What size tires and why?

[Grunion]

OK I was wondering what size tires are best and why. I have a old class 8 truck that I want to run in 8s and was told to run a 37- 17. so thats what I have. I know they give my axle better ground clearance. But what are the other pros and cons?

[sidekicker]

yok is best tires

[Grunion]

Not what brand what size.

[Brutpwr]

Your truck will always be quicker with 33's or even 35's as you lower the truck it will be faster in the turns. Also the lighter weight accelerates better and gives a better gear ratio. On our short course truck a 2" lower tire is good for 2 seconds a lap courtesy of better acceleration, braking and cornering which is all you do in short track. For desert you will want to run 37's as that is what many are running and the ruts get deep from others running similar sized tires. You need center diff/engine tranny clearance as rocks will not be cleared in the center of the road and you don't want to hit them.

Jason :)

[SimonSays]

Your truck will always be quicker with 33's or even 35's as you lower the truck it will be faster in the turns. Also the lighter weight accelerates better and gives a better gear ratio. On our short course truck a 2" lower tire is good for 2 seconds a lap courtesy of better acceleration, braking and cornering which is all you do in short track. For desert you will want to run 37's as that is what many are running and the ruts get deep from others running similar sized tires. You need center diff/engine tranny clearance as rocks will not be cleared in the center of the road and you don't want to hit them.

Jason :)

Tire size and gear ratio should always be changed at the same time!!!
If you change your gears tire size will have nothing to do with acceleration.

Same gears with bigger tires will slow you down, changing the gears will put you back to where you were before.
If you had 35's before you will need to change your gears to be able to have the same power with your 37's now. In desert you are going to want the 37's keeps from dragging the diff. For short stuff like laughlin or Corr. or Terrible cup, switch back to the 35's for cornering and stability.

[ntsqd]

snip...
If you change your gears tire size will have nothing to do with acceleration.

Um, wrong. If you increase the tire's OD you increase the unsprung weight and you increase the Moment of Inertia (flywheel effect) of the tires. Short of finding a 37" tire lighter in weight than the 35" (not bloody likely) you're stuck with this.

Now suppose that the gear ratio is changed to offset the larger OD tire. That fixes the change in leverage due to the larger diameter, but you still haven't accounted for the increase in Moment of Inertia. A very qwik & dirty SolidWorks model predicts that there will be roughly a 20% increase in Moment of Inertia by going from a 35" OD tire to a 37" OD tire, even if the rim size & tire width remains the same.
That won't translate into a 20% loss in acceleration, but it will be a noticable loss. How noticable depends on the HP of the engine.

[matt_helton]

the bigger the tire, the smaller the hole.

[Samco Fab]

the bigger the tire, the smaller the hole.

I agree, this is the biggest reason in the dirt. One more thing to consider is that your contact patch of rubber on the dirt is longer with a taller tire, and you can run a bigger wheel/ bigger brakes without a lower profile sidewall that likes to pinch flat.

It boils down to the fact that a vehicle is generally built around what tire you will run. If you want 37's, you need to build everything to handle the 37's. This includes spinning them (power), stopping them (big brakes) and the greater forces transfered to the drivetrain (probably min. 40 spline axles and a beefy diff)

Bigger performs better due to the rule mentioned at the top , it just costs more $$ to run a bigger tire.

[SimonSays]

[QUOTE=ntsqd;231580]Um, wrong. If you increase the tire's OD you increase the unsprung weight and you increase the Moment of Inertia (flywheel effect) of the tires. Short of finding a 37" tire lighter in weight than the 35" (not bloody likely) you're stuck with this.

We have done some testing in our truck 1\4 mile times and 0-60 with a 4.11 ratio and 35's and 4.89 ratio with 37 they were within tenths of a second, with a 4.56 ratio and 37's it was slower but with 4.89 it was almost exactly the same, difference was on top speed but you never reach that in the desert so it made no difference. I under stand what your saying but if you go 2 gear ratio difference it counter acts the loss of power,
with the higher ratio it alows the motor to spool up quicker and keep in the power longer. (closer shifts but you have a better power band)

[Stephen]

You just overgeared to correct for the power loss from the size. Sounds like you have a very practical real world solution and good testing to prove it works.

[rebel33]

the bigger the tire, the smaller the hole.

Here's a graphic I once made showing this point - the big tire is 5/4 the smaller one, that is like a 37 compared to 30 inch tire, the hole is the same, the red arrow exmplifies the difference in vertical accelaration. It can be fairly said that vertical accelarations are the main input that control the speed of the car through the driver. Big wheels (and long travel suspension) serve to flatten the vertical acceleration curve the driver and car undergo.

http://www.jeepolog.com/UserFiles/Image/without_watermark/buggy%20blog/hole_example.jpg

[Grunion]

Thanks for all of the input. I will only be running the truck in desert races. I have plenty of hp, The only thing stated that worries me a little is braking. I have disc brakes all the way around on the truck right now. and it doesnt stop that great it is ok but could be better. what scares me is to run my truck in 8 stock I am going to have to put drum brakes on the rear to be legal. I wish there was a way around this but I dont think there is.

[Old School 8]

[QUOTE=ntsqd;231580]
with a 4.11 ratio and 35's and 4.89 ratio with 37 they were within tenths of a second, with a 4.56 ratio and 37's it was slower but with 4.89 it was almost exactly the same,

Also keep in mind, the lower the gear ratio, the less contact each tooth has between the ring and pinion. The bigger the tire, the lower the ratio. The lower the ratio, the more stress on the gears. Broken gears = DNF Thats why TT's use the Chrisman type center section, with 5.0 and lower ratio's. More $$$ !

[SimonSays]

[QUOTE=SimonSays;231629]

Also keep in mind, the lower the gear ratio, the less contact each tooth has between the ring and pinion. The bigger the tire, the lower the ratio. The lower the ratio, the more stress on the gears. Broken gears = DNF Thats why TT's use the Chrisman type center section, with 5.0 and lower ratio's. More $$$ !

On my dad's truck he runs 5.89 gears has not had a huge problem with blowing out 3rd members.

P.s. a 4.89 ratio is a higher ratio then a 4.11, correct statment wrong terminoligy but i get what your saying (with a 4.89 ratio you are more likely to break the gears then with a 4.11 ratio) even so you should change out your gears on a race truck every season even if they arn't broken, they will. 9" gears seem to last about a year to a year and a half, good thing to be safe and not get DNFs is to change them out regularly.

[SimonSays]

Thanks for all of the input. I will only be running the truck in desert races. I have plenty of hp, The only thing stated that worries me a little is braking. I have disc brakes all the way around on the truck right now. and it doesnt stop that great it is ok but could be better. what scares me is to run my truck in 8 stock I am going to have to put drum brakes on the rear to be legal. I wish there was a way around this but I dont think there is.

I don't remember any thing in 8 stock saying you have to use stock brakes but if you do, what master cylinder are you running? changing that may help out your braking, are you using a CNC you can get different boars that will push more fluid with less pedal movement. Making for better braking.

[Grunion]

yeah I was just going to show up with my disc brakes until someone said somthing. but Im pretty sure you have to run stock brakes in BITD class 8s

[Moss2]

Go back to class Simon !!

IF you push more fluid with less pedal movement it makes for less braking power. ( Unless you are adding more power assist at the same time) It will be very firm pedal though. Braking power is not about volume its about pressure and pad area.

A 4:89 is a lower gear than a 4:11 but that could be debated with no purpose forever.

[Grunion]

is it even legal to change any of your brake system in class 8 stock?

[ntsqd]

Um, wrong. If you increase the tire's OD you increase the unsprung weight and you increase the Moment of Inertia (flywheel effect) of the tires. Short of finding a 37" tire lighter in weight than the 35" (not bloody likely) you're stuck with this.

We have done some testing in our truck 1\4 mile times and 0-60 with a 4.11 ratio and 35's and 4.89 ratio with 37 they were within tenths of a second, with a 4.56 ratio and 37's it was slower but with 4.89 it was almost exactly the same, difference was on top speed but you never reach that in the desert so it made no difference. I under stand what your saying but if you go 2 gear ratio difference it counter acts the loss of power,
with the higher ratio it alows the motor to spool up quicker and keep in the power longer. (closer shifts but you have a better power band)
I avoided that solution as it reduces your top speed potential, or requires an engine with more rev potential. Not to mention the tooth strength issue already brought up.

And yea, larger bore m/c moves more fluid volume at the expense total system leverage. So the pedal feels firmer, but the brake system pressure is lower which = less braking effectiveness for a given pedal force input. It has been my observation that frequently a firm pedal is considered a "better brake" regardless of how effective the brakes actually are. Within some small window the pedal needs to be what the driver prefers or he/she will be unhappy and not effective as a driver.

[SimonSays]

Go back to class Simon !!

IF you push more fluid with less pedal movement it makes for less braking power. ( Unless you are adding more power assist at the same time) It will be very firm pedal though. Braking power is not about volume its about pressure and pad area.

A 4:89 is a lower gear than a 4:11 but that could be debated with no purpose forever.

Power assist why use that off-road, on my truck i had a CNC pedel with 7\8 bore and it would not stop, we talked to CNC they said to switch up to the 1" we did they work great(spungy pedel but can stop like nobodys buisness). So i guess it depends on your sisutation i guess.

As far as the gear debate it can be argued either way(4.89 is a higher ratio then 4.11 but lower numericaly).

[Moss2]

We started out on 33's because that is what was available in late 2000. We got stuck in so many silt beds and had a lot of flats (we were kinda rookies too.) Going up to 35 made a world of difference. For a 2wd truck I think 37s would be good if you think your hubs and axles can take it. Dont worry about the power difference. The time you make up not getting stuck and skipping over the rock gardens more than makes up for it. The 35s made a noticable drop in top speed and acceleration but overall the truck is faster.

[ntsqd]

Power assist why use that off-road, on my truck i had a CNC pedel with 7\8 bore and it would not stop, we talked to CNC they said to switch up to the 1" we did they work great(spungy pedel but can stop like nobodys buisness). So i guess it depends on your sisutation i guess.


The going up in bore size = spongy pedal and increased bore size = increased stopping power is a hydraulic conflict. I'm not doubting this happened, but it goes against what should happen. Something else has come into play there.

[SimonSays]

The going up in bore size = spongy pedal and increased bore size = increased stopping power is a hydraulic conflict. I'm not doubting this happened, but it goes against what should happen. Something else has come into play there.

I don't know i was confused my self when they told us to go bigger, what ever happened it worked don't know why just did.

[Ramsey_ElWardani]

is it even legal to change any of your brake system in class 8 stock?
I doubt your truck will be legal in Stock Full (There is no such thing as 8s) because everything is supposed to be basically stock. If it was an old Class 8 it has had way too many modifications. The guys (and the factory support) racing the stock classes take the rules seriously because there is a lot of money at stake, especially at BITD.

[Grunion]

I doubt your truck will be legal in Stock Full (There is no such thing as 8s) because everything is supposed to be basically stock. If it was an old Class 8 it has had way too many modifications. The guys (and the factory support) racing the stock classes take the rules seriously because there is a lot of money at stake, especially at BITD.

my truck is a 1971 f150 that was raced back then and not to much is modified, at least compared to a newer 8 truck. It is however a cool old truck it still has score inspection tags from the 70s. I do have a few things I need to replace like a dash and a bumper. But all of my suspension still meets the requirements. It really wont take much to make it legal for 8s or (Stock full) for those who didnt know what I meant.:)

[Brutpwr]

Our truck almost always uses 6.50 gears (lowest they make for a 9") so you can not regear for the taller tires! Truck is faster on a short course with the smaller tires but all tests were back when you could run a 14.50" wide rear tire. CORR now restricts you to a 12.5" wide tire but still I would guess the truck is quicker with the shorter lighter tire. Remember on a short course truck you are almost always accelerating, braking and/or cornering so lighter and lower means a lot. Even in the desert we usually end up with 6.50 gears although 5.43 & 6.20's are sometimes used.

Jason :)

[moabWalker]

Something else has come into the guy's breaks. The bone stock **** *** drum brakes on my '75 Bronco will lock up all 4 37" BFG Krawlers(not a light tire) when I stomp them. Disc brakes should stop faster than drums unless pressure is being leached somewhere

[Grunion]

I belive my master cylinder is from somthing else it doesnt have a booster. I dont think a booster will work with the amount of cam I have.