Landracing Forum
Tech Information => Technical Discussion => Topic started by: entropy on May 07, 2014, 08:11:01 AM
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On our rear Goodyear 26" Front Runner, I measured rear tire circumference = 81.5",
I calculated front circumference: 3.14 * 26 = 81.64"
calculated is close to measured = good!
On our front Goodyear 28" Front Runner, I measured rear tire circumference = 90.0",
Then I calculated front circumference: 3.14 * 28 = 87.92"
2" different than measured = huh???
I will use circumference numbers for a % tire slip display I am still considering for our lil Lakester, so I don't like the discrepancy.
Has anyone actually put a tape measure on 26" & 28" Front Runners???
Karl
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I use Pi = 3.14159265359 X wheel height makes a small difference.
BR
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I use Pi = 3.14159265359 X wheel height makes a small difference.
BR
BR, i had it right when i did the calc,I SWEAR IT!!! :cheers:
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I am sure, I had a geometry teacher beat Pi' into my head...its most of what I remember, every other rule I have to look up.
I have 4 28's front runners mounted I will put a tap on them and see. What air pressure are in the tires when you measured? I measured 2 with 50 Psi one was 89.950" and the other 89.825" which makes it around 28.6" at 50 psi.
Not sure if that helps,
BR
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I am sure, I had a geometry teacher beat Pi' into my head...its most of what I remember, every other rule I have to look up.
I have 4 28's front runners mounted I will put a tap on them and see. What air pressure are in the tires when you measured? I measured 2 with 50 Psi one was 89.950" and the other 89.825" which makes it around 28.6" at 50 psi.
Not sure if that helps,
BR
OK, I'm gonna be a smart ass and suggest that you can't accurately measure the diameter or circumference to 3 significant digits and therefore it doesn't make a lot of sense to take PI to 11. :evil: :cheers:
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I am sure, I had a geometry teacher beat Pi' into my head...its most of what I remember, every other rule I have to look up.
I have 4 28's front runners mounted I will put a tap on them and see. What air pressure are in the tires when you measured? I measured 2 with 50 Psi one was 89.950" and the other 89.825" which makes it around 28.6" at 50 psi.
Not sure if that helps,
BR
wheelerdealer/BR
Your info helps ALOT!!! :cheers: I really appreciate it.
I measured front tire circumference: LF = 90.0"@55psi, RF = 90.0"@50psi and this jives well with your 89.9"
It seems the "28" tire isn't exactly 28", whereas naming the 26 a "26" is accurate.
Karl
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The first pair of Dunlop 30 inch 18s I got were so far off you could clearly see it with the naked eye. Roger Kruas didn't even try to sell them to me. The next pair were pretty close, but I don't remember exactly what they were. You need to measure any landspeed tire pair.
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...It seems the "28" tire isn't exactly 28"...Karl
I use 27.8 for them in my spreadsheets for Hooley's car and we are usually within 1-3 mph with the tach vs. mph at b'ville,
Sum
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...It seems the "28" tire isn't exactly 28"...Karl
I use 27.8 for them in my spreadsheets for Hooley's car and we are usually within 1-3 mph with the tach vs. mph at b'ville,
Sum
thanks for the data, Sumner!
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Jacksoni
For what it is worth, I quoted Pi' as the 11 digits as I remember. But I don't use all of the decimals when I do the calc manually, but my Excel spread sheet does and that is where I pulled my tire numbers from.
I did take another look at the circumference numbers and they should be 29.750 and 29.875 which is the decimal equivalent to 3/4" and 7/8". Which is easily read on a tire rollout measuring tape. I suspect the same is true for a carpenter's tape as well. But I did make a typo in my original post .950 and .825.
Karl, thanks for the feedback. Good luck. I have 2 more 28s mounted that I will pull out, air up and measure. If I recall they we off about the same .125". I'll let you know on those.
BR
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...It seems the "28" tire isn't exactly 28"...Karl
I use 27.8 for them in my spreadsheets for Hooley's car and we are usually within 1-3 mph with the tach vs. mph at b'ville,
Sum
For the stated 28" tire, it is interesting
- Sumner has "real" diameter less than stated = 28 vs 27.8
- wheeler & I have diameters more than stated = 28 vs 28.6 & 28.66
As pointed out above, it appears that you gotta measure, always. :cheers:
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.... - Sumner has "real" diameter less than stated = 28 vs 27.8....
Actually I mislead you :oops:, I don't know what the 'real' diameter is of the different 28 inch tires that we have run (we've run more than one set). I do know that Hooley has measured the circumference to make sure they were close but don't know if he recorded that or not but we do keep the tires in sets.
27.8 is what I've used since way back when we started with the spreadsheets as it seems to agree with the clocks on the salt (our rpm vs. their recorded speeds). Our license runs where we have run steady rpm for some distance through the clocks has show that we can predict our speed withing a couple mph off the tach or the data we get from our Innovate AuxBox that records the rpm on the runs.
So the 27.8 might be making up for tach error or minor wheel spin or rounding off the gear ratios in the transmission and rear. Sorry for any confusion,
Sum
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We use 28". Never have measured them. Factoring in clutch slippage, tire growth and tire slippage -- we use 28" with our $2 cardboard Dream Wheel. Usually off the tach reading by no more than 1 MPH.
Hand me a larger hammer, Willie.
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I only figure tire growth from the exit speed trap, as all other times are an average over the mile.
Been awhile but tack indicated lower rpm for time slip.
Figured 1.5 in. tire growth at 275mph with 10x28x15 Goodyear #2222 which are no longer made.
JL222
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Just got a new set of Mickey Thompson 30-9.0X18 tires yesterday, When I unwrapped them they were marked: One said 93-3/8, and the other was marked 93-1/2. Pretty damned close I'd say. Mounted up easy, and no leaks anywhere. Both tires balanced up with less than 1 oz of lead. Very smooth deal and great people to deal with.
Doug :cheers: :cheers: :cheers:
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Just a data point.
Goodyear 808-029-090
25.0 x 4.5 x 15
D2209, made in Chile in 2002
LF, 6' 6 1/4", 78.25 roll out, 24.907 Dia.
RF, 6' 5 7/8", 77.875 roll out, 24.788 Dia.
D2284, made in USA in 2010
LR, 6' 6 3/8", 78.375 roll out, 24.947 Dia.
RR, 6' 6 1/2", 78.5 roll out, 24.987 Dia.
Just measured them +/- 1/16" at 60 PSI.
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Thanks guys!
Some interesting variations, good data.
When i was running my Hayabusas, i kept very detailed info on tire circumference, ticket trap speed, logged trap speed, logged rpm and nearly always could correlate rpm to speed by less than 1/2mph.
What was interesting is that on asphalt i needed to take 3.8-4.2% off the measured tire circumference to get the calculated speed = trapped speed. In 2012 World Of Speed was very damp and we didn't get enough passes to develop a solid correlation
(although we did get enough passes to set MPS-F 1650 record... :-o)
Clutch was a lockup, so no clutch slippage.
Prob tire slippage, but definitely not spinning or it would show up in rpm variations.
I'm guessing that car tires will need a similar "adjustment" in the calculations.
The tire circumference variations are very interesting, but in the end, I will find out what the "adjustment factor" is and calibrate my data accordingly.
karl
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Karl, how much did Greenie's rear tire grow at speed?
Do LSR tire manufacturers list a growth factor?
Dean
:cheers:
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Karl, how much did Greenie's rear tire grow at speed?
Do LSR tire manufacturers list a growth factor?
Dean
:cheers:
Dean, that's the weird thing, i have seen many times tire incr in diameter on the dyno.
But my correction factor is negative, the opposite direction of tire growth. I have to give it LESS circumference (than i measure on the tire) to get the calculations to match trap speed at a specific rpm.
go figure
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Maybe holding front end on the dyno allows the bike to unload the tire, and on the track the weight transfer compresses the tire? Interacts with wheelbase, swingarm angle?
I'm setting up greenie with a hard tail and about 7/8ths clearance to the subframe, hopefully it will be enough.
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What is important is the "rolling radius" of the tire. This includes the compression of the tire due to load and is usually quoted at a given inflation air pressure. Race tire catalogs also frequently show the revolutions per mile. This is based on the tire's rolling radius.
A radial tire exhibits much less growth at speed than a bias-ply tire. Good luck on finding specs for this parameter. :x
Regards, Neil Tucson, AZ