hi all
just fitted some new rims tonite,(just got in)whats the best tyre pressures??they are 225/40/18..on a s14a..they were up 40,so im now running at 32 all round,not sure if thats too much??any ideas would be great,cheers lads..lee
hi all
just fitted some new rims tonite,(just got in)whats the best tyre pressures??they are 225/40/18..on a s14a..they were up 40,so im now running at 32 all round,not sure if thats too much??any ideas would be great,cheers lads..lee
32 is what i run at unless fully laden, then 34
Would it be the same for a 265/35/18 rears and 235/40/18 fronts?
Gona put my new alloys on at the weekend
Adam
thanks jon,ill leave it at that then,appreciate your quick reply bud!!
Iīm using 42,2PSI front and 40.5PSI back. Nice and responsive ride that keeps rims safe from potholes. A little slippier on wet but I donīt mind. Showy driving beats fast lap times
Last edited by Piplis; 23-02-2006 at 06:24.
The tyre pressures are determined by the weight of the car not the size of tyre so you should be running at 32psihaving said that I usually set mine to 34
Not quite Juz, This is a question thats bugged me ever since fitting fat 16's on my 205 gti so i just worked it out... First Imagine the tyre compresses under weight by a percentge of its wall height and the rims are correct for the width...
For wider rims + tyres (same wall size = same rim diameter) you need lower pressure to get the same effective stiffness (tyre 'squish) hence comfort. This is because the pressure is acting over a larger area - wider rim thus more force.
But for lower profile tyres you want higher pressure to get the same %compression under the cars weight. Possibly more to stop rims getting dented on potholes.
Both are linear relationships (well the wall depth isn't but can be sensibly linearised for small changes) hence for matched tyre+rim sizes (and equal rolling radius) a good approximation would be:
P= tyre pressure
RW= Rim width (inside rim eg. 8 inch) can be subbed for tyre width if you dont have streched/fat look but not ideal
TW= Tyre width (on tyre side eg. 225)
TD= tyre depth (on tyre side eg. 50)
1= original manufacturers spec
2= your new shiny wheels
P2 = P1*(RW1/RW2) -to account for wider rims
P2 = P1*(TW1*TD1)/(TW2*TD2) -to account for smaller tyre wall
Combine these and you get: P2 = P1*(RW1/RW2)*(TW1*TD1)/(TW2*TD2)
Simplified:
P2 = (P1*RW1*TW1*TD1)/(RW2*TD2*TW2)
This works for mixed units as they are fractions and the reason the tyre depth is found from width multiplied by depth is that the depth rating on tyres is actually a percentage of width. A worked example:
S14:
P1 = 32psi, RW = 6.5 inch, TW = 205, TD = 55.
New wheels:
RW = 7.5 inch, TW = 225, TD = 40
P2 = (32*6.5*205*55)/(7.5*40*225) = 37 psi
Only a quick formula i worked out but serves as a pointer. Realistically compressed volume ratios need to be matched but the results wont be far wrong
Last edited by ReTeP; 23-02-2006 at 17:35.
Erm.....................................so what would i need for my tires mentioned above then?Originally Posted by ReTeP
Not quite Juz, This is a question thats bugged me ever since fitting fat 16's on my 205 gti so i just worked it out... First Imagine the tyre compresses under weight by a percentge of its wall height and the rims are correct for the width...
For wider rims + tyres (same wall size = same rim diameter) you need lower pressure to get the same effective stiffness (tyre 'squish) hence comfort. This is because the pressure is acting over a larger area - wider rim thus more force.
But for lower profile tyres you want higher pressure to get the same %compression under the cars weight. Possibly more to stop rims getting dented on potholes.
Both are linear relationships (well the wall depth isn't but can be sensibly linearised for small changes) hence for matched tyre+rim sizes (and equal rolling radius) a good approximation would be:
P= tyre pressure
RW= Rim width (inside rim eg. 8 inch)
TW= Tyre width (on tyre side eg. 225)
TD= tyre depth (on tyre side eg. 50)
1= original manufacturers spec
2= your new shiny wheels
P2 = P1*(RW1/RW2) -to account for wider rims
P2 = P1*(TW1*TD1)/(TW2*TD2) -to account for smaller tyre wall
Combine these and you get: P2 = P1*(RW1/RW2)*(TW1*TD1)/(TW2*TD2)
Simplified:
P2 = (P1*RW1*TW1*TD1)/(RW2*TD2*TW2)
This works for mixed units as they are fractions and the reason the tyre depth is found from width multiplied by depth is that the depth rating on tyres is actually a percentage of width. A worked example:
S14:
P1 = 32psi, RW = 6.5 inch, TW = 205, TD = 55.
New wheels:
RW = 7.5 inch, TW = 225, TD = 40
P2 = (32*6.5*205*55)/(7.5*40*225) = 37 psi
Only a quick formula i worked out but serves as a pointer. Realistically compressed volume ratios need to be matched but the results wont be far wrong
I could work it out, but i've just had 2 harsh engineering exams at uni today
Bit late for exams? All mine were early january, but hope you done well.
Well the rim width could be subbed for tyre width if things are sensibly matched but best done with rim size:
265/35/18 rears:
P2 = (32*205*205*55)/(265*35*265) = 30.1 psi
235/40/18 front:
P2 = (32*205*205*55)/(235*40*235) = 33.5 psi
I gave you results to 1dp since your an engineering student like me
Just to confirm 28 psi on 300zx rims is ideal, grippy and comforts the same. 30/32 psi was just a crashy ride and gave much wheel spinnage (an ITR got a sideways surprise!)
But if I run my Z rims that low it feels wollowy and tramlines more.
I run them 32-34, control wheel spin with throttle and ride is great
Paul
I have 7.5 inch Z rims all round. Interesting comment though.....
If i run 32 psi I get less grip than original wheels, every bit of interior trim rattles and its a crashy ride! Must say although I had piss all grip it was well stable when powersliding
Maybe your used to a hard ride, Ah and Bristol has shite roads BTW???!
Hell I could have put my original rims at 36psi if i wanted less wallow
Last edited by ReTeP; 26-02-2006 at 18:11.
So what about uneven wear issues? Have they been ok?
lol this has got abit involved i think ill stick to 32 and see how they wear.
They sit just like the 205's before, can't see it causing uneven wear but only been on a day!
Apparently the Z guys think 27 ish Psi is best and there cars are heavier!, but i suspect its because they heat the tyres more which raises pressure.
Sorry for the late reply, so many threads to keep track on,lolBit late for exams? All mine were early january, but hope you done well.
Well the rim width could be subbed for tyre width if things are sensibly matched but best done with rim size:
265/35/18 rears:
P2 = (32*205*205*55)/(265*35*265) = 30.1 psi
235/40/18 front:
P2 = (32*205*205*55)/(235*40*235) = 33.5 psi
I gave you results to 1dp since your an engineering student like me
Im at Plymouth uni in my 1st year doing a Bsc in Marine and Composite Technology
I think i'll stick 32psi in and see what there like, thanks for working that out though
Adam
NP
I ran 40psi in the rears and the center of the tyre balded out first.
Now I run 35psi all round, works good!
Time to awaken this thread
I'm also running 225 40 R 18.
I checked my tyres last night and they're worn in the centre considerably more than the outer edge : Over Infalted.
BUT, I've been running at 29PSI.
So, what the hell should my pressure be?
Confused.
Shoot me down in flames if im wrong here, but ive worked that out at P2=2345200/67500= 34.7 psi not 37 psi, what am i doing wrong?
ive calculated mine which equals P2=(32x6.5x205x55)/(9x40x235)=28psi does this sound right?
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