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Tire psi relation to wheel speed
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DRR Sportsman
Picture of NEMO963
posted
no-racing stir crazy and had a few questions I haven’t totally wrapped my head around.

So with other things the same does lowering tire psi increase wheel speed at launch or decrease. Such as a dead hook shake will lowering psi increase the wheel speed to get rid of it or higher psi.

This message has been edited. Last edited by: NEMO963,


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Posts: 293 | Location: Firehouse 10 Wichita Ks | Registered: February 15, 2005Reply With QuoteReport This Post
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lowering air increases on my racepak


J.R. Baxter

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Posts: 1129 | Location: Waxahachie | Registered: July 04, 2005Reply With QuoteReport This Post
DRR Sportsman
Picture of NEMO963
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Good to know, I’ve been told both (up
And down). How much of a psi change does it take to see a change in your racepack. I’ve pulled and added timing to play with wheel speed but not played with tire pressure yet.


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Posts: 293 | Location: Firehouse 10 Wichita Ks | Registered: February 15, 2005Reply With QuoteReport This Post
DRR Pro
Picture of CURTIS REED
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Remember it's wheel speed not tire speed being discussed when you are talking about an air change. I believe that lowering air allows the wheel itself to turn easier therefore faster inside the tire which in turn allows a run at the tire. That lets it get the tire spinning faster to in effect "get up on the tire". Just my take on it.

Someone will be along later with an inane description and shock info.



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quote:
Originally posted by CURTIS REED:
Remember it's wheel speed not tire speed being discussed when you are talking about an air change. I believe that lowering air allows the wheel itself to turn easier therefore faster inside the tire which in turn allows a run at the tire. That lets it get the tire spinning faster to in effect "get up on the tire". Just my take on it.

Someone will be along later with an inane description and shock info.


Are you saying decoupling the wheel and tire? That's not going to work with screws or beadlocks. Unless I am misinterpreting what you are writing.

On my Hoosier C07's, I generally run 6.5 psi hooked up. But if I need to unhook it, I had to go down to 4 psi to get the tire deformed enough to unhook. I think the theory is that the tire cups and brings more of the middle of the tire up off the track. I was really uncomfortable with going lower since my tires aren't screwed and frankly it felt a little squirmy on the top end with that little pressure. I also have a four link in my car adding to the complexity. With the four link, keeping the wheel from heading toward the ground is generally the rule for not hitting it too hard. But with very little air pressure, that rim wants to head that direction regardless of four link settings. It's just seems odd looking at the shock data, because the shock data doesn't take this into account.


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Posts: 4147 | Location: Illinois | Registered: July 08, 2004Reply With QuoteReport This Post
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Picture of CURTIS REED
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quote:
Originally posted by Bucky:

Are you saying decoupling the wheel and tire? That's not going to work with screws or beadlocks. Unless I am misinterpreting what you are writing. You are misinterpreting it.

On my Hoosier C07's, I generally run 6.5 psi hooked up. But if I need to unhook it, I had to go down to 4 psi to get the tire deformed enough to unhook. I think the theory is that the tire cups and brings more of the middle of the tire up off the track. I was really uncomfortable with going lower since my tires aren't screwed and frankly it felt a little squirmy on the top end with that little pressure. I also have a four link in my car adding to the complexity. With the four link, keeping the wheel from heading toward the ground is generally the rule for not hitting it too hard. But with very little air pressure, that rim wants to head that direction regardless of four link settings. It's just seems odd looking at the shock data, because the shock data doesn't take this into account.


All I'm saying is that the rim turns further, quicker and easier within the tire with lower pressure before it starts to turn the tire. It does not slip within the tire.



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Posts: 1202 | Location: KIEFER, OK. | Registered: August 17, 2007Reply With QuoteReport This Post
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quote:
All I'm saying is that the rim turns further, quicker and easier within the tire with lower pressure before it starts to turn the tire. It does not slip within the tire.


Got it. thx


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Posts: 4147 | Location: Illinois | Registered: July 08, 2004Reply With QuoteReport This Post



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quote:
Originally posted by CURTIS REED:
Remember it's wheel speed not tire speed being discussed when you are talking about an air change. I believe that lowering air allows the wheel itself to turn easier therefore faster inside the tire which in turn allows a run at the tire. That lets it get the tire spinning faster to in effect "get up on the tire". Just my take on it.

Someone will be along later with an inane description and shock info.


Yes and No, initial rim speed is greater, actually it has speed when the tire doesn't at the hit, the tire needs to wrap up before it turns and the car moves. After that they basically are the same and for this tread, this area has little to do with what wheel speed tuning is about.

So if the tire has running pressure that currently maximizes traction then lowering pressure will increase wheel speed. In high power cars, to little wheel speed usually causes tire shake from the tire becoming so distorted. To much and it's just another burnout. Tuners usually look at wheel speed and given times after the launch and increase of decrease as needed to get the most performance/ quickest ET possible. For bracket racing, in this case fast bracket racing, this is not usually the most consistent way to tune.
 
Posts: 1354 | Location: Tewksbury, MA,USA | Registered: November 03, 2000Reply With QuoteReport This Post
DRR Pro
Picture of CURTIS REED
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quote:
Originally posted by TOP38:
quote:
Originally posted by CURTIS REED:
Remember it's wheel speed not tire speed being discussed when you are talking about an air change. I believe that lowering air allows the wheel itself to turn easier therefore faster inside the tire which in turn allows a run at the tire. That lets it get the tire spinning faster to in effect "get up on the tire". Just my take on it.

Someone will be along later with an inane description and shock info.


Yes and No, initial rim speed is greater, actually it has speed when the tire doesn't at the hit, the tire needs to wrap up before it turns and the car moves. After that they basically are the same and for this tread, this area has little to do with what wheel speed tuning is about.



Are we not saying the same thing?



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Posts: 1202 | Location: KIEFER, OK. | Registered: August 17, 2007Reply With QuoteReport This Post
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quote:
Originally posted by NEMO963:
no-racing stir crazy and had a few questions I haven’t totally wrapped my head around.

So with other things the same does lowering tire psi increase wheel speed at launch or decrease. Such as a dead hook shake will lowering psi increase the wheel speed to get rid of it or higher psi.


Higher or lower pressure is relative to the specific circumstance of how the tire looks (observed) planted. You could have a tire with 20 psi, the rim is nearly already hitting the ground.

You're looking for an objective answer to a subjective question/subject.


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Posts: 2599 | Location: Madeira Beach Fl. | Registered: June 12, 2018Reply With QuoteReport This Post
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We know we are talking about a rear engine dragster, and that gives us a range to work with for suspended and not and big and small blocks mostly.

Tire size has an influence over what pressure to run as well. Suspending the same weight with twice the contact patch will reduce the pressure required to hold the car up...well, the wheel up.

So of course that is all true.
But
Usually, as was said earlier, if you are set up at the ideal dead hook pressure, going down considerably on pressure will cup the tire and reduce the contact enough to increase wheel speed.


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Posts: 4147 | Location: Illinois | Registered: July 08, 2004Reply With QuoteReport This Post
DRR Pro
Picture of Mike Rietow
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quote:
Originally posted by Bucky:
We know we are talking about a rear engine dragster, and that gives us a range to work with.


I agree, there's plenty of evidence to validate it.

The fact everyone has a different approach when it comes to pressure, same tire, and still zerox tickets, is all the evidence anyone should need to know, ya'll know exactly what you're doing. But it also evidences my point, it's a subjective matter. If it weren't, everyone could call M/T and they could tell you where the start, middle extreme pressure is to be set.

I agree, typically if the track is loose you add pressure, if it's tight you take pressure off.

There's no exact right or wrong answer though. I think if you're talking a new rigid chassis doorslammer, it's probably more of an objective topic.


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Posts: 2599 | Location: Madeira Beach Fl. | Registered: June 12, 2018Reply With QuoteReport This Post
DRR Pro
Picture of 27Keith
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This could turn into a very good topic. How about posting what you have found to be your sweet spot with your own set up? Tire ? Psi ? 60 ft.? E.t. ? Leave rpm ? And your .25 and .5 numbers. Even throw up a picture of a graph for those of you who know how to do that.


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Posts: 2062 | Location: out there | Registered: March 31, 2004Reply With QuoteReport This Post
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I would gladly post if I had unhooked data. I need to transition with my car. Forever it has been set up with a lot of initial bite. Probably too much even for a car with no shake issues. My problem was often making sure it didn't spin after initial hook because of the excessive plant.
As I try to lean on the new combo, I have found myself with tire shake. Hoosiers are less forgiving with this. I borrowed some M/T's and had no shake issues. But I did see more wheel speed. IMO, it's no secret why they shake less. They bite less so it is easier to get the wheel speed. I am hoping that by taking initial bite away with the four link, that I can slip the Hoosier enough to get through the shake area. I'm going to try a fairly flat top bar with a bottom bar with a little more rake down in the front than I usually run. Keep the shock tight on extension, and start off fairly low on air. Leave with all the timing in. I'm actually hoping to completely unhook the tire, and start sneaking up on this from that side. I really don't want to weld more things back onto this car.
I have heard some say that anything less than 1900 rpm on the driveshaft at .25 second is too little. I don't have anything near that now. I have a picture on my desktop of the car leaving at about 4 feet out. Wheels are up, and the track behind the tires is steamrolled out smooth. It is stuck hard with the tire even across the treadwidth.
We will see how this goes. If I can't get this 14.5" Co7 Hoosier to respond to these changes, I probably won't have any choice but to change tires.


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Posts: 4147 | Location: Illinois | Registered: July 08, 2004Reply With QuoteReport This Post



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Keith, the stuff I'm racing now is too slow to help anyone even if I posted that stuff up. LOL



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Posts: 1202 | Location: KIEFER, OK. | Registered: August 17, 2007Reply With QuoteReport This Post
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quote:
Originally posted by Bucky:
I would gladly post if I had unhooked data. I need to transition with my car. Forever it has been set up with a lot of initial bite. Probably too much even for a car with no shake issues. My problem was often making sure it didn't spin after initial hook because of the excessive plant.
As I try to lean on the new combo, I have found myself with tire shake. Hoosiers are less forgiving with this. I borrowed some M/T's and had no shake issues. But I did see more wheel speed. IMO, it's no secret why they shake less. They bite less so it is easier to get the wheel speed. I am hoping that by taking initial bite away with the four link, that I can slip the Hoosier enough to get through the shake area. I'm going to try a fairly flat top bar with a bottom bar with a little more rake down in the front than I usually run. Keep the shock tight on extension, and start off fairly low on air. Leave with all the timing in. I'm actually hoping to completely unhook the tire, and start sneaking up on this from that side. I really don't want to weld more things back onto this car.
I have heard some say that anything less than 1900 rpm on the driveshaft at .25 second is too little. I don't have anything near that now. I have a picture on my desktop of the car leaving at about 4 feet out. Wheels are up, and the track behind the tires is steamrolled out smooth. It is stuck hard with the tire even across the treadwidth.
We will see how this goes. If I can't get this 14.5" Co7 Hoosier to respond to these changes, I probably won't have any choice but to change tires.


Bucky

I am not familiar with your combo but gonna say your likely not going to fix it with just 4 link adjustments. You will likely need a wheelie bar to help you unload the tires at the hit and possibly need weight added to the nose of the car.

As for wheel speed, remember that your really measuring driveshaft speed which is related to rear gear ratio and tire diameter so you need to consider this when you look at others and their wheel speed numbers before you used these for comparison. This also applies to you when you change tires!
 
Posts: 1354 | Location: Tewksbury, MA,USA | Registered: November 03, 2000Reply With QuoteReport This Post
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I have thought about the wheelie bar, but really wanted to avoid that if for at least the issues of trailer loading and uhooking it, getting around in the pits etc. But that certainly would be another tool to use. You may be spot on correct about it.

Point taken about the driveshaft, and I completely agree.


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Posts: 4147 | Location: Illinois | Registered: July 08, 2004Reply With QuoteReport This Post
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Leave it to me to over think something. But is there any reason the wheels couldn't be mounted right on the rear end housing instead of on a long bar extending to the rear of the car?

On edit, this isn't really my idea it seems. I thought I had seen this before. Top fuel cars I think I read use a skid on the frame to limit tire squat. The wheelie bar is still there to limit chassis rotation.

This message has been edited. Last edited by: Bucky,


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Posts: 4147 | Location: Illinois | Registered: July 08, 2004Reply With QuoteReport This Post
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My son Curtis and I have been Crew Chiefing two Big Tire Street Outlaw cars the last two years. And have learned more about wheel speed and getting down a race track than the previous 60 years of racing. In "general" more traction,..increase tire pressure, reverse to decrease traction. There is a window for each Track and particulate tire and condition. You can get outside of that window and all bets are off.


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Posts: 1003 | Location: Visalia, Calif. 93292 | Registered: November 23, 2000Reply With QuoteReport This Post
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quote:
Originally posted by 27Keith:
This could turn into a very good topic. How about posting what you have found to be your sweet spot with your own set up? Tire ? Psi ? 60 ft.? E.t. ? Leave rpm ? And your .25 and .5 numbers. Even throw up a picture of a graph for those of you who know how to do that.


I would love to but not sure i could find those DatA logs to show it....


J.R. Baxter

""Fathom the hypocrisy of a Government that requires every citizen to prove they are insured ..but not everyone must prove they are a citizen."

2016 Miller
Rolla Competition Engines
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Posts: 1129 | Location: Waxahachie | Registered: July 04, 2005Reply With QuoteReport This Post
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