Landracing Forum
Tech Information => Steering - Suspension - Rear End => Topic started by: RebekahsZ on May 14, 2015, 07:28:28 AM
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Track is ECTA at Wilmington, Ohio-nice clean concrete. Car is 2500 pound 400hp 240z in C/GMS class. Front tires are Mickey Thompson ET Fronts 24.0/4.5/15 (2-yrs old, stored in bags in A/C storage between races) on 3.5" aluminum wheels with sidewall-recommended pressure of 35psi. Car has gone 167, looking to go 174 in June. I am running brand new MT 275/60/15 Drag radial Pros in back at 35 psi (traction and wear pattern looked great at May meet). Question: what tire pressures do you guys run and recommend?
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I ran the same size tires at Wilmington on my Lakester front tire presure is 36 the rear is 38 PSI the car weight is just under 2500 pounds. Wilminton speed has been over 170. Our other team car is a roadster the front tire is the same size the rear tire is a drag radial just a lot taller and wider once again same tire preasures roadster has been over 212 at Wilmington.
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I have a friend, who has records at several tracks from Maine to Utah, telling me to go up to 65-70 psi....says it stiffens the sidewall and makes car wander less. But, I'm afraid to pump up that high on a tire with such a low manufacturer's recommendation...
My car ran straight but was a little fishy on brake application. Nothing I can't handle with judicious application of the middle pedal.
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I have a friend, who has records at several tracks from Maine to Utah, telling me to go up to 65-70 psi....says it stiffens the sidewall and makes car wander less. But, I'm afraid to pump up that high on a tire with such a low manufacturer's recommendation...
My car ran straight but was a little fishy on brake application. Nothing I can't handle with judicious application of the middle pedal.
I'm not sold on raising the tire pressure so high above what the tire is rated for on the sidewall. I know some high speed rated tires for the salt have pressures high like 60 or more but that's what the tire is rated for that pressure. I find at Wilmington my rear tires grow about an inch in height from the speed the roadster tire diameter grows even more that since the speed is greater. The tire diameter increase in size is giving us an added benefit of a taller gear which helps gain more MPH.
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Another thing to keep in mind when playing with tire pressures is knowing WHY you might want to add or delete air pressure.
One of the things to know is what is your rim width? What size tire should be run on this rim? How fast might you go? Etc...
As ronnie posted above, his tire diameter grows at speed. This in turn well pull the tire inward at the bead. If your rim is on the wide side, lets say 7", and the tire co. recommends a rim width between 6" & 7" you MIGHT want to add air pressure to help keep the tire bead seated. This varies greatly depending on tire type, recommended rim widths, tire diameter vs. rim diameter, maximum speed run, vehicle weight, Etc.....
This is posted as food for thought. I feel know one knows your combination better than you, so you you need to try to understand as best you can WHY you might want to change anything on your vehicle for better performance.
Good Luck on you project :-)
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Mount a camera in a location to see how the tire deforms at high speeds, and use it on a "test" run. You might be surprised. Make changes accordingly.
Higher tire pressures in the front tires can prevent "squirrellyness" under "heavy" braking. As the driver, you need to be "comfortable" with how the car handles. Forget what anybody else says about how you should drive it. When you are constantly "wary" of the handling, it is difficult to extract the last bit of performance.
:cheers:
Fordboy
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That sounds like very sound advise from two very experienced "crew guys". :-D :-D :-D
Pete
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Ok, I appreciate the coaching, but we are kinda dancing around the subject (and that may be the way it's got to be). Let me rephrase my question: what the highest tire pressure you would feel comfortable inflating a front runner to? Or maybe: how high do you run your front runners? Is 65 psi asking for a blown tire?
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Ok, I appreciate the coaching, but we are kinda dancing around the subject (and that may be the way it's got to be). Let me rephrase my question: what the highest tire pressure you would feel comfortable inflating a front runner to? Or maybe: how high do you run your front runners? Is 65 psi asking for a blown tire?
Your tires have a speed rating, chances are very good that the manufacturer tested those tires within that rating, so call the manufacture and ask for their recommendations.
As a general rule, higher pressure helps keep the tire beaded and reduces or limits speed growth. The folks that made your tires should be able to give you good information as long as you get it from their engineering and not some sales guru.
Rouse
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Ok, I appreciate the coaching, but we are kinda dancing around the subject (and that may be the way it's got to be). Let me rephrase my question: what the highest tire pressure you would feel comfortable inflating a front runner to? Or maybe: how high do you run your front runners? Is 65 psi asking for a blown tire?
We run front runners on both ends of a Loring, ME car in XO/GCT. It's a 57 Chevy ex Stock car racer. We have only been to 116 mph but we run 65 lbs front and rear. The driver says it's straight as an arrow, no steering necessary. We run a spool and the rears are almost perfect at 1/8 difference in rollout.
Rear tires are new Goodyear 28's and fronts are ancient dry Goodyear 24's.
Front runners are accepted without a speed rating because they are made to go way faster than we can all go in the mile and mileandahalf.
Frank
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Notice the OP said Mickey Thompson front runners....... :roll:
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Thanks, gas pumper! That's two guys who are running front runners of some kind or another (even dry-rotted ones) to 65psi, so I'm gonna try increasing to at least 50psi. If I look at the video of my run, the car only gets fishy (and not terribly so) when I transition from full throttle to braking. That's either due to needing more toe-in, or the soft bias ply tires in front. Its a lot easier to add tire pressure than to go get an alignment! I'm using this site to learn from other guys' experience. Rule books don't discuss car handling, or car performance-they just state rules. I called Mickey Thompson before posting and they said: "Our tires are not approved for landspeed racing." (and) "The recommended pressure is whatever it says on the sidewall." Manufacturers aren't much help when we exceed the design criteria of their equipment. That's when we either need each other, or just to buck-up and go for it. Thanks again, fellas!
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A toe-in check can be done almost as easy (sometimes) as an air pressure adjustment. I'd think it might have a bigger effect than a pressure change.
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the car only gets fishy (and not terribly so) when I transition from full throttle to braking.
Have you checked the bump steer characteristics?
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the car only gets fishy (and not terribly so) when I transition from full throttle to braking.
Have you checked the bump steer characteristics?
I agree- bump steer is a definite suspect here.
Regards, Neil Tucson, AZ
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Rebek...""I called Mickey Thompson before posting and they said: "Our tires are not approved for landspeed racing." (and) "The recommended pressure is whatever it says on the sidewall." Manufacturers aren't much help when we exceed the design criteria of their equipment. That's when we either need each other, or just to buck-up and go for it.""
there has been much SAGE advice given, yet you seem to want direct answers/direction about very
serious issues....that cant be blanket answered, TOO MANY VARIABLES
front runners should NOT be grouped together with respect to how they are utilized....
they ARE NOT apples to apples....!!!!
goodyear for example has standard front runners built and designed for one set of variables,
and other "front runners" made from same mold and built very differently, they both say
front runner on sidewall, but other designations reveal much more like built for sustained high
speeds, with much different carcass and bead structure and inflation specs
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There you guys go off on tangents again...
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Joe just gave you some very good advise which you seem to choose to ignore or even berate. Go into the Goodyear racing catalog and look at their landracing tires. They come from the same moulds as frontrunners but are a totally different tire.
I was on your side up until now. Smarten up.
Pete
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There you guys go off on tangents again...
Contrary to the direction we travel, LSR is NOT linear.
One more variable - and I don't know the answer, but it may germane. Are you running tubes?
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Rebek....your post
""That's two guys who are running front runners of some kind or another (even dry-rotted ones) to 65psi, so I'm gonna try increasing to at least 50psi.""
compelled me to try to communicate my previous information, ie that there can be vast differences
in front runners, not apples to apples etc
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OK, just tell us what you want someone to tell you...
Do guys run tires above sidewall pressure number...? sure all the time
Do you want us to tell you that your problem is tire pressure and not bump steer and you should try lots more pressure before you look into bump steer.., probably aint going to happen... I can tell you that a little bump steer twitch can look insignificant until you transition from power to parachute and end up 1/4 mile right of the course... luckily that year, that was the correct way to go if you had a problem :-o
When you ask, try to listen, even if it is not what you want to hear
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Thanks guys, I am listening, and I really do appreciate your advice. I will take a look at the Goodyear catalog. I'm not running tubes, but I'm open to it, especially with these two piece wheels. I have searched for small, skinny tubes before and couldn't find a vendor, any suggestions?
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Scroll down to page 4 on this link. http://www.racegoodyear.com/tires/pdf/Goodyear_Drag_Catalog_2015.pdf
This should give you the information you want other than they don't offer a maximum tire pressure, only a minimum. These tires are a different construction than the drag tire so the recommendations are for lsr tires only.
Pete
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I wonder why they only list a minimum pressure....thanks so much for the link.
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That chart is super informative, thanks so much! The minimums are all either 50 psi or 70 psi. Also interesting that the narrow tires are $600 each and the largest tire is $260. I'm sure you can't judge apples to oranges, but I will look up the weight on the MT front runners and see how it compares to the Goodyear Eagle Landspeed tire in weight.
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One more thing to think about is tire circumference. Before you make a pass, lay your fronts and rears in the Sun for say 15 minutes minimum.
Then check the circumference at the psi you wish to run. If they are more than 1" in circumference apart (I am not a tire engineer) I would advise you to add additional air in the smaller tire to bring it up to as close as you can to the mate tire or better yet, buy another pair of tires.
A word of warning to those trying to replace a single tire. It is not uncommon for variations in "listed" tire size due to multiple molds, and perhaps other production factors. I know that M/T matches their tires in pairs (I believe aired up) before shipping.
One Run Bob :dhorse:
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Also be wary - not trying to change subjects again but some front runners are bias ply while others are radials and that dramatically changes inflation pressures.
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Just back from June ECTA meet with feedback. Ran with 50psi cold in MT front runners in front and 40 cold in MT 275/60/15 Drag Radial Pros in back. Did not check air pressures on Saturday when it was cool, but on Sunday when it was hot, the fronts had risen to 60 and the rears to 45 coming off the track after a run. Ran an airdam that increased my top speed from 167 to 174. But, the airdam increased downforce and changed my dynamic alignment settings. Also found that the suspension is very active (too active). With the front end down from the airdam and splitter, the added nose down of braking made the car a little scary at the transition point and i found myself using the second turn-out most of the weekend, just so I could brake easier. Clearly I had bumpsteer from the toe-out induced by the nose dropping from downforce, further aggrevated by braking (I have road racing brakes). Spend Saturday afternoon cutting off the splitter and Sunday experimenting with different toe settings. Increasing toe helped A LOT in braking, but made the car more twitchy during the speed section of each run. On the last two runs, I dropped my air pressure down to 35 all the way around and ripped into a big headwind and ran 167. Car felt better, although my video shows more driver input at speed than my mind remembered. I think perhaps the softer tires allowed the tires to deflect instead of the suspension (maybe?). Lots to learn and lots of TnT to do next year. Will probably wait till June meet due to 2 daughters graduating next year. I have a couple of autocross/road course events to do this summer, so the toe is getting pulled back out. Then this winter, I'm going to zero camber, as much caster as I can get into it and DOUBLING the spring rate. Gonna make a variety of aero devices to try out in June. Will aim to stay in the 170mph range next year, but get the car handling before doing anything that would increase speed. Will plan to stay at the 35psi tire pressure as a starting point for next year. When surveying drivers at the meet, tire pressures were all over the map. Thanks everybody for all the encouragement and advice. Even though I started this thread with the specific intended goal of a tire pressure survey, it was super educational and I read every post and I really appreciate everybody's advice.
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Major kudos to you for you results and efforts !!
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A very analytical and logical approach. Thanks for sharing, you have me thinking deeper about a couple things.
Ed
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While you have the front end apart for springs, I would (if you haven't already) check bump steer and scrub radius. Both of those can cause handling problems.
Ron
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I'm with Ron on this. Check your bump steer-- toe-in should not change much as the nose of the car dips down from braking or aero forces.
Regards, Neil Tucson, AZ
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I'm with Ron on this. Check your bump steer-- toe-in should not change much as the nose of the car dips down from braking or aero forces.
Regards, Neil Tucson, AZ
If the front suspension "toes out" in bump travel, it will make the car "twichy" (at least) under braking. Large amounts of "toe out" change can make the car "wander" or "hunt" unpredictably/uncontrollably.
Try to limit the toe change to less than .03" per wheel through the dynamic ride height range you are using. If you have "large" changes in toe dimensions, revise the suspension geometry to minimize the changes. Carroll Smith's book "Tune to Win" (although road racing centric) has a great practical chapter on toe/geometry and how to measure and alter toe characteristics. The book is available on Amazon, etc.
:cheers:
Fordboy
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Here is more information that may be useful when choosing tire pressure and tire type to achieve a balance among speed, safety, and cost.
One reason to increase tire pressure is to reduce rolling resistance, which along with aero drag and peak engine power pretty much determine maximum top speed. Rolling resistance can be as important as aero drag (details are on the Design Topics page on my website). Higher tire inflation pressure reduces rolling resistance by reducing tire flexure (which dissipates power) where the rubber meets the road. A caveat is that increased tire pressure reduces the contact (footprint) area, which reduces available traction for steering, acceleration, and braking.
Actually achieving the maximum possible top speed also depends on several other factors.
Re Goodyear Frontrunner tires, see the 2015 list at URL
http://www.racegoodyear.com/tires/pdf/Goodyear_Drag_Catalog_2015.pdf
Goodyear makes “Eagle Land Speed” tires rated for 300 mph (with load and inflation pressure specifications) and “Frontrunner” tires that are used on dragsters and list no speed rating. Goodyear and SCTA have authorized the new D2904 Frontrunner for LSR use up to 300 mph at 90 psi and less than 1200 lbs radial load. It is my understanding that the other Frontrunner tires (at least the 17”) are restricted by SCTA to use only at considerably lower speeds and lighter loading because the tire construction may not reliably stand up under sustained high LSR speeds.
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Not trying to hijack... on topic ...
What is the max psi for LSR's that Ill have to run for 150 to get a speed rated with load capasity.
Curent weight (add all safety for 150 and up )
3500 front / 2500 rear so Goodyear LSR # 2850 and custom 18" 8x6.5 wheels front 70psi min =25sqin - hysteresis contact patch
2015 55T 91.5 dodgeD250 87.14mph
Dammit Dave I can't do that, this means you can't either