# Tire pressure causes bounce



## mrpercussive (Apr 4, 2006)

I've just found out the reason for my bike bouncing when i lift the rear wheel up and drop it... It's cause i'm running too low a tire pressure. I run a 22.5psi. Which got me thinking... Tire absorbs the impact but has no damping and thus the bouncing. Of course it could also be the axle path but when i pumped it up, it didnt bounce anymore just some pedal feedback from the tight chain... Also, when i drop both wheels at the same time it doesnt bounce. Hmmmmmm... Anyone facing this problem??? What are the pros and cons of running a low pressure like that. Anyone care to share some experiences?


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## .Danno. (Jun 18, 2005)

mrpercussive said:


> I've just found out the reason for my bike bouncing when i lift the rear wheel up and drop it... It's cause i'm running too low a tire pressure. I run a 22.5psi. Which got me thinking... Tire absorbs the impact but has no damping and thus the bouncing. Of course it could also be the axle path but when i pumped it up, it didnt bounce anymore just some pedal feedback from the tight chain... Also, when i drop both wheels at the same time it doesnt bounce. Hmmmmmm... Anyone facing this problem??? What are the pros and cons of running a low pressure like that. Anyone care to share some experiences?


My bikes didn't used to bounce before platform shocks came along. I figure it's not particularily relevent though without a rider in the equation.

FWIW, I like as low of a tire pressure that I can run without pinch flats, dented rims, or sloppy handling.


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## XSL_WiLL (Nov 2, 2004)

Low tire pressure allows the tire to deform a bit, tracks the ground a bit better. But if you have thin sidewalls, they can fold over. It CAN be detrimental to handling. And if you aren't running tubeless, you risk pinchflats. Always run enough PSI to keep from flatspotting.


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## dogonfr (Jan 6, 2005)

You should be at about 24-25psi with a DH tube. :thumbsup:


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## mrpercussive (Apr 4, 2006)

i'm running Kenda heavy duty tubes 26X2.35-2.75 1.2mm wall... I dont think there's a flat spot as the tube imo seems to really push the tyre out and also i'm featherweight... Also, running at 22.5psi. Just pumped it up to 25psi though. There's still the bounce although it's not as noticeble...


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## SHIVER ME TIMBERS (Jan 12, 2004)

your analogy is wrong

take a basketball for instance......if you add more air it bounces more......take away air it won't bounce as good.....ponder that for a few minutes.....


Here is your shirt


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## mrpercussive (Apr 4, 2006)




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## SHIVER ME TIMBERS (Jan 12, 2004)

mrpercussive said:


>


you need to drink a whole lot when you turn 21....and start chasing girls too


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## XSL_WiLL (Nov 2, 2004)

Flatspotting doesn't have to do with the tube. Flatspotting is when your tire bottoms and the rim smacks the ground. The rim dents, and you get a flat spot.


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## Master_Jako (Mar 27, 2004)

SHIVER ME TIMBERS said:


> you need to drink a whole lot when you turn 21....and start chasing girls too


yesssirrrrr

mrpercussive-
i dont see whats the problem with ur bike. more air pressure = more bounce. ur thinking is wackyness.


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## Banshee Rider (Jan 19, 2004)

mrpercussive said:


> Tire absorbs the impact but has no damping and thus the bouncing. Of course it could also be the axle path


What does your axle path have to do with anything?


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## ScottJensen (Nov 24, 2006)

im soooooo glad im an engineer, and your not.


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## mrpercussive (Apr 4, 2006)

my wheelpath is different from most FS long travel bikes. It's a curve starting from the inside of the bike to upwards unlike most which cresnt from upward to inward the frame. So i think i got a falling rate suspesion. Initial hits need to be spikier for it to absorb. And thus letting the force go straight into the shocks and the oil dampening the spring and not the force going into the tyres and being dampened by the floor.

And scott... if you've got any other explanation... i would love to hear them... this is an open discussion and i would really want to get my bike at it's best... And no, i'm no engineer... Just a kid who thinks too much...

SMT - Yeap, i want beer and girls!!!
Will - I dont think i've ever flat spotted before, except on one i cased really bad at 20psi in the tyres... never again. Now running 25psi
MasterJako - Watch the video... i think it's wacky too... lol


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## 10Clams (Jul 31, 2006)

*Tire pressure..*



ScottJensen said:


> im soooooo glad im an engineer, and your not.


I am so jealous, I always wanted to drive a train.


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## keen (Jan 13, 2004)

{QUOTE} my wheelpath is different from most FS long travel bikes. It's a curve starting from the inside of the bike to upwards unlike most which cresnt from upward to inward the frame. So i think i got a falling rate suspesion. Initial hits need to be spikier for it to absorb. And thus letting the force go straight into the shocks and the oil dampening the spring and not the force going into the tyres and being dampened by the floor

For discussion sake i'd contact Ibex to see if they can shed some light on the suspension rate - or there must be someone that can plot / calculate your wheelpath. My Heckler has a falling rate rear - it pedals well for a single pivot but suffers from bottoming much as you have described w/ your Ibex. In order to run a coil on my Heckler I have to run a heavier spring to limit bottoming even with a DHX coil w/ B.O. control. I think an air shock suits my Heckler better - the air is more progressive than the linear coil. If I was dead set on a coil for my Heckler i'd have Push custom tune the DHX.

As far as suspension & tire pressures & the bouce testing, hmm a little vague.


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## Zonk0u (Jun 3, 2004)

I dont see the problem. of course a bike is going to bounce when you drop the rear end like that... unless your suspension is setup for someone whoweighs 120lbs....this thread is retarded


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## Freerydejunky (Sep 21, 2006)

Yeah Poh you put more pressure in the tires its gonna bounce more.... Like a basketball.... come on now. And zonk0u poh is 120 lbs  Im 105 and my bike doesnt even as much giggle when i drop it on the rear tire thank you very much. Poh your bike is gonna bounce get over it lol.


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## FloridaFish (Mar 29, 2004)

mrpercussive said:


>


Well, I am an engineer and your argument sounds pretty good. There's definately some interactions going on with the suspension at the higher pressure. does your shock compress more or less at higher pressure?? or does it have lockout?

*****The reason a basketball bounces more with more air pressure is that there is less energy dissapation due to deformation of the basketball material, the force of the ball hitting the ground is directed almost entirely back to the ball. With lower air pressure in the ball the "skin" deforms, the ball contacts the ground with a larger footprint, and the air inside the ball is "squished" more.

 whoa, i think i need a beer after that nerd attack.

edit: look at the frame vs. wheels at high and low.


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## TheProphet (May 31, 2005)

For once I actually agree with one of SMT's statement.

Ive got a good idea, why wont you just got out and ride the thing.


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## SHIVER ME TIMBERS (Jan 12, 2004)

TheProphet said:


> For once I actually agree with one of SMT's statement.
> 
> Ive got a good idea, why wont you just got out and ride the thing.


RIDE,... how they ride..................


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## dwnhlldav (Feb 2, 2006)

Yeah dude, you're picking pepper out of fly ****. Go ride your bike and stop thinking.


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## mrpercussive (Apr 4, 2006)

i sooooo wish i can ride right now.... but i'm in school from 9am to about 9pm... so i cant on the weekdays... gotta feed my brain some bike too on those 30 minute breaks... hahaha


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## Tim F. (May 22, 2006)

mrpercussive said:


> i sooooo wish i can ride right now.... but i'm in school from 9am to about 9pm... so i cant on the weekdays... gotta feed my brain some bike too on those 30 minute breaks... hahaha


So quit feeding your brain "bike dookie", and feed it more "book dookie".

> brainwork during the weekdays = < brainwork on weekends, thus > dooking around on (rather than thinking about) the bike all weekend long. :idea:


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## Pableras (Aug 8, 2006)

It's very simple

More pressure -> More bounce -> Less tire absorption -> Less comfort -> Less risk of pinch-flat

Less pressure -> Less bounce -> More tire absorption -> More comfort -> More risk of pinch-flat


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## .Danno. (Jun 18, 2005)

mrpercussive said:


> my wheelpath is different from most FS long travel bikes. It's a curve starting from the inside of the bike to upwards unlike most which cresnt from upward to inward the frame. So i think i got a falling rate suspesion.


You can't look just at wheelpath and tell anything about suspension rate.


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## austinb89 (Nov 6, 2004)

how abotu you pump ur tires up and go ride, who cares if u bounce or not. what are u gonan do ride without tires?

and i just rode in 8 degree weather, plus snow so yeah


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