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Comfortable cruising RPMs on an XR250?

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Hey there everyone!

Please brace yourself for one of those rambling, "thinking out loud" posts. :-)

I know I'm in a minority here, since I use my XR250R as a dual-sport machine, but I'd like to turn to the community for advice. Unfortunately I think a true dirt rider would have never given these questions much thought, since they probably aren't relevant to him or her.

In a few months I will probably be changing my sprockets and chain.

I am wondering about buying a smaller rear sprocket to help with highway riding, specifically, to be able to cruise at about 110 km/h tops without stressing the engine (more on this subjective criteria below).

I'm not doing this blind, however. I'm using the following Excel spreadsheet that I found on the web and customized for my bike, sprockets and tyres.

http://individual61.awardspace.com/files/Gearing_V12-altered.xls

It has several fields that I'm not sure about.

Minimum RPM, Max RPM, Gear change down RPM, Gear change up RPM, Max Power RPM.

If I had a tachometer, everything would be oh-so-easy. Instead, I have to trust the transmission specs and tyre sizes, plus the speedo (which is accurate to within 3 km/h at 90 km/h, tested via a friend's GPS).

Does anyone here have a tachometer on their bike?

If so, can you tell me what the minimum RPM value would be? I don't mean "minimum so I can ride up a hill", but rather "minimum so I can cruise on the street at a moderate speed and not have the engine suffer".

I find that the engine doesn't sound happy under 50 km/h in 6th gear. Accodring to the spreadsheet,

48 km/h = 3000 RPM

So this should probably be the minimum RPM.

Similarly, one could say that the rev limiter determines the bike's max RPM, but what I'm trying to determine is something different. *I want this engine to last*. So what I'm trying to determine is a reasonable highway cruise RPM.

I have this to offer: when riding on the highway with my current setup, I subconciously select 90 km/h as a riding speed, even if I have decided to ride at 95 or more. I feel squeamish about taking the engine up past 95 km/h. Now don't get me wrong: I've done it, and I'll do it again, but I'm not happy about it.

According to the spreadsheet, in 6th gear:

90 km/h = 6850 RPM

95 km/h = 7200 RPM

118 km/h = 9000 RPM

That's with a 13/48 setup.

If I were to switch to a 13/39,

6850 RPM = 111 km/h

7200 RPM = 116 km/h

9000 RPM = 145 km/h :thumbsup:

However, I cannot express how much I enjoy the great acceleration I get in the low gears and at low speed, and I know I'll sacrifice this by going so low on the rear sprocket. So I'd probably want a rear sprocket somewhere in between.

Just for completeness, let's see what the speed is at 7200 RPM (my current RPM at 95 km/h) with a range of rear sprockets

48 -> 95 km/h

46 -> 99 km/h

44 -> 103 km/h

42 -> 108 km/h

40 -> 113 km/h

39 -> 116 km/h

Now, on to the question of torque. Being extremely naive, I might assume the change in torque at the rear wheel will be related to the change in radius of the rear sprocket. Indeed, the ratio between the before and after sprocket numbers should give a ratio of the radiuses, and therefore a ratio of the torque, before and after the swap.

For example, factor by which the torque changes going from 48 to 44 teeth should be about an 8% drop. On the other hand, going to 39 teeth is a considerable 19% drop.

I could probably make up for an 8% drop in torque subconciously, since I rarely start off at full throttle from a red light, and therefore will end up opening the throttle slighly more on all gears, but 19% would probably be noticeable.

In any case, it would seem that a 44 or 45 is more like what I'm looking for.

A 44 rear sprocket gives:

6850 RPM = 98 km/h <- Cruising speed

7200 RPM = 103 km/h <- Higher cruising speed

9000 RPM = 129 km/h <- LUDICROUS SPEED! :ride:

which is certainly short of my original idea of cruising at 110 km/h, but you can't have your cake and eat it.

I've been calling around, to see what's available, but all I can find is either a 39 or 48 rear sprocket. I'll see what's available locally tomorrow.

Sorry for the long-winded, rambling post, but I'd appreciate comments on my reasoning, and any insight people might offer.

Cheers,

d.

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You're way over-thinking this. :ride:

Put a 14 tooth front sprocket on. You won't have to change your chain and it will be cheaper and easier than changing the rear sprocket. You'll get about 8% change.

Ride for a while and see how it feels. Then, if you want more top end, try a 15 tooth front. I admit, I don't know if the 15 front will fit, but you'll be able to judge when you mount up the 14.

PS Do all your calculations based on the ratios of teeth on the sprockets. For example, changing from 13 to 14 is about 8%. 1/13 = 7.7% :thumbsup:

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You're way over-thinking this. :ride:

Put a 14 tooth front sprocket on. You won't have to change your chain and it will be cheaper and easier than changing the rear sprocket. You'll get about 8% change.

Ride for a while and see how it feels. Then, if you want more top end, try a 15 tooth front. I admit, I don't know if the 15 front will fit, but you'll be able to judge when you mount up the 14.

PS Do all your calculateion based on the ratios of teeth on the sprockets. For example, changing from 13 to 14 is about 8%. 1/13 = 7.7% :thumbsup:

I assume the case saver will need to be shaved down? My xr250 doesn't have much room to spare up there for a bigger sprocket.

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Thanks for the suggestion. On Friday I called a Honda dealer and he said that he didn't have anything other than the 13 tooth front sprocket listed as an option for a 96 250R.

With respect to shaving down the case saver... hmm... me no like!

d.

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Thanks for the suggestion. On Friday I called a Honda dealer and he said that he didn't have anything other than the 13 tooth front sprocket listed as an option for a 96 250R.

With respect to shaving down the case saver... hmm... me no like!

d.

Dealers generally don't know their head from their ass....don't know about down there? The case saver is a pretty simple piece to modify. I'd like to hear if it's necessary to do so

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Chaparral lists in their Dirt catalog #75 for 2006 - Sprocket Specialists steel front sprocket: 500-585 in 12-15 tooth range for $22.96 USD. They ship international.

ChaparralMotorsports.com

800-841-2960

909-884-3183

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Thanks for all the replies.

With respect to clueless dealers, I've been lucky, but I have a friend who would agree 100% with you, after trying, on separate occasions, to obtian a new sticker for his Transalp, a user manual for it, and crash bars. He now gets a nervous tic whenever he has to call one.

I wonder if anyone here can confirm that such modification is necessary, and if so, what the metal protector looks like afterwards: perhaps one can moidfy it and still have a functional protector.

d.

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the old XL250's 1972 era cruise at 50MPH 5K RPM - 60 -6K RPM and they will do it all day long! and those motors are nothing compared to the newer technolgy of the XR i.e needle bearing cam - vice plain journals. Fresh oil is key to longevity and if you feel your abusing it - you probably are, back off the throttle, or get something bigger that does not have to windup so tight, a 250 used to be good midsized bike for the street, not anymore. Cars like to travel everywhere at 50 plus, if your on something small - they will run you over. (Washington state). I crusie back from a trip to the market on my 650L at 55-60 and get a car about 6-10 feet from me every time. That is why I ride 95% off road only. Too many nutso's out there...

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Hi Durandal,

I bought my 2003 XR250R about 3 months ago and it was already setup for the road with 13 front/39 rear - sprockets. At the moment this suits me as the majority of my riding is on the road where I like to cruise at around 90kph

when I'm able (majority of speed limits off the highway in Oz are either 60 or

80 kph). I've opened it up to about 120 to 125kph so far but I couldn't imagine cruising at any more than 100 kph. I intend to change my rear

sprocket to a 43 or 44 when I replace the chain etc at 10K in a few months. Fuel economy's good though - about 30k/litre so that's a factor also, though

it would be nice to be able to pull the ocassional mono - not much hope with a 39 rear. Sorry, can't help with the RPM's but good luck with the mods.

Dave

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Cars like to travel everywhere at 50 plus, if your on something small - they will run you over. (Washington state). I crusie back from a trip to the market on my 650L at 55-60 and get a car about 6-10 feet from me every time. That is why I ride 95% off road only. Too many nutso's out there...

That's very true. So far I've been lucky, and haven't had that many lunatics be aggressive to me in the city or the highway.

A bigger bike for now is not an option, and besides, I'm happy with the one I have!

I'm looking into the 14-tooth front sprocket option, it sounds like a good compromise.

d.

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Thanks djrowe, that's precisely what I'd feared about a 39 rear sprocket.

One interesting thing, I get 30 km/litre in the city, very relaxed riding in 6th, cruising along, no racing starts from traffic lights, etc.

That should probably mean that with a 39 one moves the "economy spot" up towards 90 km/h.

d.

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Well, I shopped around locally, and the 14-tooth front sprocket is nowhere to be found. Renthal makes one, and so do others. I checked the Chaparral site that someone recommended, and could not find the right sprocket (my bike isn't even mentioned in the front sprockets list).

I emailed XRs Only, and asked about shipping the sprocket plus a magnetic oil plug and a temperature disptick to Chile. I didn't have high hopes about getting an answer, since my last email inquiry went unanswered, but this time I got a one-liner (no hello, no regards, nothing): "US mail $40.00 UPS $120.00".

Ouch 40 bucks just for shipping. Well, I'll sleep on it. I'll probably end up doing it.

d.

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Send me an e-mail at ramz [at] amigo [dot] net and we'll work something out.

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