DIY experiments on my Gixxer 250: Suspension, brakes and more

So far I have done about 500kms after the change and I can confirm that the forks are much improved for my weight (90kgs).

BHPian b16h22 recently shared this with other enthusiasts.

Sorry for hijacking Neil's thread. Didn't want to create a separate ownership thread so I'm writing down some of my DIY experiments on my Gixxer 250 here. Thought it would be better if all the info about the 250 twins were available in one place.

O2 Sensor update

Removed the o2 sensor from the exhaust to have a look. Didn't look particularly dirty or damaged. Cleaning the tip was a futile exercise as it made no difference to the fuelling when I plugged it back. So I decided to run the bike without the sensor until I got a new one. It has been about 1000kms of running without the sensor and the bike feels nice to ride. It was a feeling I missed ever since I picked it up back on April 23. The bike still has a buzz when you get into the upper ranges of the revs but it's nothing like before. But it pulls so clean all the way to the top and I absolutely love the power delivery. In fact, I've been having a lot of fun with this thing inside the city premises. The only drawback is the constant check engine light and F1 error (or is it FI) warning on the dash not letting me see the ODO

Is it made by Denso? Can't seem to find any manufacturer name on it except a logo that says D.

New brake pads and fluids

Went to the local dealer to pick up the Gixxer 250 front brake pads. They did not have it in stock as expected (250s are rare here). Got the V Strom 250 brake pads. 700 INR for them. They were not 100% identical and the parts number was also different. But it is a straight fit. There was a backing plate sort of thing missing on the Strom pads I bought. Initially, I thought these lacked the bite of the previous pads but they had matching bite after bedding in for a couple of 100kms.

Cleaned up the brake dust and pushed the pistons back with old pads as a spacer. Cleaned the sliding pins and greased them.

The front brakes were decent but the lever travel was too much. The rear brakes basically had no bite from the day I bought the bike from the previous owner. I don't use the rear a lot but it is nice to have it working properly. The fluid in the rear brake fluid reservoir started to smell bad as well. Bled both front and rear brakes with new Bajaj Dot4 fluid. Front brakes felt good for 4 days but the dreaded spongy feel came back again.

Fork oil replacement and dialing in the fork sag

Gixxers have good suspension from the factory. Not overly stiff and sporty but just enough to have good support for spirited street riding. The bike has run 18k and the forks were starting to feel a little soggy. And I had this long-standing issue of a misaligned front end which was robbing the fun of riding. It was the perfect opportunity to replace the fork oil and make some changes to dial the forks to my liking.

The initial travel of the forks always felt too soft to me and this didn't give me much confidence. There was too much sag. These forks are basic damper rod units and they don't have any kind of adjustability to them to set ideal preload at least. There are no specific preload adjustable fork caps available for these bikes. So I got these 35mm OD, 5mm thick washers to add more preload onto the stock preload spacers.

Taking apart the front end took longer than expected because somebody decided to go King Kong while tightening the right fork cap. His left cap looked pristine compared to the right one so someone was here before. In the end, used the lower clamp as a vice and put all my effort into it to get it off.

Suzuki recommends endurance fork oil in the Spanish service manual I have. There is no mention of any weight grade. The Suzuki Ecstar bottle doesn't have any weight grade on it either. However I found out from a Japanese blog that the G10 Ecstar fork oil they are using in the 250s is a 10w oil. I decided to stick with Suzuki's stuff as I didn't find the compression or rebound damping to be poor.

Pepsi blue aka Ecstar fork oil going in

Stock fork oil quantity for the Gixxer naked is 438ml

Stock fork oil quantity for the Gixxer SF is 463ml

Stock fork oil height for the Gixxer naked is 108mm

Stock fork oil height for the Gixxer SF is 112mm

This was news to me. I was under the impression that the forks were identical for the Naked and SF. It seems that there is a difference in tuning. It is probably due to the difference in fork tube length as the SF forks extend almost 2cms over the top clamp to fit the clipons. I had already decided to up the oil height in my forks.

Fork oil height determines the height of the air column inside the forks. The shorter the air column is, the stiffer the combined spring rate. It makes a non linear change in the effective stiffness of the forks (springs + air spring) especially in the second half of the stroke. Found a few articles and a neat graph that explained the effects of oil height.

Fork oil height is measured from the top with springs out and the inner fork tube fully compressed. I decided to increase the fork oil height by about 1.5 cm from stock as an experiment (108-15=93mm).

Made this contraption to measure the fork oil height. It worked pretty accurately for the use case. Measuring the height of the oil is a better method compared to measuring the quantity as the quantity of oil that comes out of each fork leg might not be identical when we drain it without full disassembly.

Springs, washers, and stock preload spacers were put back in. The 5mm extra preload spacer went in over the stock spacer. It was a snug fit inside the fork tube. I started out with a conservative 5mm spacer and might add another 5mm if necessary.

When you fit the forks back, the height on the clamps is different for both the SF and the Naked 250.

For the Naked - 2mm measured from the bottom of the fork cap.

For the SF - 21mm measured from the bottom of the fork cap.

Greasing the steering stem bearings aka cone set

This was not a planned job. While the forks were out to get the front-end alignment correct, I noticed the steering was heavy on both extremes of its sweep. And there was a notchy feeling in the center. Removed the clamps and to my surprise, 3-4 loose ball bearings fell down. I was not expecting Suzuki to use loose ball bearings in the cone set. The bikes I have owned before used a caged bearing setup. It is much easier to take apart and assemble. Anyway carefully collected the bearings that fell down and left inside the race. They were in okay condition. There was not enough grease and some rust was starting to settle. Ideally, I should've replaced the bearings but it was evening and the SVC was closed. And finding spares for the 250s can be a frustrating process. So I cleaned up everything and greased the bearings and then put it back. So far it seems to have fixed the notchy steering. I'll replace the bearings in the near future to be sure

Observations on the new fork setup

So far I have done about 500kms after the change and I can confirm that the forks are much improved for my weight (90kgs) They don't dive too much and I get increased feedback and support from the front end. They have stiffened up a bit more but not enough to make the ride any harsher

I measured the rider sag by Motorcyclistonline's method and it was 45mm. I weigh around 90 kgs. It is a little outside the ideal 30-40mm range but I'll take it. Penske shocks say 35-45mm for street riding but MCN and a few other sources say 30-40mm for a fork with almost 120mm travel. Unfortunately, I forgot to measure the sag before the mod. Probably was much worse before

A short ride to the twisties

Was my first 100+ km ride after a tailbone injury back in 2022 Jan. Bike worked flawlessly and I was able to ride 120-130kms with a heavy pillion after taking constant breaks. The pain didn't play spoilsport too much but it reared its head by the end of it. Gixxer handled the twisties very well with a pillion onboard. Rear preload was on 4th notch but it felt inadequate. The rear shock is a bit of a pogo stick with two heavy persons on it. But then I'm asking too much from a budget naked. It works well with a solo rider

Spongy brakes again

After the ride, the front brake got spongy again. The bike stopped adequately but there was too much lever travel and it felt spongy. I assumed that the air had gotten into the system somehow while taking apart the forks and handlebar. Bled the front brake again but this time with Bosch DOT4 fluid. Double-checked to make sure there was no air in the system. Worked fine for 2 days and the same story again. My friend who owns a V Strom 250 was complaining of the same issue after 4000kms. He bled the system and it felt good for 2-3 days. At this point, it reminds me of my first gen 390's master cylinder. It has probably gone weak which is a Bybre trait from my experience. I'll consider the options to improve or upgrade the stock MC in the future.

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