Low tyre pressures?

[kiwi cowboy]

can to low tire pressures in a front tire cause the suspension to shake under hard braking from speed??.
I have just put a set of ah12 dunlops on my race bike and ran 27r and 27f hot last weekend at ruapuna for the first time and noticed the front end shake\pogo a little under brakes at the end of the straight under brakes.
I ran the front at 25 yesterday on the b track to see if i could get more grip and thinking about it now i was getting a little shake into the hairpin.
Just wondering if the pressures are to low and deforming the tire badly under brakeing causing the chatter.
I diddnt notice anything like that with the power ones i had on before at 27 hot front.

[tigertim20]

can to low tire pressures in a front tire cause the suspension to shake under hard braking from speed??.
I have just put a set of ah12 dunlops on my race bike and ran 27r and 27f hot last weekend at ruapuna for the first time and noticed the front end shake\pogo a little under brakes at the end of the straight under brakes.
I ran the front at 25 yesterday on the b track to see if i could get more grip and thinking about it now i was getting a little shake into the hairpin.
Just wondering if the pressures are to low and deforming the tire badly under brakeing causing the chatter.
I diddnt notice anything like that with the power ones i had on before at 27 hot front.

I would expect that tyre pressures being too low would make the front feel soggy, and heavy and not provide a great deal of feedback, rather than a heap of shake a whole lot.

I may stand corrected though, Im no suspension god by any means, but 25 does sound a bit low to me.

[kiwi cowboy]

I would expect that tyre pressures being too low would make the front feel soggy, and heavy and not provide a great deal of feedback, rather than a heap of shake a whole lot.

I may stand corrected though, Im no suspension god by any means, but 25 does sound a bit low to me.

The front felt good cornering and on power but was only under hard brakeing but yer i think i might have dropped them to low.

[sinfull]

The front felt good cornering and on power but was only under hard brakeing but yer i think i might have dropped them to low.

Go and ask again in the racing section !

Most who would have the knowledge of race pressures for the tyres you have, wouldn't even wander into any other section of the forum more than once a year !

[p.dath]

I don't know the answer, but I'm thinking next time you are on the track try an experiment, and bump the pressure by 2 or 3 psi, change nothing else, and see what the difference is.

[kiwi cowboy]

I don't know the answer, but I'm thinking next time you are on the track try an experiment, and bump the pressure by 2 or 3 psi, change nothing else, and see what the difference is.

Going to but not racing till burt so may have to test elswere

[kiwi cowboy]

Go and ask again in the racing section !

Most who would have the knowledge of race pressures for the tyres you have, wouldn't even wander into any other section of the forum more than once a year !

Yer wondered that so will do cheers.

[actungbaby]

can to low tire pressures in a front tire cause the suspension to shake under hard braking from speed??.
I have just put a set of ah12 dunlops on my race bike and ran 27r and 27f hot last weekend at ruapuna for the first time and noticed the front end shake\pogo a little under brakes at the end of the straight under brakes.
I ran the front at 25 yesterday on the b track to see if i could get more grip and thinking about it now i was getting a little shake into the hairpin.
Just wondering if the pressures are to low and deforming the tire badly under brakeing causing the chatter.
I diddnt notice anything like that with the power ones i had on before at 27 hot front.

Am not sure with road tires but dont slicks with heat or very hot day cause tire pressure to increase slighty

or is this alot of rubbish , i whould thought to high pressure be more likey to cause this , lower presure might make it

Move or squim around but might give little more grip too

[mossy1200]

You did the front axle nuts up while front on a stand.
Slack nut bounce front end a few times and do up while wheel on the ground.
27-29 hot isnt to bad for most race tires.

[kiwi cowboy]

You did the front axle nuts up while front on a stand.
Slack nut bounce front end a few times and do up while wheel on the ground.
27-29 hot isnt to bad for most race tires.

No i did bounce on the front before tightening the fork nut or do you mean the axel nut?.

The front had about 25lb so may been to low.

[mossy1200]

No i did bounce on the front before tightening the fork nut or do you mean the axel nut?.

The front had about 25lb so may been to low.

axel nut.
Often the bottom of the forks are at different heights (one side is longer under the axel).
When the bike is on front fork stands and you tighten the axel the alignment is wrong and the suspension doesnt work well.
In extreme cases it can make your pads sit on disc on a angle and cause vibrations due to them flexing when braking hard.
Its just a guess as I have done it before when I first started racing.

Depending on the track temp 25 isnt to bad.
I used to run 25 cold front and 23 cold rear looking for 27 hot on the warmers at 80deg on a average winter series fine day.(slicks)

Some tires respond to different pressures so finding someone with the same brand will help you alot if they have a bike same weight and power or close to.

[kiwi cowboy]

axel nut.
Often the bottom of the forks are at different heights (one side is longer under the axel).
When the bike is on front fork stands and you tighten the axel the alignment is wrong and the suspension doesnt work well.
Its just a guess as I have done it before when I first started racing.

Depending on the track temp 25 isnt to bad.
I used to run 25 cold front and 23 cold rear looking for 27 hot on the warmers at 80deg on a average winter series fine day.(slicks)

Some tires respond to different pressures so finding someone with the same brand will help you alot if they have a bike same weight and power or close to.

Ok thanks moss i will redo the front nut as i thought it only mattered with the fork nut lol we live and learn.
The problem only started after i put the new tires on - there a12h's and there quite stiff on the caseing compared to the power ones i had on.
I had 25 hot so about 22 cold to low?.
On another topic the bike seems to be vauge mid corner and drift wide on exit on the track and wondered if i raised the rear shock hieght weather it would help thoughts?.

[sinfull]

Ok thanks moss i will redo the front nut as i thought it only mattered with the fork nut lol we live and learn.
The problem only started after i put the new tires on - there a12h's and there quite stiff on the caseing compared to the power ones i had on.
I had 25 hot so about 22 cold to low?.
On another topic the bike seems to be vauge mid corner and drift wide on exit on the track and wondered if i raised the rear shock hieght weather it would help thoughts?.

Don't know about raising the rear shock, as that'll prob just have it tipping in faster, or are you just talking preload ?

Running wide on exit screams to me that the front is rearing when you get on the gas increasing the wheel base !
Be a fine line between having the rebound in front too fast so it leaps out of corners and so slow you have no tire contact when getting on the gas and losing the front (no one likes that feeling)

Got to ask what suspension ya bike's got in it ?

I guess the rear squatting too quick could create running wide too ! Hows ya set up, your not too heavy for the rear spring rate are ya ?

Robert Taylor be the best to talk to but i'll post this link anyway as it really helped get my head around my suspension !
http://www.gostar-racing.com/information/motorcycle_suspension_set-up.htm

[kiwi cowboy]

Don't know about raising the rear shock, as that'll prob just have it tipping in faster, or are you just talking preload ?

Running wide on exit screams to me that the front is rearing when you get on the gas increasing the wheel base !
Be a fine line between having the rebound in front too fast so it leaps out of corners and so slow you have no tire contact when getting on the gas and losing the front (no one likes that feeling)

Got to ask what suspension ya bike's got in it ?

I guess the rear squatting too quick could create running wide too ! Hows ya set up, your not too heavy for the rear spring rate are ya ?

Robert Taylor be the best to talk to but i'll post this link anyway as it really helped get my head around my suspension !
http://www.gostar-racing.com/information/motorcycle_suspension_set-up.htm

LOL its stock front and a 06r6 yamaha rear shock on a gsxr400 1987 year.
Probably are to heavy but not realy into spending lots if i can help and yes suspencion can get expensive lol.
I had a stock rear shock off an impulse in it when the original shit itself and had it wound to the max on the preload which seemed to work well and made the front feel quite good but it shit itself to as was old anyway so thats why i put the r6 shock in.
All them are the same length eye to eye.

[sinfull]

LOL its stock front and a 06r6 yamaha rear shock on a gsxr400 1987 year.
Probably are to heavy but not realy into spending lots if i can help and yes suspencion can get expensive lol.
I had a stock rear shock off an impulse in it when the original shit itself and had it wound to the max on the preload which seemed to work well and made the front feel quite good but it shit itself to as was old anyway so thats why i put the r6 shock in.
All them are the same length eye to eye.
Use that site to guage where yr at, but springs vary in density (for a better word) wish robert would step in now lol (like closer coils give stronger spring rate and wider spaced gives lighter) could be only a matter of a fraction of mm changes 20 kg weight capabilities !

If you use that site to get the right sags ya need and find you're having to crank the preload to close to its max it could be a new spring for the R6 shock and only a matter of a hundy or so through RT !
The front well hehe shiiite, does them Gixer 400's have clickers top and bottom ie compression and rebound ? if so ya might get lucky with settin them up, if not, ahem revalving perhaps or maybe some racetech emulators might be the shot at a couple of hundy if you aint got the clickers to control that side !

May the force be with you !!!

[Biggles08]

can to low tire pressures in a front tire cause the suspension to shake under hard braking from speed??.
I have just put a set of ah12 dunlops on my race bike and ran 27r and 27f hot last weekend at ruapuna for the first time and noticed the front end shake\pogo a little under brakes at the end of the straight under brakes.
I ran the front at 25 yesterday on the b track to see if i could get more grip and thinking about it now i was getting a little shake into the hairpin.
Just wondering if the pressures are to low and deforming the tire badly under brakeing causing the chatter.
I diddnt notice anything like that with the power ones i had on before at 27 hot front.

Shaking under braking is usually caused by your front discs being warped...I have had very few race bikes in which the brakes don't shake a bit from hard braking...if its not too bad just ignore it. Also, 'pogo' or a heavy 'shake' under very heavy braking is probably you bottoming out your suspension on the rubber...when you don't have enough preload you will use all your fork travel and bottom out making your front tire start to give instead of your front suspension...this can feel somewhat un-ideal. Does it reduce shaking when you let the brakes off a bit when its doing it? If so this is the culprit.

On Dunlop 211GP's I was running rear 20Psi and front 28Psi hot off the track. 27 both front and rear isn't too low I wouldn't imagine...but I don't know what ah12 Dunlops are so couldn't comment.

[kiwi cowboy]

Shaking under braking is usually caused by your front discs being warped...I have had very few race bikes in which the brakes don't shake a bit from hard braking...if its not too bad just ignore it. Also, 'pogo' or a heavy 'shake' under very heavy braking is probably you bottoming out your suspension on the rubber...when you don't have enough preload you will use all your fork travel and bottom out making your front tire start to give instead of your front suspension...this can feel somewhat un-ideal. Does it reduce shaking when you let the brakes off a bit when its doing it? If so this is the culprit.

On Dunlop 211GP's I was running rear 20Psi and front 28Psi hot off the track. 27 both front and rear isn't too low I wouldn't imagine...but I don't know what ah12 Dunlops are so couldn't comment.

Hey Biggles, thanks for the reply,

no its not the brakes i dont think as i have had that befor and it comes through the lever.

This seems to come on after i have been on the brakes for a second or so and i didnt notice this with the power one i had on before.

I only had 25-hot off the track and with the a12h being a harder carcase tire i wonder if that was too low and made the tire deform under stress from braking and crinkeled on the sides causing shudder.


Its hard to explain but it isnt there "bang" when i hit the brakes but sort of comes on after being on them for maybe a second and then it sort of comes in from just noticeable to a shudder and come right after i let the brakes go so maybe suspension but then why did i not get this problem with other tires?.

[Ocean1]

Just a thought, check your steering head bearings, a bit of play there can cause this.

If that's OK it might be a disk or pad issue, and they can be hard to nail down. Big thing is to change just one thing at a time eh? Take the disks off and lightly sand them all over. Clean 'em with brakeclean and test ride. If there's any change you're in the right area, if it's different but still not OK then clean the disks again and try different pads.

Luck mate.