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Discussion Starter #1 (Edited)
I will in this thread add postings regarding the KTM 2 stroke MX bikes and what we have learnt that has helped us be competitive. Most of the stuff here will apply to KTM 2 strokes from 50cc to 125cc. But if it is specific to one/some of the models only I will mention that.

Please also feel free to add info/knowledge/experience you have that will help 2 stroke MX bikes perform better!

My intention with this thread is to help KTM 2 stroke MX riders make their bike as competitive as possible!

NOTE: The KTM bikes we have been using have all (from 65cc onwards) had HGS pipes and mufflers. (We have also tested other additions and will cover them later on in this thread). The HGS pipes have allowed our bikes to rev out longer in addition to giving more snappy power through the whole rev range. This is in comparison to original KTM exhaust systems!
 

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Discussion Starter #2 (Edited)
Holeshots are important in MX. When riding a 2 stroker there are things we can do to make it easier to achieve this. Here we will discuss how the squish affects this:

You can adjust the squish (the gap between the piston when in TDC and the cylinder head) to help you get holeshots. Sadly nothing comes for free and by doing this you will loose in other areas. (We normally have the squish between 0.9 and 1.1 mm for club day racing on the 125sx. We measure the squish by using solder that we put on the top of the piston, assemble everything, turn motor over a couple of times, then take head off and measure the solder. You will find videos of how to do this on YouTube. ONLY use solder with a soft core!!)

When reading info re your 2 stroke MX bike you will find "squish" measurements. We played with them to find the ultimate and learned the following:
  • When increasing the squish gap by fitting thicker cylinder base gaskets (making the gap bigger) we found that the motor would rev higher but loose some power.
  • When reducing the squish gap (making the gap smaller by fitting thinner cylinder base gaskets) we found that we got more power but lost top revs.
So when we would race a track with long straights and sweeping corners, including TT tracks, we would increase the squish gap a fraction (make the gap bigger) as we wanted the motor to really rev out. Sure, we lost some power at the lower end of the revs, but we gained the ability to rev higher and change gears later and when going down a long straight the motor would just keep on increasing the revs!

If we were racing a track with many turns and jumps and really no long straights we would decrease the squish gap. This also helped with the holeshot as we had good power from low revs. The motor would not rev out in same way as when we had a bigger squish gap, but that did not matter as we would never get to those revs on these tracks.

By playing with the squish you will not find huge increase in performance. But when you are doing all right at the start and others just beat you to the holeshot, it might be worth a few hours to play with the squish to find out what works best.

There is much more you can do to the bike to help you get holeshots and we will provide some more suggestions soon. Stay tuned!

NOTE:
The comments we post here are from our own experience with the 2 stroke KTM bikes. There is no scientific researh gone in to this. You might have opposing experiences. And that is OK as there are so many variables that will add to the results.
 

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Discussion Starter #3 (Edited)
Gearing

When racing 2 stroke bikes in MX we live in the top rev range. (I remember the first competitive 50sx we got. I had only started the motor once where I bought it from to check that it did run and I did not rev it very high. Next day was time for my then 7yo to have his first race with a bike that would be on same level as the other kids ones. I put the bike on the stand in the pits and checked out things, filled fuel. And started her. But I could not get the rear wheel to spin! Concerned I asked another dad to come and check it out. He kicked the bike over, revved the motor to a level where I expected it to blow in to pieces, hit the kill switch and told me she was all good! I was also told that they needed 9000 revs before the clutch would grab... I had never had a bike that got even close to that!!).

12 - 13000 revs is where the power band (PB) starts to drop. And it is narrow not starting before 9 - 9,500 revs. So if we want to have a chance to be at the front, that is where the motor needs to work!

When out on the track there are only three options to get to that rev range:
  • Slip the clutch if the speed is not high enough. This is not as bad as it sounds. By slipping the clutch you can get the motor in to the PB where the motor is at it strongest. Doing this normally increases the speed faster than when you only try to use the throttle. But this will wear out the clutch plates faster. My boy adapted this technique for a while and he did OK actually. But we kept on buying clutch plates on regular basis so we changed his style.
  • Drop a gear (or two) so that the motor gets in to the PB. Probably the best of the options for a beginner to a medium rider.
  • Make sure you keep the revs in the PB all the time from start to finish by actively changing gears as soon as you start to drop revs and when you end up in the top part of the PB where the power will start to disappear. This can be quite scary as the speed will often be very high so it all demands your full focus.
So that you do not end up in any of these situations you need to gear your bike correctly. On a MX track you will find that many riders will have their 2 stroke bikes fitted with a to small rear wheel sprocket resulting in it taking longer to get in to the PB. This will also cause you to not get that holeshot as the rider next to you, who has the right gearing, will always out-gate you!

Best thing you can do is to take a few different size rear sprocket with you to the track on a practice day. Do this after you have found that "sweet spot" by setting the perfect squish (see above in earlier post) so you can start testing different rear sprockets. One tooth bigger at the rear will get your motor a little faster in to the PB (and we just agreed that this is where you want to be!). Or perhaps 2 teeth bigger is even better... Only by testing (and having a mate doing some timing) can you find that out. Fit a rear sprocket that is too big and you will end up doing nothing but changing gears all the time.

If you have never worried about the sprocket sizes I promise that this will open your eyes and you will wonder why you did not do this earlier!!

I have attached sprocket ratios. Note that you can achieve almost the same ratio with two smaller sprockets (front and back) as with two bigger ones, example; 12/43 ratio 3.58 and 18/64 ratio 3.56. What to prefer from these two? The 12/43 setup will have less chain and smaller sprockets meaning there is a weight reduction. The 18/64 setup will be heavier and the chain will move 33% faster than 12/43 creating more of a "gyro" effect. But you will not really notice this as with the wheel on the ground it will be minimal difference.
93023
 

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Discussion Starter #4
Side Note: I had an interesting discussion with one of New Zealands past MX World Champions (Katherine Oberlin-Brown [nee Prumm] ) who won the Womens MX World Champs in 2006 and 2007. I asked her what the next level would be if a rider wanted to make sure he/she would want to guarantee that he/she would win. She straight away said: "Pinn the throttle and use the clutch, gears and brakes to adjust the speed...". Katherine told me that she had started working on this but sadly an accident where she broke her spine stopped all that. (Katherine recovered and is now working at Sport NZ but did stop riding MX after the accident)

But it made me think... a 2 stroke bike that is in the PB 100% of the time... that would be hard to beat!
 

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Discussion Starter #5
Rear wheel position

I have a background in Solo Speedway racing. I was at best a top B grade rider. But I loved how small adjustments to the bike would make a noticeable change in how it performed on the track. Much of it is not transferable to MX, but one thing I use in MX is the positioning of the backwheel.

On a MX bike the backwheel is normally moved to adjust the chain tension. But as we have approx 5cm (2") available to position the rear wheel we should use that to improve the bikes performance!

By cutting the chain so that you need two chain links plus a piece of the chain you cut off, you can quickly move the wheel forwards or backwards. By making yourself small pieces of chain of different lenghts that you can add to the chain while using two chain links, you can move the wheel backwards and forwards and also compensate when changing sprockets.

- If you place the wheel as far forward as it goes the wheel will get more traction as it now sits closer to the riders body weight. This makes the bike a little trickier to ride as it will be easier to do an unintentional wheely and as your body weight will be closer to top on the backwheel the tracktion also increases for when you turn on the throttle. This can make the bike harder to ride for an unexperienced rider as the bike tends to get more "twitchy".

- If you move the rear wheel as far back as you can get it the bike becomes easier to ride for a beginner. But there will also be a tendency for understeer resulting in tight cornering becoming a fraction harder to do. The bike will be more stable in high speeds and this is how we set the bike up for Long Track and TT racing. On a track with short straights and tight corners we tend to place the rear wheel forward resulting in quicker response in cornering.

With practice you can learn where you need to have your body to get the most out of your rear wheel placing position.

You might not have considered that the placing of the rear wheel will alter how the bike performs on the track, and as a technique to win a race this would have a very small impact. But if you are interested in maximising your chances perhaps add this to your box of tricks as it might just give you that split second that was the difference between a win and a 2'nd placing...

Again, I recommend that you spend some time testing this on a practice day to see what difference it makes.
 

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Discussion Starter #6 (Edited)
Power Wing (also called X - Wing)

When we moved up from 65cc to 85cc MX racing my son was hesitant in keeping the throttle "pinned" and tended to initially ride part of his riding below the PB. I knew this would change soon enough, but wanted to see if there was something that would help here. I came across the Boysen Power Wing and decided to test it as the cost was minimal.

Straight away after his first test ride my son told me that the throttle response was much crisper and he could feel an improvement in the power below the PB. We used the Power Wing through his 85cc riding and it served us well.

When we moved to 125cc riding I tried to find one that would fit, but I was not able to do so. As his riding had now changed and he spent much more of the time in the PB range and he used the gears to achieve this, it did not really worry me. But in the workshop I have a half completed Power Wing, modelled on the Boysen one we had, that I still intend to finish and test. We have now finished the 125 riding and are looking for a late model 150sx that we will race in MX2 classes. So my plan is to complete the Power Wing and fit it before we do any other modifications to see if there also is a benefit with it in bigger bikes.

We used the Power Wing for close to 4 years in 85cc racing and the few times we left it out from the race bike my son could always feel that it was missing. Perhaps the Power Wing is not needed in 125cc top grade racing as the riders have by then become quite good in staying inside the PB. But in 85cc it made a difference for us!


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Discussion Starter #7 (Edited)
One ring or a two ring piston in a 2 stroke MX bike?

We had always used a one ring piston in our main race KTM. The reason was that one ring should create 1/2 the drag compared to a two ring piston. But we had never tested this theory.

We had since we started racing 65cc always had two bikes. One was called "race bike" and the other "practice bike". But most of the time they would be set up the same and ready for the track we would race at. (Or at times they were set up a fraction differently to allow for changes in weather/track conditions) This worked well and when there was a problem with the "race bike" it was a quick job to move the transponder over to the "practice bike" and go racing. (I then had time to try and sort whatever was wrong with the bike before the next race as the "practice bike" would be the older of the two bikes).

We replace pistons at 40 hours and the ring at 20 hours meaning we "freshen" the compression every 20 hours. I know of top 2 stroke riders who will fit a new piston at 12 hours. But we were always riding on a tight budget and after 20 hours there was never any real notieable difference, so it was what we stayed with.

It was time for a new piston for the "race bike" as a big race meeting was coming up. I ordered one from our main sponsor, but was told that they only had the two ring pistons in stock until next week. (We use Vertex pistons). I was told by the parts guy that nobody could feel the difference between a one and a two ring piston anyhow! So considering we had another bike with a one ring piston in it and ready to be raced, I went with the two ring piston. (We had at times used a 2 ring piston in the "practice bike" but never compared the performance between a one ring and two ring piston)

I warmed up the motor and allowed the new piston to "settle in" at home and we were ready for the racing!

After the practice/timed practice my son told me the bike felt different and a fraction sluggish. I suggested it could be the track (was quite deep and damp) and we left it at that. But after the first race he came in and told me that nope, it was the bike. So for Race 2 he went out on the "practice bike" and when he came in he informed that this bike felt good. He also raced Race 3 on the "practice bike" and from memory he placed in top three. (I had checked the "race bike" while he was out racing but could not find any isses.)

When back home I gave the "race bike" a more thorough check over, but could not fault it. The only thing that I had really changed on the bike was the piston, so I ordered a one ring piston, fitted it and we went racing the next w/e. I was told straight after the first race that now the bike felt like it should!

So I can only conlude that there is a difference that is noticeable, when riding MX, between a one ring and a two ring piston. Based on these observations (and the logic that says that a two ring piston has twice the drag) we reverted back to always racing with one ring pistons in the "race bike". I have since this discussed it with others and I get support in my thinking.

Again, we have done no scientific testing here, but we feel strongly that there is enough of a difference between a one and a two ring piston to always use a one ring piston in competitive 2 stroke MX racing.

93064
 

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Discussion Starter #8 (Edited)
Will a muffler increase power in a 2 stroke?

We have replaced the KTM header/chamber with HGS ones on our 65 - 125sx KTM's. There has always been an instant increase in response and power, and the motor will also rev out a fraction higher (approx 500 additional revs) before the PB starts to drop. This has always been an advantage when racing TT and Flat Track races.

But what about the muffler? Does it do anything else apart from look cool and perhaps change the sound?

We fitted HGS mufflers on the 65sx, but not much changed.

We fitted a FMF shorty muffler on to the HGS pipe when on the 85sx. And apart from giving the bike more of a "bark" that made her sound like there was more "umph" we could actually not feel any improvements.

When moving up to 125 racing we again got HGS chambers for our bikes and straight away there was an improvement very much in the same way as with the 85cc. We left the factory muffler on the bikes and happily raced with that setup. Then we again found a shorty FMF that we fitted on to the practice bike and a little later we got a cool orange HGS muffler for the racing bike, but there was no obvious gains anywhere that my son could feel when riding. Again the sound changed, but that was all.

So from our experience the money you spend on getting a muffler that matches the header/chamber would be better spent somewhere else if you are looking for gains. But if you want some "bling" then a colourful muffler does look cool!
93089

93088
 

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Discussion Starter #9 (Edited)
Ignition timing and fuel

When we got our first 125sx it was a 2012 model. We raced it as it was for a few race meetings, but felt that the motor was holding back some power. We played around with jetting withouth success and cleaned and adjusted the Power Valve but not gaining anything. So we started to look at the timing of the ignition. As these bikes have a CDI (a black box that sets the ignition timing) that has only two settings: one for beginners and one for experienced, there was not much we could do there to gain some power. So we looked at the ignition pick-up-sensor that sits next to the stator and determines when the spark comes.

The 2012 had a plate the sensor was mounted on and the three mounting screws were in slotted holes so the plate could be adjusted. There was two lines that could be lined up to the one on the motor casing. The plate was set to the line that made the spark come later of the two, so we losened the three screws and turned the plate so that the sensor would give spark earlier. My boy went out for a test and came back telling me that this was now a totally different bike that was much quicker to respond and had more power. Brilliant! We raced like this for a couple of months, but I had started wondering what would happen if I made the sensor give the spark even earlier... so I modified the slotted holes so I could turn the pick-up-sensor a few millimeters further. With no gear in she revved well, so we went testing. But now the motor started "pinging" as clearly we had gone to far and the ignition was now too advanced.

This annoyed me and I started thinking what I could do to solve this. We were already using 98 octane fuel withouth Ethanol in it so I decided to test Aviation Fuel (AvGas) that in NZ is close to 110 octane and still has led in it. (We are not allowed to use any type of Racing Fuel in New Zealand racing!) Once I filled the tank with the AvGas the "pinging" disappered and I could play with the ignition setting until we found the "sweet" spot for the bike. We raced like this for approx a year. We then sold the 85cc bike, that had been the backup bike, and bought a 2016 125sx.

The 2016 did not have the ability for us to adjust the pick-up-sensor setting as the sensor is rigidly mounted in it's place. I modified the setup so that I again could move the timing for the spark. Again we found a "sweet" spot and kept on racing. We were using straight AvGas at this stage. Others I spoke with would use 50/50 AvGas and 95 Octane. I started to look in to this and learned why: As the AvGas was 110 Octane with led in it it would burn much slower than what the motor needed. By mixing it 50/50 with 95 Octane we could increase the burning speed of the fuel but still have the benefits from the AvGas. Once we did this we had to retard the spark, from where we had it, a fraction to get the best result as the fuel now burned faster.

We feel that by using 50/50 AvGas and 95 Octane plus setting the ignition just a fraction earlier than the 2016 and later bikes rigidly mounted pick-up-sensor allows (as we still have the modification in place) we have gained power in our 125sx. I assume you can achieve similar results by fitting an after market CDI that has a multitude of settings (up to 10). These CDI's are quite expensive (over US$ 500). We feel we have achieved same result, maybe even a better one, by doing what I have here described.

Picture below of a 2012 stator showing the plate that allows for the timing adjustment (NOTE: The one in the picture has additional windings on it for lights. The sx one only has the two white ones). The 2016 onward setup does not have the backing plate with the three slots that allows us to adjust the timing position manually. This is now only done through the CDI. But as this does not allow us to setup the bike for the 50/50 AvGas/95 Octena fuel I modified the setup so that I can manually adjust the pick-up-sensors position.
93116
 

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Discussion Starter #10
Pro Lever Linking from 2012 onwards.

From 2012 the 125 rear shock was moved to the middle of the swingarm. Before 2012 the rear shock had been on the right hand side of the frame/rear swingarm and mounted straight in to the swingarm. This was an disadvantage when KTM wanted to get the rear working better, but an advantage for the air intake to the carbi as the shock was out of the way. For 2012 this changed as KTM concluded that the lack of motor performance by fitting the rear shock in middle of the frame (and in the way of the air intake) was outweighed by the improvement in handling as now the swingarm could be fitted with the Pro Lever Link. I have seen information that by doing this the 2012 125sx lost 2 Hp compared to the 2011, but I have no proof of that. (Thou when we got the 2012 there was still many of 2008 - 2011 125sx bikes we raced against and they did seem to have more power that our 2012 bike. But our 2012 was much easier to ride in loose sand, in mud and in the wet. She also performed better in jumps, whoops and corners evening out the 2Hp we lost.

We had the suspension worked on based on my sons weight and his riding style by one of NZ's top suspension gurus (and one of our sponsors). He also told us about after market pull rods and in particular the ones made by RIDE Engineering. So we did get one and fitted it. The difference was straight away obvious! The bike has always been stable and easy to handle, but now it got even better! The RIDE pull rod is a fraction longer than the original one. This drops the rear with 6mm but also changes the position of the Triangle Lever changing the leaverage. This all provided a more comfortable riding, better control of the rear and a more "settled" suspension. Our intention had been to move the pull rod to the 2016 bike. But sadly it did not fit after more changes made by KTM, and with the ongoing issues we had with the 2016 we could not justify spending money on her suspension as we more often than not were using the 2012. With us soon moving to a 150sx in the near future we will get one of the RIDE pull rods for the new bike as the 2012 was by far the bike we have owned that had the best handling and my son preferred it over the 2016.

Again, this is not a "magic potion" that makes you/your kid a winner. But it does make the rider more confident in jumps, ruts and whoops resulting in increased speed. (Well, it worked for us...)

NOTE: RIDE Engineering is not one of our sponsors and I know that there are also other companies that supply their own after market pull roads that I suspect might work as well as this one.
93230


93231
 

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All good information here. I heard the horsepower losses were anywhere from 1 to 2 horsepower between a linkage rear suspension bike, but as it were horsepower is just a number to boast but a good handling bike wins races, all else being equal. A small change in leverage applied to shock can be magical and also disastrous when applied incorrectly. Too many riders fail to apply this theory when lowering a bike or adding aftermarket link without proper shock settings to adjust for changes to leverage rate. I have seen some riders install link and cause a harsh rear suspension and fork diving under braking and observed others causing rear shock to wallow and pack during stutter bumps and make forks ride harsh and stiff. Careful study and tuning is required during suspension changes. The manufacturer of quality suspension modification linkages and parts can usually supply baseline settings for usage but testing and study is usually required for extracting optimum performance. The universal positive comment on a good aftermarket linkage has been a plush feel while allowing a firmness to follow uneven terrain without hopping around or using too much shock travel. This allows front end to remain balanced and stable. The rider has to spend less energy fighting to keep front wheel planted. A confident rider is always faster when he can trust the front wheel is going to stay where he expects it to stay.
 

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Discussion Starter #12
Thanks for your input @augiedoggie. It is always nice to have you supporting stuff I discuss/inform about as you have many more years of info tucked away in your head than me. Suspension is probably my weakest point. I can do the clickers to improve an issue that pops up and also replace fork seals and oil. But that is as far my suspension knowledge goes. Therefore we had one of our sponsors, who is brilliant at suspension, do the changes required when we fitted this link.

The funny thing was that as the 2012 bike was running well when we got it we did not even consider doing this before this option was discussed on another forum. That made me wonder what if... and as I said above in the post, it really made a difference. As we have never had a bucket full of money we always have to justify any spending with looking at the advantages. This one was one I almost said "NO" to. But after discussing it with the sponsor I changed my mind. (It did mean that my son had to use his worn out boots for a little longer...) And we never regretted it! Reason the 150sx, when she arrives sometime in 2020, will also get this upgrade (including all the ones I have sofar discussed in this thread, obviously apart from the muffler...).

Here a link to a video of my son Vinny (#333) racing the 2012 (after the 2016 had once again refused to run after us initially using it here.). The 2012 (that has the blue number plates) was very low on compression so he could not rely on it's power but on the handling. It is a lenghty video and if you are short of time go to the final race that starts at 7.30. This was a 2 day event and riders had already dropped out after bikes failing and from injuries by this the last race, that became a battle between the top 250f rider and Vinny the top 125 rider where Vinny made a few mistakes trying his all to be first!

 

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Discussion Starter #13
We did jumps over and over and....

My son struggled with jumps. So we worked on different ways to get rid of the fear. As it turned out, he was never scared to jump... he was scared of the landing! So we spent money on setting up the suspension for the landing. Once we got that right things changed and his riding improvead heaps! I can not tell you what setting to go to as all riders are different and so are the tracks. But what I can tell you is to get the bike set up for your rider!! And if you do not have the knowledge, pay someone! You will never regret it!!

Here a short video of my sons riding. Notice the jump ( At 4 seonds) where he adjusts the bike in middle of air! We spent quite some time sorting that one out!! (Turn on the throttle in the air and the front end goes up, turn off the throttle and hit the rear brake in the air and the rear comes up!)

Oh... and if you think his rear-wheel comes out "by mistake" at the holeshot corner in that final part of the video, I tell you, there was no "mistake" there...

 

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Discussion Starter #14
Aaaand another thing we do (you can see it at the beginning of the video above) is grabbing the front brake just before the gates drop, turning on the throttle while both feets are on the ground (rear wheel starts to spin). Once the gate starts to drop we let go of the front brake and plant the bum on the set and...
 
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