After much torment, I have taken Kevin's advice and let go of first and made 6th useable on the road. This set up rock's the dog's nuts.

15 or 16 front, 42 rear plus engine mods giving 70 hp rear wheel is heaven.
 
#14 ·
Boldor - Is your mate's 2008 running an KTM Akra? These bikes are very sensitive to the exhaust length/volume/flow - I'm positive if you added the TI Akra exhaust you would out run all DUKE 3s....you have the cam, airbox, Acra map....and a straight pipe eliminating the cat...not sure if this is a good thing...for optimal power/torque with the Akra map...

What gearing (front/rear) is that fast DUKE 3 running ?
 
#21 · (Edited)
I'm cammed up, yes ;)
My only concern for 42 teeth is my commute on highway. What revvs are you seeing at 110kph (65mph) in 6th?

If that's bearable, then I'll try swapping up to a 42...

Edit: Checked gearingcommander, will increase by roughly 220rpm. I think I will get a 42 rear!
 
#27 ·
Whoa !!! "cocked"??? who uses that word??? :)

I am glad that you and Roadrogue confirmed that a new chain was not required!

I will try utilizing the all the adjustment available (without flipping).

When you used the 42 in the rear, it fit with the OEM 16 front?
 
#31 ·
BTW, the Iron Man sprocket is chrome moly steel, over 2x stronger than stainless and 4x aluminum, and quite nice. The 42 weighed 10 grams less than than my OEM 40 tooth. A good option for those that don't want more orange.

I also recently put on that EK chain which has black side plates with chrome/nickel pins and rollers. Looks good but I'm finding it rusts very easily so next time I think I'll go back to DID.
 
#32 · (Edited)
Well I have done a few hundred clicks on the 15/42 set up now, so I am confident to report my opinions.

All speeds are given in kilometres per hour.

I cannot overstate how much this mod improves the whole feel of the bike. I have to wholeheartedly agree with Kevin; the bike is over-geared to buggery from stock. It is sooo out that I am loosing faith in KTM as the ultimate bike engineers.

To try and sort my "stutter" frustrations I was offered an aluminium 38 rear for nix, so I fitted it. With the 16/40 stock front/rear cog ratio, riding about town meant mostly using 2nd and 3rd gears. With my 16/38 set up there were upsides and downsides but no real fixes. 4th was not really viable until you were travelling at 75. Travelling below 75 in 4th meant "the stutters" even with Kev's O2 mod.

If you study the gearbox ratios, it is evident immediately why Kevin had it right. I feel like a twit not referring to this chart or listening to Kev earlier.



5th AND 6th gear are OVERDRIVE!

The ratio changes through the lower gears are more pronounced than the ratio changes through the higher gears; that is, the biggest jump is from 1st to 2nd and the smallest is from 5th to 6th. My initial logic was to use the 38 toothed rear to make 1st gear useable and "write off" 6th for anything much except the straight at track days. But my logic was very flawed due to the ratios in the gearbox itself. It makes much better sense to "write off" 1st gear and use the taller gears for around town.

Where as before using the stock ratios, accelerating from 50 to 80 km/h, I had to use 3 gears (2nd, 3rd and 4th), with the 15/42 mod fitted, I can travel from 50 to 80 using just 4th gear. Due to the lessening difference between ratios in the top half of the gearbox, there is significant overlap between gears so staying in the correct rev range for any speed is so much easier.

The "stutters" are pretty much completely fixed now! Each of the following mods contributed to the fix.

1. Tuning the engine with go-fast bits like the racing box, cam and map. They all contribute to low end torque which assisted with the stutter.

2. Kev's O2 mod. Easily the best bang for buck regarding improving low end torque.

3. Front/Rear cog ratio gearing. Easily the best bang for buck regarding keeping the engine in the right rev range for your speed.
 
#34 · (Edited)
I don't know the top speed and I am considering it less and less relevant.

The increased pleasure of riding at 0 - 130, without stutters (which must also cause damage to the engine and gearbox over time) and with snappier performance means I couldn't give a rat's about the 150 - 200 zone.

I may refit the 40 or 38 on the rear, while I am changing tyres, for track outings. I may not too. The tight stuff will be even more fun.

It does accelerate snappier Owlie, but there are strengths of each set up, the 16/38 and the 15/42, which is why I also liked the 16/38 set up but for limited reasons. The 16/38 made the most of 1st 2nd and 3rd gears, off the line it was lethal. Accelerating from stationary with the 15/42 set up means the little orange tacho-needle rips around like a dying blowfly through 1st and 2nd, which translates to really sharp gear changes cause there are more of them. Get lazy and you'll hit all those false neutrals, or miss changing the gear altogether with clutchless changes - which sounds just as ugly.

BUT

Accelerating from 60 - 100 or 30 - 80 is much better with the 15/42 set up because 1st and 2nd are not used.
 
#41 ·
Got up to 160kph yesterday (100mph) and it still had more to give with the 15/40 ratio. Tried to keep up with an R6 passing me on it's rear wheel on the highway. :p
Sneaky, see the bold section ;)
But when you gear it down you lose top speed. Reckon it would do about 180 or so before hitting the limiter.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top