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Old 07-25-08, 04:30 AM   #1
Al Downie
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Tyre pressure - loaded or unloaded?

Having noticed that there's a maximum recommended tyre pressure of 55psi on my supposedly bombproof Mavic 729s, I'm wondering if I should be concerned about the increase in pressure when I get on the bike with all my luggage.

If you're inflating to a target air pressure, do you stop a little short to make allowances for extra weight on the bike, or just ignore it and hit the target bang on?
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Old 07-25-08, 05:03 AM   #2
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Having noticed that there's a maximum recommended tyre pressure of 55psi on my supposedly bombproof Mavic 729s, I'm wondering if I should be concerned about the increase in pressure when I get on the bike with all my luggage.

If you're inflating to a target air pressure, do you stop a little short to make allowances for extra weight on the bike, or just ignore it and hit the target bang on?
I have not heard of rims with a pressure rating, is it the rims or your tyres that are rated? 55 psi is not high at all even the really cheap tyres can go up to 65 psi. If the mavics rims could only go to 55 psi it would make it a bit pointless.

Myself for load carrying i'd go to higher psi tyres so you get less rolling resistance. As for the tyre pressure ballooning; the manufacturer has already factored that in.
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Old 07-25-08, 05:08 AM   #3
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55 sounds very low i inflate my marathons to 90psi on sun rhyno wheels ,running soft tyres can cause problems like pinch flats ,damage your rims, so put the max in them even with the full load on.
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Old 07-25-08, 05:12 AM   #4
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is it the rims or your tyres that are rated?
Hi rustguard - the tyres are rated 30-75psi, but the rims have a label attached saying 'Max tyre pressure: 55psi'.

Thanks for the note about the manufacturers already factoring-in the weight of rider 7 luggage - that's reassured me a wee bit.

As it happens we'll be rolling on unpaved roads, so with 2" tyres, 55psi will help cushion some of the bumps, and after the tour I'll be rejigging the bike as a full-on MTB again, and will be unlikely to ever want higher than 55psi on my knobblies. I'm wondering if that wee sticker is something that Mavic put on all their new rims now, to cover their backs since their reputation for cracks is growing.
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Old 07-25-08, 06:10 AM   #5
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Yea, I've never seen a tire pressure rating on a rim before either???

On my loaded touring bike I run the back tire higher than the front, back near the high side of the recommended range and the front near the low number. This gives a little softer ride and stays away from pinch flats. Rolling resistance is not an issue, the difference in resistance between the low and high pressure tire is so small that you could not measure it.

On my unloaded bike, both tires are at the low side of the pressure range.
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Old 07-25-08, 06:30 AM   #6
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Tire pressure on rims is something I haven't seen before.

I have normally inflated my tires to the maximum specified on the tire. I used to pump them up even more, but found with the tires I was using then (continental top touring) that I was getting sidewall tears, particularly if I was fully loaded and it was a warm afternoon.
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Old 07-25-08, 06:39 AM   #7
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I've just looked at Mavic's website, and they do in fact issue blanket maximum pressure ratings across their whole rim range, dependent upon tyre width rather than rim model (there's a separate page for Road use), and my 2.0' tyre width should be good for 65psi rather than the 55psi on the rim (The rim is supposed to be used with 2.3"+ tyres).

I'm also banking on the assumption that my 'Extreme MTB' rims will withstand a wee bit more pressure than their more lightweight rims.
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Old 07-25-08, 09:28 AM   #8
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Interesting I looked at the mavic websight, interesting you learn something new everyday, but 55 seems very low. I wouldnt worry myself i'd go with the tyre recomendations, they are solid welded rims so strength isn't going to be a problem. I'd be interested to know why they are rated at such a low max pressure.

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the difference in resistance between the low and high pressure tire is so small that you could not measure it.
When I'm running a load I notice a big improvement when i went from a 65psi max to a 80psi max. Mainly I just like the way the rim wasn't so close to the ground.
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Old 07-25-08, 10:19 AM   #9
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When I'm running a load I notice a big improvement when i went from a 65psi max to a 80psi max. Mainly I just like the way the rim wasn't so close to the ground.
Tire deflection under load is the criteria you should be using to determine what pressure to run on your bike. You can assess this yourself or get a friend to help you. If you have to use a higher pressure than is safe with your current tire/rim combo than you need to move to wider/higher volume tires and use a lower pressure. More pressure once you're supporting your weight and cargo does not make your bike faster or do anything useful for you.

Personally I don't sweat this too much and just eyeball it when I get on the bike. A few psi either way isn't going to matter.

On one tour I let some pressure out of my tires for the plane trip and forgot to add it back in on the other end. I rode for more than a week at 15-20psi less than I normally put in those tires. The bike handled great, was fast and very comfortable. When I realized what was going on I added some pressure to the tires and all that happened was the bike vibrated a whole lot more. These were 35mm 700c Marathon XRs on a LHT - so reasonably high volume.
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Old 07-25-08, 11:03 AM   #10
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This is only my preference and I'm not giving this as advice, But I run everything on bitumen maxed out on pressure, the main reason is performance, especially in the corners, for me cornering precisely is very important. As my gf says "as soon as you get on a bike you are like a little kid". Before I got my panniers, I was asking a guy at one of the lbs how panniers handle when you have the bike on the back wheel? Well; you should have seen the look he gave me, He told me you just don't do that. So I explained that I know what I am like and that I wouldn't be able to help myself. One time when i was riding home from the markets I had about 30 kgs of fruit and vegie in 2 panniers and one backpack. Still I was riding on the back wheel, balanced the front wheel was only 6" in the air. Its more a mindset, but comfort dosn't enter into my head much. I think as I get older I'm changing. Ive even been thinking that a stand would be nice
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Old 07-27-08, 12:48 PM   #11
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I hope it's more than 65lbs,I run a 120 in mine,haven't exploded ...yet.I hope aluminum can withstand more than 65 pounds per sq/in.LOL!

Pure aluminum has a tensile strength about 15,000psi and alloys up to about 90,000psi,I think more air will be OK.You should be able to blow the tires up until they pop.

Sounds like the lawyers talking.

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Old 07-27-08, 09:03 PM   #12
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Sounds like the lawyers talking.
YES!!! my thoughts exactly, thats why the stronger downhill rims have a lower psi rating. How many riders know the exact psi just before their rim goes oval?
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