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 50T to 48T Report

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PostSubject: 50T to 48T Report   Sat Jul 16, 2016 11:24 pm

I bought my bike new in 2012 but had the ratio changed to 13/50 from the standard 13/43 to assist with steep hills and my limited riding capability (yes I am a hack). So I had never ridden it with anything other than the 50T. While I have enjoyed the 50T for the zing effect what I was losing was fuel economy, speedo accuracy and top end speed. While they may not seem that important for the dirt in fact when you have the chance to sprint on a clear track the 450s that I generally go out with get away from me without that better top end speed, and the engine screams. So I fitted a 48T and hope the following helps some readers.

Firstly I only ever change one thing at a time to make sure each change can be assessed. Sadly, my rear tyre is a bit worn so today's first run with new sprocket probably was not at  peak bike performance. However, today in Toowoomba is just miserable with gale force winds, raining and bloody cold. Tracks were slippery and muddy. Perfect stuff.

I found that my normal tracks have been eradicated by a new freeway cutting its way across the country so went to Murphys Creek and found some out of the way tracks. I had never ridden these tracks and came across a couple of steepish and slippery hills and the bike went very well. Lower rpm, less slip at the rear and still had the crawler capability. So I anticipate very little loss of hill climb ability. I did do a dumb mistake and went down a muddy track I had never been down before and had a realisation that I would struggle to get back up it with bald tyre - so I did do a few yards pushing back up but overall I was surprised how well the bike went.

What I did notice though was the additional engine breaking I got with the 48T. With the 50T I tended to need to snap the throttle off all of the way when heading into tight stuff, and a fist full of break, but with the 48T I found that with the additional engine breaking I was much more smooth with the throttle roll off and power on, making cornering much more pleasurable. I did get the impression that I was carrying a smoother rpm into and out of the corners. I didn't have anyone with me so could not gauge the speed against the 450s but I feel I was smoother and faster than before.

I did tend to labour the engine a little on uphill hairpins but that is generally good for a four stroke. I think with experience and drier tracks I will be able to correct the rpm and clutch/throttle manipulation. Some good hints on this forum re clutch use helped me today.

From a standing start while the 50T allowed the engine to rapidly reach high rpm in fact forward momentum was lost to fishtailing. Impressive fun but still not catching the bigger bikes. The 48T I hardly noticed the impact on standing start but that was more to do with the mud. I will need to re-assess that bit in the dry. I did feel that I got more momentum from first gear before the up change on the main roads as I went to and from the tracks.

Doughnuts no problem. Still plenty of zing to spin the bike.

Wheelies... I bit of an issue here. I have never wheelied the wrr250 well. I can get it up but sustaining the wheelie has been a problem, and I put that down to not finding a low rpm balance point, so I tended to pop it up and down a bit too quickly. Today with bald tyre and mud no real chance to compare that but I did feel that the less slip at the rear helped me skip the front wheel on the uphills with lower engine speed and better traction. A couple of modest hills I was able to hold the front up easily but need to wait for the dry before I can compare the two sprockets for wheelies.

Finally, I see lots on forums about what changing the sprockets does and there is a lot of misunderstanding so using my wealth of aeronautical engineering experience I thought I would explain it to those that still may doubt the influence. Here goes:

1. If you pull two roles of toilet paper at the some speed, one full and one say half full, the full one spins slower. Therefore to make the bigger one spin at the same speed as the smaller you have to pull it quicker. 43T to 50T is going to a bigger diameter so engine must run quicker to get same wheel rpm. Make sense?
2. If you have a frozen bolt maybe you might reach for a tube to extend your spanner handle. Thus extending the moment. That is, force at greater distance gives greater mechanical advantage. So going from 43T to 50T gives bigger radius, and therefore better torque and hence hill climb capability (you would think).
3. Sadly, the combination of higher engine rpm and the easy ability of the engine to spin the rear wheel (due to better mechanical advantage) and additional torque in my case equaled much higher loss of traction - which was counterproductive for hill climb.

So for the moment the 48T has turned out to be a much better option over the 50T for better economy, less engine speed, smoother engine breaking, less slip at the rear end and speedo closer to correct.

Next mod is speedo correcting add-on and better exhaust. I think the exhaust will unclog the engine and help with the labouring issue.

The real lesson is to play around with your set-up and look for the best outcome. Too many riders get their bike and never play with settings to their own disadvantage. As a hint though, don't make too many changes at once as one change may undo the others and you will not gauge the net effect.

Ok, I am happy to be disproved on the above but for now remember to ride it like ya stole it.
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PostSubject: Re: 50T to 48T Report   Sun Jul 17, 2016 1:38 pm

Your butt dyno is apparently a lot more sensitive than mine. Dropping from 50T to 48T results in a speed increase of a mere 3 km/h (2 MPH) at 6000 RPM in 6th gear. At lower gears or RPM the difference is exponentially less.
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PostSubject: Re: 50T to 48T Report   Mon Jul 18, 2016 4:48 am

Yep sensitive but I purposely kept away from qualitative info and kept to 'felt like better speed etc'. However, I followed a cop and traffic to the tracks on Sunday and and in 100kph zone my speedo pace of 110kph kept up. A few weeks ago I struggled to keep up with 100kph traffic at 120kph. Note that the biggest error is speedo error so that just adds to the speed comparison difficulty. Before the change I had to do 70 to 75kph to keep up with 60kph traffic. Today I was doing 65 to 68 for same. So maybe a modest improvement.

I had seen your recommendations to consult the ratio table in other posts and using proportionality the ratio difference between 13/43 at 3.whatever, 13/48 at 3.69 and 13/50 at 3.85 is small so my expectation for much higher top end was not high. However, in 3rd and 4th gear on the tracks my track speed appeared to be better or at least the same at lower rpm. The truth will be next visit to enduro tracks at Cooyar and to see if my times go down.

Satnav is certainly needed for accurate measures but my post was more about how I felt. My mates on 450s will let me know the real comparison I am sure.

Overall I am happier with the 48T than the 50T at this time and will test it out on hill climbs. If I face plant maybe the 50T will go back on.


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