Changing rear sproket

[duckman]

Hi all,

I'm thinking about changing the rear sprocket (adding teeth) so that 1st gear is shorter and I can get to 2nd and still be under the 50 kmh speed limit.

At present the revs sit at 5,000 in first while doing 50 and if I change gear the revs drop too much and the bike doesn't like it.

Anything lower than 4,000 and the bike splutters a bit and just feels wrong.
I'm running a power commander and it's been dyno'd to set up the air fuel mixture correctly.

Just wandering if anyone has had any experience with this sort of thing. I did change it a while back (just a couple of teeth) and the difference wasn't significant enough. So how many teeth do you think before I can hit second and stay under 50 ??

I know this will reduce my top speed but I'm not really too concerned.

Appreciate all comments. (that are to the point)

[merv]

Just wandering if anyone has had any experience with this sort of thing. I did change it a while back (just a couple of teeth) and the difference wasn't significant enough. So how many teeth do you think before I can hit second and stay under 50 ??

I know this will reduce my top speed but I'm not really too concerned.

Appreciate all comments. (that are to the point)

You know what sprockets you've got now and you know what revs the engine doing so do a simple calc and see how many you need on the back or off the fron to make it do 4,000 at 50k in 2nd, then figure out what speed it will still do in top and then decide if you would be happy with that. Just use a calculator and do simple ratios. e.g. Let's say you got 15T on the front and 45T on the back and its doing only 3,500 in 2nd at 50k then you need 4000/3500 x 45 = about 51 on the back - that's a big sprocket on a road bike. Or you need 3500/4000 x 15 = 13 on the front. Hey this is just an example - you work it out as I don't know the actual revs it is doing in 2nd. If it was like my example then the calc is 87.5% change - so you bike would go 12.5% less top speed at the same revs in any gear including top - that would be a big change.

[jrandom]

So how many teeth do you think before I can hit second and stay under 50 ??

As per merv's post, depends on the current diameter of the sprockets, number of teeth and your actual gear ratios, dunnit?

I suppose, ideally, if you've already had the bike on the dyno and have the output from a base run to look at, given that your gearing is presumably fixed, you should be able to figure out precisely what level of torque multiplication you want to apply at the rear wheel to get a nice peak at your desired 2nd gear cruising revs and work backwards to your ideal rear sprocket diameter.

Want to post a scan of your bike's dyno chart?

[duckman]

As per merv's post, depends on the current diameter of the sprockets, number of teeth and your actual gear ratios, dunnit?

I suppose, ideally, if you've already had the bike on the dyno and have the output from a base run to look at, given that your gearing is presumably fixed, you should be able to figure out precisely what level of torque multiplication you want to apply at the rear wheel to get a nice peak at your desired 2nd gear cruising revs and work backwards to your ideal rear sprocket diameter.

Want to post a scan of your bike's dyno chart?

Yea thats a good point. As you can tell, I'm not the most technically competant individual here

I'm a little embarrissed to post the dyno results... but here they are

HP - 63.03 and Torque 45.22 This is after the power commander was fitted. I don't have a printout of the baseline..

[merv]

OK to help you further if this page can be believed http://www.onyerbike.net/2002/ducati/ss750.htm and I'll see if I can attach an Excel worksheet showing gear calcs. First one on stock ratios shows at 4,000rpm in 2nd that Duc would be doing 56km/hr. To do 50km/hr at 4,000rpm looks like you need a 45T rear giving 49km/hr in fact - 44T gives 51km/hr. You said you had already changed it by a couple of teeth - looks like you need 4 more. But that will knock your top speed into the 180's if the Duc does about 8,500rpm - my guess.

Damn it won't do Excel I try converting them to Word quickly.

[merv]

p.s. I did those sheets quickly and just cut and paste them when Excel wouldn't come over so you won't be able to use them to do your own calcs but remember everything is linear and you can just ratio off the calcs I've given you. That sucker is high geared and only a 5 speed doesn't help with giving you a good spread of ratios if the website I got the figures from is correct. Hope this helps.

[duckman]

p.s. I did those sheets quickly and just cut and paste them when Excel wouldn't come over so you won't be able to use them to do your own calcs but remember everything is linear and you can just ratio off the calcs I've given you. That sucker is high geared and only a 5 speed doesn't help with giving you a good spread of ratios if the website I got the figures from is correct. Hope this helps.

Your a bloody star Merv Thanks for your help!!

[merv]

Your a bloody star Merv Thanks for your help!!

Only thing I wasn't sure of after reading the web page on the 750ss is what does it redline at? I guessed 8,500rpm but is that on the low side?

This is where us VFR riders can smirk (and no doubt those with even bigger Jap bikes) as we have such a wide power band, 6 gears and a smooth engine. The same calcs done on my VFR confirm what it does on the road - normal riding I would keep it above 4,000 to keep good power on tap, but riding in 50km/hr zones it easily trickles along at not much above idle in the 2-3,000 range so you can ride in 3rd or 4th. At 3,000 rpm VFR is doing in 1st through 6th - 24, 33, 42, 52, 60, 67 km/hr. At 11,500rpm redline it is good for 93, 128, 162, 198, 229, 256. It easily revs past redline if necessary up to rev limiter at just over 12,000. Mind you on the dyno the VFR 750 according to bike tests are about 94-95hp (haven't had mine tested to confirm it) so you can see they are quite a different bike to the Duc. All that and it will do 400kms to a tank of gas. There was me thinking the twin should have been nice and tractable, but what you have told us about it not wanting to run at 50 in 2nd is the opposite of what I expected, me thinking it would easily have trickled along at 2,000rpm.

[duckman]

Hey Merv,

Unfortunately I would say these faults are inherent with the lower end Ducati range. (750 ss, 900 ss, monster e.t.c) Certainly all the two valve Ducatis felt the same to me.

Anways I solved my problem on Saturday - While picking up my bike from Haldanes from getting it's 40,000k's service and there was a shiny TL1000R waiting for me to dribble on. To cut a long story short, I bought it and I'm in love.

I'll open another thread for this in more detail.

[wkid_one]

Did mine on the Yam to dull the acceleration - worked a treat....stopped the front lifting so much coming out of corners - plus added to breath to the top end as well.

Well worth the changes - but remember, it will fuck your speedo reading up.

[White trash]

Good mate of mine geared his TL1KS down by 2 teeth on the front. From 17t to 15t. Purely for the wheelies. Objective acheived.

How after a particularly quick over rev on wet roads, promptly threw a valve in the dirrection of the front piston.

A whole shit load of parts and labour later, it's finally ready to roll again.

just my $0.02

[duckman]

Good mate of mine geared his TL1KS down by 2 teeth on the front. From 17t to 15t. Purely for the wheelies. Objective acheived.

How after a particularly quick over rev on wet roads, promptly threw a valve in the dirrection of the front piston.

A whole shit load of parts and labour later, it's finally ready to roll again.

just my $0.02

Heya White trash,

Youchy, sounds expensive!! After a whole 4 days with my new TL I think I'll leave as it is until I have tamed the beast. (if I ever will)

[Hitcher]

Well worth the changes - but remember, it will fuck your speedo reading up.

Depending where the sending unit for your speedo reads. If it's off the front wheel it won't make any difference at all??

[merv]

Hey Merv,

Unfortunately I would say these faults are inherent with the lower end Ducati range. (750 ss, 900 ss, monster e.t.c) Certainly all the two valve Ducatis felt the same to me.

Anways I solved my problem on Saturday - While picking up my bike from Haldanes from getting it's 40,000k's service and there was a shiny TL1000R waiting for me to dribble on. To cut a long story short, I bought it and I'm in love.

I'll open another thread for this in more detail.

Geez I didn't know that giving you those spreadsheets would put you off the Duc so badly. How's the TLR go, does it happily run smoothly at less than 4,000rpm OK?

[duckman]

Geez I didn't know that giving you those spreadsheets would put you off the Duc so badly. How's the TLR go, does it happily run smoothly at less than 4,000rpm OK?

Hahaha Merv, no it's wasn't quite THAT bad. However the TL has been an absolute pleasure to ride. It burbles along at 50ish in second and pulls harder than a 12 year old school boy. (oops probably a bit inappropriate)

So far I cant fault it. even the weight issue hasn't bothered me yet. I find it sits on the road in a more stable manner. (no horse jokes now)