Daytona gear ratio for stock 750

Started by kl3640, February 23, 2008, 01:53:20 AM

0 Members and 1 Guest are viewing this topic.

kl3640

Any gear ratio suggestions for the Spring Daytona config (next week)?  I was going to go with 2.750 or 2.765 based on last year's experience, but now I'm re-thinking that because friend of mine is going with 2.705 and another with 2.875.  I don't think that .05 will make that much difference, but I'm wondering about the .125 difference.

Any suggestions?

Thanks.


bambam

start with stock and be ready to go up or down a couple of teeth on the rear. most likely down in the rear for a little more on the top end.

kl3640

#2
Quote from: bambam on February 23, 2008, 02:02:20 AM
start with stock and be ready to go up or down a couple of teeth on the rear. most likely down in the rear for a little more on the top end.

Thanks, but I'm not going to start with stock, which is something like 2.64.  I started with that last year at Daytona single-bank and now I've got it down to a range, but I'm wondering specifically, for this bike and this track config, if anyone has tried 2.705 up to 2.875 and how those ratios felt for them, because I was planning on starting out at 2.75 but now I'm not so sure.  I would think that 2.875 or there abouts is to short, but that's why I'm asking - I'm really just curious what others are planning.  I weight about 200# without gear, and the bike is pretty close to standard weight for a stock 750 in race trim.  My friend who's going with 2.705 weighs a lot less than I.

GSXR RACER MIKE

#3
Power to Weight Ratio (P/WR) is such a factor in this type of racing, there is a definate 'range' where power is at that fine balance with the weight it's propelling. A great example of this is the 600's today, with an average weight rider on an average Hp 600 the ratio is close to being in the balanced zone as far as P/WR goes (but 600's are reaching the ceiling of that zone now). Something I've seen many times are riders who are crazy fast on a lower Hp bike jump up to a higher Hp bike and start crashing, not be really any faster, or even have trouble going the same speed they were going on the lower Hp bike. Why? Because as your P/WR increases it becomes more difficult to effectively transfer that power to the ground while still leaned over and effecting your drive out of turns, individual throttle control becomes very important at this point.

With that being said you brought up a good point about the difference in your weight with the other rider. If you had the same gearing as the lighter weight rider your P/WR would be less which would effect your ability to accelerate out of the turns. Realize that P/WR limits your Top speed as well, the same Hp carrying more weight will lower the top speed that can be achieved. Throttle control still plays a significant role, but if that's not an issue in your case I would figure out the P/WR of him on his bike and you on your bike and adjust your gearing ratio to reflect that difference.

Here's an example assuming both of you having a bike with 140Hp and using the combined bike weight / rider:

*Him: (Bike with fuel) 400 lbs. + (Rider with gear) 150 lbs. = 550 lbs. (total weight)

*You: (Bike with fuel) 400 lbs. + (Rider with gear) 200 lbs. = 600 lbs. (total weight)

Power to Weight Ratio:

*Him: 550 lbs. divided by 140 Hp = 3.92857 Pounds per Hp

*You: 600 lbs. divided by 140 Hp = 4.28571 Pounds per Hp

If you divide your P/WR by his you will then have a ratio, 4.28571 divided by 3.92857 = 1.09091, this is the P/WR difference between the two of you. If the above weights and power were accurate in your case and you wanted to try the same effective gear ratio as him then you would multiply his gear ratio by the P/WR difference between the two of you. (His gear ratio) 2.705 times 1.09091 = 2.951 gear ratio.

This is just an example using rider weights and Hp that I guessed, you need to use the actual weights and Hp to come up with the identical gearing as him thru P/WR - even then what works for him may not work well for you because of your difference in riding style, bike set-up, tires, etc.

Hope this helps you out, good luck!  :thumb:
Smites are a cowards way of feeling brave!   :jerkoff:
Mike Williams - 2 GSXR 750's
Former MW Region Expert #58
Racing exclusively with CCS since '96
MODERATOR

Super Dave

Quote from: kl3640 on February 23, 2008, 03:32:29 AM
My friend who's going with 2.705 weighs a lot less than I.
Is he running the same tire as you?  That needs to be accounted for in your thought process.
Super Dave

kl3640

Mike, that's a great analysis, thank you - you should make it a sticky post.  That's exactly why I included weights in my question.  Also, for SuperDave, we're both running the new Dunlop 190/55-17's.

My bike puts out 141 whp on race gas, and I weigh <200# without gear.  It's a 750 with race bodywork, stock engine, full SS system, BMC, PCIII, and clip-ons, rearsets, stay, etc., no ceramic bearings or aluminum subframe or anything like that, so whatever that weigh's.

Anyone with a similar setup have a ratio that works for them at the single-bank config?

Thanks.

Super Dave

Either way, you're dealing with going across start/finish regardless at Datyona.  That's going to be the spot you don't want to run out of gearing.

How was it before, and were you and your buddy turning the same lap times?
Super Dave

kl3640

Quote from: Super Dave on February 25, 2008, 10:03:42 AM
Either way, you're dealing with going across start/finish regardless at Datyona.  That's going to be the spot you don't want to run out of gearing.

How was it before, and were you and your buddy turning the same lap times?

Dave, previously (last year, when the Dunlops were 60's, not 55's), I felt like the gearing was good at the tri-oval, but since then my bike is making a bit more power (new full system, PCIII, tuning), it weighs about 10# less and I weigh about 20# less, so that's why I was thinking a little taller.  Last year I ran a 2.87+, so that's why this year I was thinking a 2.75 or 2.70.  As for lap times, I can't really compare to my buddy because he's a fast EX and I'm a mid-pack AM.

So looking at last year's HP/#, I get that this year's ratio should be about 2.60, but that seems like it might be overly tall, so I'd like to start in the 2.7's and then substract a tooth or two from the rear, if necessary.

GSXR RACER MIKE

#8
The '55' and '60' dimension is describing the height of the tire from the rim as compared by percentage to the width of the tire. For example a tire marked '180/60/17' should be (in a perfect world) 108 from the rim to the top of the tire (60% of 180). To figure out tire diameter you would multiply that 108 dimension times two because the tire is on the top and bottom of the rim to come up with 216, divide 216 by 25.4 (25.4 is the metric to inch conversion) to come up with the amount in inches: 216mm = 8.504 inches, now add the 17" wheel diameter to get total height: 17 + 8.504 = 25.504 diameter. Doing this for the '190/55/17' tire it would be 55% of 190 = 104.5, multiplied times 2 = 209, divided by 25.4 = 8.228", added to the 17" wheel = 25.228" diameter. Now according to this the '190/55/17' would actually be a little over a 1/4" (.276") shorter than the '180/60/17' labelled tire, but in reality it's probably different than what it's labelled.

If we take the original '180/60/17' labelled tire's diameter (25.504") and figure out it's circumference (25.504 x pie {3.14159} = 80.123") this gives us the distance traveled by 1 rotation of the tire, the '190/55/17' tire would have a circumference of 79.256. If we divide the smaller tire by the larger one we have the percentage of difference traveled by each tire per rotation, in this case 25.228 divided by 25.504 = .9892, this means if you went to the smaller circumference tire you would have to multiply your gear ratio by .9892 to gear it the same as you had it with the larger circumference tire (you need a smaller sprocket for the lesser circumference tire to rotate more times to travel the same distance per mile as the larger circumference tire does per rotation). So in this example if you were using a 2.87 ratio last year you would have to run .9892 of that to have an identical ratio as with the taller tire, that would be a 2.84 ratio. I realize your looking to also make a change from the gear ratio you had last year, but this gives you a baseline.

I think what Dave is getting at with the tire size is not what it's LABELLED, but what it ACTUALLY MEASURES in true diameter. Take a string or flexible tape measure and measure around the outside at the largest diameter of the mounted and inflated to proper pressure tire (to get the circumference), comparing the old and new tire circumferences should be figured into your gear ratio as well. I bet when you measure both of those tires neither will be what they should be (the numbers I have above) according to the size their labelled.
Smites are a cowards way of feeling brave!   :jerkoff:
Mike Williams - 2 GSXR 750's
Former MW Region Expert #58
Racing exclusively with CCS since '96
MODERATOR