mikecart1

Hi. I have noticed that on my new Trek ALR4 (already had one flat due to under pressure) that the back tire seems to flatten slightly when I'm riding the bike. Is this normal? I know it should flatten some from my bodyweight (~170-180lbs with hydration pack), but it seems like I'm always looking to see that the back tire is a little more flat to the road that the front tire. I always check my tire pressure before riding now and it's exactly 50 psi like it says on the tire. But do all tires do this? I'm watching the 2020 Tour De France and it looks like their tires have no give at all to the road. It just feels like my back tire makes it that much harder to pick up speed.

Advice? Is this something normal for gravel tires?

datlas 

So-called experts say the tire should drop 15% loaded vs unloaded. There are too many variables such as tire width and road conditions and rider weight to give the “best” or “ideal” tire pressure without it getting complicated.

mikecart1

Ok thanks! I know if you over-pressure the tire, you risk blowouts and bad accidents. But it sucks looking down while riding and seeing the back tire smushed to the ground. It might just be the tires are too wide for the listed max pressure. They also might be better for gravel. I've put over 200 miles already on roads. Feels like I'm riding in quick sand sometimes though.

79pmooney

Yes, what you are seeing is straight physics and "right" in God's (and Newton's) world. On almost all bikes, the rider puts 55-65% of the weight on the rear tire. That's 25-80% more weight on the rear. For that reason, most of us put more air in the rear tire. How much is a bit controversial Some vary by weight percentage on each wheel or simply by visual inspection to have equal sidewall flex. Others keep front and rear closer for more reserve in front and not quite as bouncy standing and leaning forward climbing. (I keep in mind that the best pressure depends on the tire size and to a lesser degree how supple the tire casing is. I run 90 psi on tires that are 28c and have very supple casings. All my riding is on pavement. I don't go over 60 psi on 38c tires on pavement, a lot lower off road.)

Did you buy this bike at a shop? If so, ask a mechanic there if running say 60 psi in back is safe with those rims. (40c tires are pretty big. Big tires put much more "spreading force" on the rim sidewalls than skinny tires. Blowing the sidewall off is something you do not want to do.) If you didn't buy the bike at a shop, consider contacting Trek or Bontranger.

There's nothing magic about the psi labeled on the tire. It is a max based on tire construction, rim considerations and our lawsuit happy society. That a quality Bontranger tire cannot handle more than 50 psi would amaze me. Do a little homework and see if rims are up to say 60 psi. If so, try 60 in the rear and 45-50 in front, Ride it and see if you like it. Tweak those pressures until the ride is "Ahhh!"

Ben

big john

What a 130 pound TDF rider on sew-ups does has nothing to do with a 180 pound recreational rider on gravel tires.

If you're riding road only you could try a set of road tires. Maybe even switch when you plan to go dirt.

I run 100 psi in my road tires (25mm) and on my mtb only 15psi front and 20 psi rear. You could probably go a little over the max on the tire but not too much.

Iride01 

Ride a particular pressure for a half dozen rides. Then ride another pressure for another half dozen of preferably the same routes. Rinse and repeat until you know what works for you.

Any data you can collect to help you compare as well as just noting what your perceptions were by writing them down for each will help you.

Smooth roads you can run higher pressures on and rough roads you'll want lower pressures.

I ride very smooth roads and paved trails. I only weigh 170 lbs. I'm very happy with the rear at 120 and the front at 100 psi in 25 mm wide tires. Others tell me I'm wrong, but they don't ride my roads. I lowered them to 90 psi a few months ago for a dozen rides. I was slower and more tired after every ride.

MntnMan62

The back tire "flattens" slightly because most of your weight is placed on the back tire. I usually fill my rear tire to about 102 lbs while my front tire is at around 98. It works for me. And I'm a fat tub of lard. 5'9", 215 lbs.

mikecart1

I bought it at a Trek dealership. I will ask them next time I bring it in for maintenance. I might try putting slightly over 50 psi on the back tire the next few times I ride.
Yeah I might ask the shop how much it would be to get road tires.

Yeah on hills, it feels like quick sand. I rarely go above 30 mph on downhills and it's probably because of the back tire.

That makes sense. You are probably a bodybuilder.

Iride01 

I had to look to see what an ALR 4 is. I assumed it was a road bike. Appears to be more of a gravel bike and guessing 35 maybe 40 mm tires. So you don't want to inflate those tires anywhere near what I do. There is a max tire pressure on the sidewall somewhere so don't exceed that.

surak

If you want to go really deep on tire pressure and its effect on optimal speed, you can start with with the Silca Pressure Calculator. Josh from Silca made an interesting video about how the numbers feeding the calculations are from actual race performance data.

jbucky1

you dont say what tires you are using or riding style, so I assumed what it came with - 40c. Given your weight I would use 70psi if using an inner tube tube.

james
www.buckyrides.com

mikecart1

Yeah it's 50 psi LOL!

Thanks! I will look at this!

Yeah I'm using an inner tube. I will try 55psi next time. While 50psi is what is written on the tire, I don't see how that makes sense to be basically riding on a tire that is like 20% flat.

Iride01 

Tire pressure gauges are notoriously inaccurate. They are consistent, so even if it's not the correct number you'll still be able to figure out what you want to inflate it too. It's just that with another gauge it might be a different number. So unless you've checked one gauge against another, then use the same gauge each time you fill them up.

mikecart1

Makes sense. All I know is that I'm nowhere near the max weight for the bike frame itself and I just don't see how or why anyone would ride a gravel bike if the back tire flattens to the ground. I had a mountain bike in college I used to get to and from campus. I rode mostly on roads. Never saw the rear tire flatten like this. I also just pumped until the tire was hard like a rock too though LOL! Never had a flat hah!

79pmooney

Another thought - the tires. I know nothing about the those tires. But tires that are decent but slow riding have been sold on bikes forever. My full-on racing bike had very mediocre tires. It may be that just replacing the tires you've got with tires with more supple sidewalls and casing can make a large difference. You might try the 38c Panasonic Paselas which I find a surprising lively tire for the size and a tire that feels fairly fast to low pressures. (The Paselas are not a gravel tire but have served me quite well off pavement. I rode a 38c in front and 35c in back on a rather epic day with about a dozen other forum members 3 years ago. 60 miles on pavement, 30 on hardcore logging roads. 50-60 psi on the pavement, 30 or less on the gravel. I was one of the very few to have no flats.)

One word of warning on the Paselas. Don't let the sidewalls scrape brake shoes, chainstays, curbs, etc. Those sidewalls are strong, but not designed to be abrasion resistant. The tires are very good all-arounders. Not superb at anything, but not bad at anything either.

Ben

jbucky1

5psi extra is not gonna make much difference, but like what someone else said dont go over the max printed on the side wall for safety reasons
If you are riding mainly road, just buy some road tires. Thats what I would do. The thickness, quality and your weight will determine how much pressure you should use.

for example, I am 165lb, using a 30mm tire rated to max 100psi max. Mainly on road I use 70psi but could lower for gravel if I liked.

james
www.buckyrides.com

jbucky1

+1 on the Pasela's

James
www.buckyrides.com

MntnMan62

Hardly. I'm just as I described myself. A fat tub of lard. My gut hangs over my belt. The image staring back at me in the mirror is not one that I am proud of. I need to stop eating and ride more. Period. And frankly, bodybuilding never appealed to me.

mikecart1

Ok hold up! I just watched a YouTube video of how to pump up bike tires with a presto valve. So you have to take off the cap AND unscrew something on the presto valve for it to pump? I'm either the dumbest biker of all time or I am not understanding it correctly?

EDIT: Oh man does this explain everything. So since I got my new bike pump, every time I use it to pump air before rides, it always read 50psi. Well that's how far I let the gauge read before stopped. However, I never had to plunge all the way down. I just assumed I had enough air. What was happening was that there was no where for air to go because I didn't know about the 2nd screw to release before air went INSIDE the tire. I just pumped air this time with the presto valve open. I got to around 35 psi before the tire is solid like steel LOL. Oh man. I am in the Wall of Shame here. I will see how this goes. The tire by feel is at a legit 35 psi is way more solid than it was at 50 psi when I didn't know how to inflate tires. What's sadder is I am on a 2nd Master's degree and I have a Professional Engineering license LOL!

mikecart1

Well this explains everything. Just did both tires. I got them both to 35-40 psi. Didn't want to go near the 50 psi on the tire. But at least I now know what was wrong. The ride should be way better next time I go out. Just put my weight on the bike and nearly no flex on the back tire like it was before. Basically when I started pumping it was at around 20 psi (if the gauge was correct). Had to pump several times just to get to the 30s.

rubiksoval

That was an excellent conclusion to this thread.

Glad you got it sorted out.

drewtk

Good, glad you figured it out. To recap, the numbers on the side of the tire provide a very general range that frankly I disregard. And definitely don't just inflate to the max number.

Use that Silca tire pressure calculator referenced above. I have found that to be pretty accurate. And when it comes to speed from a tire, the big influences are rolling resistance and aerodynamic drag.

When it comes to rolling resistance, you're better off having too little pressure than too much. There's a huge increase in resistance if the pressure is too high. And BONUS: lower tire pressures are also more comfortable.

Bah Humbug

We all go through things like that. Glad you asked, and glad it’s sorted.

gsa103

Bravo! Glad you got it sorted. One quick tip, I always manually burp the valve before I put the pump out, that way if the valve is a little sticky it opens easier.

mikecart1

Hopefully I'm not the only one that had this happen on here (in before crickets LOL).

Yeah I was watching that video when I just randomly typed into YouTube how to inflate tires. Most of the comments on that video were saying what dummy doesn't know how to do this. This Guy!

Yeah I would have kept getting flats for months until I figured it out LOL!

Yeah that makes sense.