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  1. #1
    PMK
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    Tire Pressure???

    Being the curious PIA type I am, I did some searching to try and see where others dial in their tire pressures. The search included here but also some other sites. Possibly I didn't look in the right spot, but I could not find any real mention of recommendation of

    tire brand vs team weight vs tire size vs inflation pressure.

    I did find this article on Precision Tandems site. It deals a lot with weight bias.

    http://www.precisiontandems.com/arttiresbymark.htm

    I grabbed one quote from it that indicates the possibility of inflating the tire above the sidewall rating.

    "That said, my favorite tire for teams in this weight range is the 28mm Continental Ultra Gatorskin. We have used the Gatorskin on triplets, quads and quints at 140 psi and find the sidewall wear at the rim junction is far less than that of other tires."

    Not saying this is for everyone or something we need.

    But I do ask, what method do you use to determine your best pressure? Is there some kind of guideline chart floating around here or on the net? Any ideas on how much heat and pressure increase is seen on a ride of a couple hours.

    Thanks
    PK
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    You don't want the pressure too low, since you will get pinch flats and such at potholes. You don't want it too high, since it is uncomfortable, and at some point, will be enough to blow the tube out. That said, the nominal max pressure is "cold", and the pressure can go way up from there. The max pressure marked on the tire allows for that. So you are safe there with pretty serious use. But the Precision Tandems thing mentions going well above the max. That is probably ok because they probably aren't descending the alps in that quint using rim brakes.

    I put the rear at the recommended max, and the front at a little less. But I don't think I would have a problem with higher numbers, though the stoker would almost certainly complain on 140psi. That is track level pressure.

    I should also mention that rim/tire fit makes a big difference here. I think there are various tire and rim charts that discuss "best fit" sort of things, but I have not heard of scientific systematic testing.
    Last edited by jgg3; 03-06-09 at 05:49 AM.

  3. #3
    Senior Member WebsterBikeMan's Avatar
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    Sheldon Brown has a chart showing, for two wheel loads, and a series of tire widths, appropriate pressures. The page is not tandem-specific so he notes that rear wheels "normally have more load than front". I plan to measure our relative front-rear loads soon, but it wouldn't surprise me to have them equal or front biased, given that I weigh 50% more than my wife.

    It only gives two wheel loads, but the equation is linear, so you can extrapolate and interpolate at will.

  4. #4
    pan y agua merlinextraligh's Avatar
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    Quote Originally Posted by PMK View Post
    ]

    But I do ask, what method do you use to determine your best pressure?

    PK
    Years of trial and error. A pressure that provides a reasonably comfortable ride, and doesn't pinch flat.

    For us at 340lbs ,and running 25c's that's typically 110psi. The charts I've seen would indicate that pressure is too low, but we're not pinch flatting, so I question how accurate the charts are.
    Last edited by merlinextraligh; 03-06-09 at 08:31 AM.
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  5. #5
    TWilkins
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    Over time, we've come to believe that 110-120 psi on our 700 x 25's is about right for us. Our team weight is 285, and we've never had problems with pinch flats. If we get down to around 100 psi, the bike seems a little sluggish to me, but Pam doesn't notice it.

    I tend to run between 100 and 110 on my single with the same tires.
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  6. #6
    Senior Member CGinOhio's Avatar
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    Another data point for you: we have 700x28 Gatorskins. 290 lb team weight + 40lb bike. Run 100 psi in front, 100 - 110 in rear. No flats in 1800 miles on some pretty rough pavement.

  7. #7
    Senior Member rishardh's Avatar
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    As per the Precesion Tandems formula (700x25, 240 team weight)... Rear = 128 and Front = 116
    And we have been riding... Rear = 110 and Front = 100
    On my single (700x23, 135 weight)... Rear = 100 and Front = 90

    Like Merlin came up with these after trying out different combinations.

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    I'll run 100psi front, 115psi rear on our tandem. Both well above the manufacturers max, but it's not caused a failure yet. No pinch punctures either

  9. #9
    Senior Member Retro Grouch's Avatar
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    Quote Originally Posted by CGinOhio View Post
    Another data point for you: we have 700x28 Gatorskins. 290 lb team weight + 40lb bike. Run 100 psi in front, 100 - 110 in rear. No flats in 1800 miles on some pretty rough pavement.
    That's us too. 700x28 Gatorskins, 100psi front, 110psi rear. We don't pinch flat at that even though our team weight pushes 400 lbs.

    We used to run 120psi front and rear. When we lowered the air pressure we immediately noticed an improvement in ride characteristic. Subjectively, I'd say our speed is about the same.

  10. #10
    PMK
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    Lot of good information, thanks. We had been adjusting pressure around a bit on the Speedster and now Roadster. Both have 28mm Gatorskins. As luck would be, 115 rear 110 front. Seems we're in the hunt, and methods described here pretty much run side by side, with similar results.

    For grins I might set up scales for weight bias. Maybe I overlooked it, or possibly it was just common sense, but is the article I linked saying to run a 1:1 weight to PSI conversion regardless of tire size. Is there a chart that's missing?

    Thanks
    PK
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  11. #11
    Senior Member zonatandem's Avatar
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    We run Maxxis Re-Fuse 700x25; tire pressure listed on tire sidewall is 120 psi and that's what we normally run. Have run them 10 psi over, no problems.
    On single, recommended pressure is 135 psi for Maxxis Courchevel 700x 23 tires. A bit harsher ride but then am not sitting in the middle of the frame like on our tandem.
    Can't go wrong with pressure listed on the tire, but can easily run 10% more air depending on your team weight and ride comfort level.
    We are a sub-250 lb. duo.
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  12. #12
    Senior Member WebsterBikeMan's Avatar
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    Quote Originally Posted by WebsterBikeMan View Post
    Sheldon Brown has a chart showing, for two wheel loads, and a series of tire widths, appropriate pressures. The page is not tandem-specific so he notes that rear wheels "normally have more load than front". I plan to measure our relative front-rear loads soon, but it wouldn't surprise me to have them equal or front biased, given that I weigh 50% more than my wife.

    It only gives two wheel loads, but the equation is linear, so you can extrapolate and interpolate at will.
    OK. I measured. With a lightly loaded bike (1 rack, two water bottles (half full), spare tube/tools beneath seat), and both of us, the load on the front is 165.5 pounds (bathroom scale under front wheel; rear in trainer). Total load on both wheels (me holding bike on scale, then adding in her weight) 352 pounds.

    Front load 47%, rear load 53%. Your mileage will vary. I was surprised not to have a higher front load, given our weight difference.

  13. #13
    PMK
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    then adding in her weight) 352 pounds.

    You might carefully rephrase these words...just in case she starts to read these forums.

    So ultimately was the bike weighed at each end with both riders and gear, or did this adding in become more of interpolating?

    PK
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  14. #14
    Senior Member WebsterBikeMan's Avatar
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    Quote Originally Posted by PMK View Post
    then adding in her weight) 352 pounds.

    You might carefully rephrase these words...just in case she starts to read these forums.

    So ultimately was the bike weighed at each end with both riders and gear, or did this adding in become more of interpolating?

    PK
    The procedure was as follows:
    1) put bike on trainer (rear axle fixed, rear wheel on roller).
    2) put bathroom scale on tile beneath front wheel (bike now approximately level).
    3) captain astride bike, stoker mounts (as if to ride off).
    4) lift front wheel, tap scale (durned digital scales...)
    5) set front wheel on scale, and captain mounts quickly.
    6) lean way forward to read scale once it has stabilized.
    ---
    This gives front tire load.

    7) ride for awhile (which of course throws everything off, since we lose so much weight )
    8) remove bike from trainer. I pick up bike, climb on scale: 232 pounds.
    9) I get off; she climbs on scale: 120 pounds.
    10) add weights to obtain 352.

    Divide front tire load by 352 to get 47%. 53% is therefore rear tire load. Sheldon Brown says the front tire load puts me right around 80 PSI with 1.75 inch tires. And 47% isn't much different from 53%. I'll probably redo the procedure before heavily loaded touring, as neither the total nor the ratio will stay the same.

    With this trainer we could in principle put the front wheel in the trainer to weigh the back but this would require the stoker lifting the rear wheel before setting it on the scale and hurriedly mounting, and then trying to read the scale behind her. Or getting a scale that operates differently.

    I think what you are calling interpolating I call trusting the law of conservation of mass. And assuming the force of gravity doesn't change between measurements. I know every number in the calculation has a measurement error associated with it, but for figuring out how to inflate tires it should be close enough.

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    Quote Originally Posted by WebsterBikeMan View Post

    Front load 47%, rear load 53%. Your mileage will vary. I was surprised not to have a higher front load, given our weight difference.
    At first I was surprised that the rear load for your team was only 53%. But considering that you weigh, give or take, 70 to 75 lbs more than your stoker, I guess that makes up for the center of gravity of both ridders being about a foot towards the rear from the middle of both wheels.

  16. #16
    PMK
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    Makes sense now, the statement you held the bike on the scale somehow had me thinking she was seated while you just steadied it and took a reading.

    I agree, you would think the difference in each riders weight would tend to shift the bias further towards the front.

    PK
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  17. #17
    hors category TandemGeek's Avatar
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    But I do ask, what method do you use to determine your best pressure?

    I pump them up to max recommended PSI for the tire or rim and then fine tune. Our 23mm Fortezza tires have a 145psi recommendation and we run them around 140psi. If we venture off our very smooth roads to some place that has chip seal I'll fit a slightly larger diameter (25mm) tire and run about 130 psi.

    Is there some kind of guideline chart floating around here or on the net?

    Sheldon Brown's site has several plus some basic-sense guidelines for adjusting tire pressure based on tire shape.

    Any ideas on how much heat and pressure increase is seen on a ride of a couple hours.

    With the exception of significant downhill braking, not a whole lot.

  18. #18
    Senior Member zonatandem's Avatar
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    In summer time in Arizona with temps at 100+ degrees, blacktop pavement can easily add another 30 degrees.
    Warmest I've ridden in was 117 degrees . . . no tire issues but did see a sidewalk buckling!

  19. #19
    Senior Member WebsterBikeMan's Avatar
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    Quote Originally Posted by cornucopia72 View Post
    At first I was surprised that the rear load for your team was only 53%. But considering that you weigh, give or take, 70 to 75 lbs more than your stoker, I guess that makes up for the center of gravity of both riders being about a foot towards the rear from the middle of both wheels.
    Well I didn't weigh myself, fully clothed. But we're more like 50-60 pounds apart. I haven't re-weighed the bike, as I'm still waiting for the rear rack, (currently have the front one mounted). But I'm guessing it's around 46 pounds with fenders, rack, tools, bottle cages, pump, bottles, and about a litre of water.

    And I'm about 17% taller, which changes the centre of gravity somewhat. I didn't lean way forward, which would have biased it more. I was actually surprised that the rear load was more than the front load, given the difference in weight and the amount of head tube vs rear seat tube, presence of front rack.

  20. #20
    just another gosling Carbonfiberboy's Avatar
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    300# team. Run 23c Fortezza Tricomps at 145. Max is 160. Seem comfy to me and stoker. Finally had a pinch flat last week - ran over a thumb-sized rock with one side of the rear tire in the dark. No tire damage. But they don't pinch in potholes or RR tracks, so must be about right for those tires. Roll and corner well, too.

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