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-   -   Tires for Clydes (Technical) (http://www.bikeforums.net/clydesdales-athenas-200-lb-91-kg/433230-tires-clydes-technical.html)

tomdaniels 06-24-08 08:21 AM

Tires for Clydes (Technical)
 
I had a full blown rear blowout yesterday and had to call the wife to get home. I can't quite figure out how the LBS and I missed the tire wear that I can now see. I was running 26x1.5 kevlar primo comets on the rear of my stratus and when it blew, it went BANG.
Post mortem, the tire had a line of wear at the edge of the kevlar belting and had several spots where the white threads were showing. I'm very glad this happened on a slow section of MUP instead of the 36mph downhill on Saturday.

Anyway, I bought a cheap tire from the LBS to get back on the road (26x1.5 Bontrager slick), but I'm trying to work out the dynamics of choosing a tire for us heavier riders.

Here Goes: Weight on tire = Pressure (lb/in^2) * contact patch (in^2) * some constant

So, let's say there's (340 (my weight) + 50 (bike+stuff) ) *3/4 of weight on the rear tire leading to roughly 300lbs. (Holy Crap!)

300 lbs = pressure * contact patch (we'll assume the constant is same for all tires.)

Now the width of the tire is variable and so is the max pressure so: 300 = pressure * width * length (dropping constants again and treating the patch as rectangular)
So now, I want to minimize the length of the contact patch to drop my rolling resistance.
My 100psi, 1.5 wide comets would come out to a length of 2 inches. Ok, now, looking at some 80 psi, 1.6 inch tires, the length goes to 2.34. And if the 2.5 inch tires at 60 psi will take the length to 2 inches, the same as my primos.

In the end, it looks like comparing the product of width and pressure (more is better for clydes anyway) is a good rule of thumb. Now, I wish I could find some 26x1.7s that went to 120 PSI or ever 100 PSI. :)

What do you think? As an engineer, this is a napkin-level analysis, but it seems valid. Anyone know of any 100PSI tires wider than 1.5in?

bdinger 06-24-08 09:07 AM

I don't know about that, but I do know I've had excellent luck with my 26x1.5 Nimbus Armadillos. They show little wear after 500+ miles, and roll fantastically (is that a word? :)).

Another option known to hold weight are the Schwalbe Marathon. I'll be putting those on my LHT when the stock Continentals finally wear out.

tomdaniels 06-24-08 09:26 AM

what pressure do you run in those nimbus tires? if you can use 1.5s at 65psi then i'm confused.

bdinger 06-24-08 09:46 AM

Quote:

Originally Posted by tomdaniels (Post 6936491)
what pressure do you run in those nimbus tires? if you can use 1.5s at 65psi then i'm confused.

I run mine at 80 psi, which I *think* is what they are rated to. Even with my substantial girth, there's not a lot of "squatting".

tomdaniels 06-24-08 10:40 AM

Quote:

Originally Posted by bdinger (Post 6936653)
I run mine at 80 psi, which I *think* is what they are rated to. Even with my substantial girth, there's not a lot of "squatting".

My bad, I thought they were only rated to 65. Hmmm... I see some folks in the forums running 100 psi in them. I may have to get some ordered before RAGBRAI.

I also noticed upon further inspection that I had highly uneven tread wear. A line towards the edge of the kevlar belting on the comets is very worn and that whole side is more worn than the other. I will keep a closer eye on this from now on!

meanwhile 06-24-08 12:53 PM

Quote:

Originally Posted by tomdaniels (Post 6936052)
So now, I want to minimize the length of the contact patch to drop my rolling resistance.

Doesn't work that way. Rolling resistance is created by the energy needed to flex the material of the tyre; it isn't eg drag between the contact patch and ground. So the size of the patch is irrelevant; it's the rate at which material enters and leaves the patch that matters. You'd think this would mean that width is key, but there can be situations when a wider lower pressure tyre (and hence a larger patch) has lower rr; see http://www.bicicletta.co.za/Download...llustrated.pdf
- very useful paper if you mountain bike, or even ride gravel roads.


This might be useful too -

Quote:

http://www.schwalbetires.com/tech_in...ing_resistance
Why do wide tires roll better than narrow ones?

The answer to this question lies in tire deflection. Each tire is flattened a little under load. This creates a flat contact area.
At the same tire pressure, a wide and a narrow tire have the same contact area. A wide tire is flattened over its width whereas a narrow tire has a slimmer but longer contact area.


The flattened area can be considered as a counterweight to tire rotation. Because of the longer flattened area of the narrow tire, the wheel loses more of its “roundness” and produces more deformation during rotation. However, in the wide tire, the radial length of the flattened area is shorter, making the tire “rounder” and so it rolls better.


...Wide tires only roll better at the same inflation pressure, but narrow tires can be inflated to higher pressures than wide tires. However, they then obviously give a less comfortable ride.
In addition to this, narrow tires have an advantage over wide ones at higher speeds, as they provide less air resistance.
Above all, a bicycle with narrow tires is much easier to accelerate because the rotating mass of the wheels is lower and the bicycle is much more agile.
At constant speeds of around 20 km/h, the ride is better with wider tires. In practice, the energy saving is even greater than in theory as the elasticity of the tires absorbs road shocks, which would otherwise be transferred to the rider and so saves energy.
I've seen similar stuff from other manufacturers, who seem quite frustrated that most people are buying narrower tyres than is optimal.


On a good road surface the material of the tyre is the main thing to worry about. Schwalbe and Conti now make some very low rr wide tyres, eg the 38mm Marathon Supremes.

Tom Stormcrowe 06-24-08 12:55 PM

Uneven wear? Rear wheel? Check your wheel dish.....

Quote:

Originally Posted by tomdaniels (Post 6936980)
My bad, I thought they were only rated to 65. Hmmm... I see some folks in the forums running 100 psi in them. I may have to get some ordered before RAGBRAI.

I also noticed upon further inspection that I had highly uneven tread wear. A line towards the edge of the kevlar belting on the comets is very worn and that whole side is more worn than the other. I will keep a closer eye on this from now on!


tomdaniels 06-24-08 01:33 PM

Quote:

Originally Posted by Tom Stormcrowe (Post 6937871)
Uneven wear? Rear wheel? Check your wheel dish.....

Will do! Time to go see the Mechanic again! I have noted that the cassette side spokes are much tighter than the other.

Quote:

Originally Posted by meanwhile (Post 6937855)
Doesn't work that way. Rolling resistance is created by the energy needed to flex the material of the tyre; it isn't eg drag between the contact patch and ground. So the size of the patch is irrelevant;

Meanwhile: Thanks for the schooling. That makes quite a bit of sense. I can see how a 1.75 or 1.95 might be ok.

scelia 06-24-08 02:37 PM

You did not mention which bike this was with. It might help to know the width of your rim. If it is to narrow or wide for the tire, it could cause unnatural tire wear.

tomdaniels 06-24-08 02:54 PM

It's the stratus long wheelbase recumbent. It is a sun mammoth rim (36h) which is a pretty wide rim for the 1.5in comets.

scelia 06-24-08 03:04 PM

I weigh 290 and for slicks I run the Continental Town & Country 2.1" @65 psi. I have not had a flat with them yet. They roll smooth and grip the road well. They do not grip off road very well at all. I do not ride a recumbent, but do ride my mountain bike "like it's stolen"...

Bill Kapaun 06-24-08 05:17 PM

Quote:

Originally Posted by tomdaniels (Post 6938111)
Will do! Time to go see the Mechanic again! I have noted that the cassette side spokes are much tighter than the other.



Meanwhile: Thanks for the schooling. That makes quite a bit of sense. I can see how a 1.75 or 1.95 might be ok.

IF your spokes were the same tension, you wouldn't have any dish in the wheel. Like the front.

I think you're over thinking tire pressure. Max + 5PSI!

Richard_Rides 06-24-08 06:29 PM

Yeah, I think the drive side is supposed to be tighter...

Pupsocket 06-24-08 07:37 PM

Tom,

I'm not sure what the PSI rating is at that size, but I had Maxxis Hookworms on the EZ I used to ride. 110 PSI, 1.95 x 406. I know they're made in other diameters and widths. Large, firm, and durable, the penalty is weight (over 400g for the one I was riding!)

Karl

tomdaniels 06-25-08 05:35 AM

2 Attachment(s)
Took the tire to my wheel builder mechanic. He pointed out that the wear was fairly even except for the circle where the wear was extreme. He was certain that something was rubbing.

We were stumped until I remembered I broke a rear seat support a couple hundred miles back. We decided that the loose support tube pushed on the fender and the fender mount rubbed the tire intermittently.
I was somewhat confused as this rubbing coincided with the edge of the anti-flat strips and there was some shading difference left to right.

jswilson64 06-25-08 03:15 PM

Quote:

Originally Posted by Bill Kapaun (Post 6939441)
IF your spokes were the same tension, you wouldn't have any dish in the wheel. Like the front.

Not so fast there. Dish is achieved, in part, by using shorter spokes on the drive side. It's been a while since I built a wheel (like 15 years or so), but seems like you don't want too much of a tension difference (that's why the drive side spokes are shorter). Otherwise, sproing, you've got a potato chip instead of a wheel.


And here's a question for everyone: I ride a hardtail, with slicks (1.5 inch, I think), primarily on the road. Would going to a fatter slick at higher pressure give me a large effective wheel diameter, and hence a bit more speed? Or just bog me down more due to greater tire squish?

superslomo 06-25-08 04:45 PM

Even so, you still (I think) need to have higher tension in the drive side to keep the rim in line at the much flatter angle of the spokes due to the dish of the wheel. At least I think that's so...


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