There are two primary choices in life: to accept conditions as they exist, or accept the responsibility for changing them. So from day one the main thing was to lose the weight of Duke. Mostly because the engine is limited by law and there is not much to gain. But the weight is different thing. By making bikes lighter, the only limit is you and your wallet basically. First you should think about the amount you want to spend and the value you get. If you have really deep pockets and are willing to put around 10.000 Eur additionally to 125 Duke, I think you can lose a lot of the weight, but at the end of the day the 390 will be quicker and from same money you could buy the 790 and tuned it. So if you just want to be quicker, buy bigger bike - there is more value for normal people then tuning the 125. But for me the obsession is really how good I can make the little beast. I could have bought the 790 and enjoyed the speed, but I did not find it as interesting as riding the 125 which has been made for me for having fun. I really like the agile feeling of 125-s and there is also more action going on when riding smaller bikes.
When building my duke, I try to think through the options and the cost/value of them, but I also try to think that If I want to upgrade something in future then I have to do it in first place, as there is no point of buying things I would replace in future if I had extra money, so cutting corners for me is not an option. And also the other big thing is that the bike has to be street legal, so I can't remove some of the heavy stuff in default. But for me its okay, as the main goal is to have ability to ride it as daily commuter and sometimes to ride on track. So no radical weight loss and race only parts are no options either.
The things to consider for weight loss.
Battery - go for lithium. Easy and cheapest 2.5 kg loss.
Number plate holder - I think we all have seen the KTM original number plate holder, the big plastic stick which has steel frame inside. It does just not a bit ugly, but it actually weights quite a bit. Luckily there are many Tail tidy options that will eliminate the big thing and make the look much better. They also save a approx 300g-500g of weight. Win-Win.
Titanium exhaust muffler or full set - depends on the model, but KTM Original silencer is around 2.3 kg. I did not weight the full pipes when i took them off, but with the OEM muffler and silencer system, I think its around 5-6 kg. The heaviest part is the big muffler between the swing-arm, the end silencer is actually quite light.
Aftermarket ones you can find for racing are as low as 0.7 kg and if you could go with full titanium for race only purpose, I think it will be around 2-3 kg without the Cat. But don't forget, now days the original exhaust is quite good from factory. If you remove the system and replace it with something else, you also have to add new adjustable ECU and make the map for it, as the whole fuelling will change.
I decided to go with just titanium silencer which was EU homologated for street use. I saved around 600g, got removable DB killer, and just a bit more outstanding look.
Aftermarket sprockets - there are many good lightweight options to choose from, even from KTM Powerparts directly. Original ones are made of steel, the tend to last way longer but they weight around 30% more usually. So I recommend to go with aluminium one in rear, but remember they wear quicker. For front I use lightened steal one. Also there are options with dual materials, which are also beneficial. All just depends what you want. I guess you can lose around 300-500g on set.
Aftermarket brake rotors - From stock they fully made from steal and do not float. Front one weights 1.7kg and rear one 790g. I replaced them with KTM original powerparts which are made by Galfer. A big name in brakes. The front disc has aluminium body and it floats and I also chose the 320mm variant. The first one should weight around 1.2kg and the rear is 570g (found the numbers online), so the approximately saving I made was around 700g.
Aftermarket Chain - The Duke 125 has 520 X-ring chain which is great for riding around in general. But as KTM has approach the 125 as big bike style, I would personally say the 520 chain is too much for 125. But as the 415 or 428 kit is not commonly available, I would like to go with 520 race chain, which is no-ring type and weights approx 500g to 700g less. But remember, they also wear out much quicker and you need to maintenance them after every ride.
Aftermarket shocks - the original shocks are quite good on 2019 model for my weight (63kg), but they lack the adjustability as most of the original shocks. For front I have seen no direct modified replacement or upgrade options, but my theory is that the Andreani 390 duke kit will fit them also. But have no real confirmed information about it. (Andreani said they are not sure) Also I think you can let some companies to make custom titanium springs for front. But the price will be quite high.
For rear shock, you can go with titanium/aluminium body shock which will help to reduce the weight a bit. There are many aftermarket shocks that are lighter, but as I have not changed it and there are many options, I can't really estimate the numbers.
Aftermarket Wheels - there is option to buy the forged Marchesini wheels for 125 duke, The estimated weight loss would be 20% of one wheel. This 20% is rotating mass, which will affect the handling more than just regular weight. But this option also comes with high end price - around 1500 to 2000 euros. Costing almost half of the new Duke.
Titanium Bolts and accessories - For Duke 125, there are many options to change the regular bolts to titanium ones. You can buy a whole preset packages or you can buy them separately. The whole bike weight saving can be around 1.5kg I estimate, but the price of titanium is also high. For example, the engine bolts only can cost around 200 Eur. Also you can buy different Titanium accessories like handlebars, levers etc., but they all cost more than any other material.
Carbon panels - there are many kits available or in single parts that are made from 100% carbon. The estimated weight saving for whole carbon body would be around max 1kg as duke does not have big panels. But the carbon means that the parts are often handmade and they are again expensive. If you decided to change just some parts for the looks, I think the weight saving is not that big over the OEM plastic.
Custom frame and frame parts - If you like to go to more advance level with weight savings, you can make customise parts for the frame or its parts separately. Either using more aluminium, titanium or carbon, this can reduce the weight drastically, but this needs some high-end engineering for better riding result then stock bike, but it's definitely doable with deep pockets. Can you imagine Duke 125 with carbon composite monocoque?
Tires - Everyone kind of likes the wider rear tires for the look, but they tend to weigh a lot more than people think. The 125 Duke has 150 rear, but actually it does not need it to be that wide for the bike or the rim. Many racers are choosing the 140 as it will be better fit for the 4" rim and it will be lighter option with better profile for turning.
If you want to keep the 150 tire, then even changing for sportier tire set could save some rotating weight.
Conclusion - There are no limits to make 125cc bike lighter. Just think through the amount you want to spend and the reason you want to do it. Then calculate the cost and find best solution for the money. The different end results can make the riding experience by far better as long as you know what you want. If money would be not question, I would go with everything above, just for passion of building it. But this would cost me 3x times the new KTM 125 motorcycle and I think I'm too young to spend all my income there. But step by step and piece by piece - some of it could and will be possible.
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