Tire Pressure Calculator

[benb]

So..

I took some of the data from Bicycle Quarterly here:

https://www.compasscycle.com/wp-content/uploads/2015/03/BQTireDrop.pdf

And made an Excel Spreadsheet that lets you input your weight, weight of bike, weight of gear, weight distribution, etc.. and then tells you the recommended pressures based on 15% tire drop for a range of tire sizes (20c,23c,25c,28c,32c,35c,37c)

Is anyone interested if I was to attach it here?

I realize Excel files can be loaded with viruses, etc.. so I'm not just throwing it up here.

I didn't do anything with figuring this out for panniers, etc.. their data doesn't include that. I could actually measure my own bike to figure that out and add it too, at least for rear panniers. (I don't have a front rack or luggage.) I could also pretty easily figure out how to add additional 700c sizes, but I have no idea how I'd figure it out for 650b or something like that.

In another life when I had lots of time I'd make this an iPhone/Android app.

It's an approximation, but it seems to work pretty well for me, although it seems like it's semi-important to get a scale and actually measure your weight distribution for real. (I've had a lot of issues figuring out the right pressures for bigger tires and this seems to work.)

[ergott]

Sure thing!

[Quilts]

Definitely sounds like it could be useful, I'd certainly check it out. I say go ahead and post it here!

[ergott]

I think uploading to Dropbox or similar with a link for us might be easiest.

[benb]

If someone is comfortable sharing off their dropbox/whatever go ahead, I'm just going to upload it here.

The thing with this is you need to edit it to use it, so I'm not sure what the best way to share it would be and I don't really want to open up sharing on my Google Drive.

I should probably re-implement it as a web page and share it that way.

[benb]

I gotta get the scale out to the garage. I don't actually know exactly what my bike weighs or the weight distribution but using this today I had almost the perfect pressure for a mixed pavement/gravel ride.. plenty of traction in the dirt but not feeling too slow on the pavement, and comfortable everywhere.

[bikinchris]

Not to minimize your work, but there is already an app called Berto tire pressure and it also takes into account the geometry of the bike as well as loading like panniers.

[eddief]

50 in the front, 90 in the back. I'm gonna try it and see if I live to tell the story.

what's a kg :).

[benb]

Thanks for finding that Android app. This was not a lot of work to make the spreadsheet. An App is a lot more work. I've done an Android app that was more complex than this but have no desire to do it again. If I was to take on something like that it'd be iPhone for sure.

Unfortunately no iPhone app and there is not much transparency in the app. I'd rather be able to manually set the front/rear weight distribution.

The Berto graphs are the same thing I used. They don't offer a direct model to add front and rear loads so I'm not sure what the app does.

If I was to do something to add loads to my spreadsheet I'd actually load up my panniers and try and get some data on how much a given amount of weight shifts the weight distribution.

[benb]

50 in the front, 90 in the back. I'm gonna try it and see if I live to tell the story.

what's a kg :).

Wow that's a pretty big differential. My bike is a porky 25-26lbs or so and I don't think I'd try 25c tires on it.

[benb]

Heh.. I actually measured weight distribution tonight.

I'll have to see what a cycling kit weighs. My street clothes add 10lbs to my weight, and my bike came in at a crazy 27lbs. (No wonder I'm craving another bike, my previous road bike was 17lbs.)

Weight distribution came out to 37% front, 63% rear using Zinn's method.. had one arm on the hoods and one trying to balance.

I've been underinflating my rear tire.

Next ride will be 40 psi front, 70psi rear. Pretty big increase in the rear. (Using 35s)

Edit: I'm actually concerned these huge spreads are too much and I'm going to fiddle with the front/rear weight distributions. I know if I stick 40psi in my front tire with these tires it will be way too squishy climbing out of the saddle. I think it's a fault in these guys model that they don't account for the change in weight distribution in different riding positions. Going to the drops might change it a little but climbing standing changes it a lot.