Colorado tire load question

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So I have a bit of an odd idea, I want to autocross my 2017 Chevy Colorado. Per SCCA regulations, track width must be wider than vehicle height, which is a bit of a tough one considering the stock height is 70.7 in vs 62.4 in track width. I have ordered a lowering kit to drop me down ~3.5 in (maybe more, 4 rear, 3 front and the truck comes from the factory with a raked stance). I am also moving from a +33 offset to 0 for an additional 2.6 in track width. Alongside the new wheels I am going to go with shorter tires to get me the rest of the way there, but I have a question about this. Stock comes with a 110 load rating (I think), which is 2337 lbs per corner. Most tires I can find in the new size are 97 load rating at 1609 lbs. GVWR of the truck is 5800 lbs. Truck weight should be somewhere around ~4200 lbs with driver, fluids, etc. I don't ever expect to carry anything near the payload capacity in the bed, and I don't expect to ever really tow anything more than 5000 lbs. Trailering is a pretty infrequent occurrence for me. I'm not looking for someone to give an absolute "yes this is fine" since we are talking less than the manufacturer's recommendations, but does anyone see any issues with moving to the lower load rating as long as I am understanding that my total payload would be impacted, and I shouldn't go crazy with towing at the max 7000 lbs the truck is rated for? And yes if anyone is keeping track I did account for the size difference needed to get that 1:1 ratio, along with the new effective final drive ratio. I know it's not the right "tool for the job", but I think I'll have a lot of fun with it if I can make this work.
 
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It's on road, it's usually a timed run in a parking lot. I am looking to switch to a performance summer tire ideally.
Originally Posted by Kira
autocross is off road...or at least harsher driving? Could a snow tire work?
 
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It is illegal most places to run tires with lower than OE load ratings on the street. That said, other than getting the tires installed on the truck, no one will check. I ran 85 load rated tires on my focus(87 is OE) but they had a 150mph speed rating so they had no issues on the street. For autocross though I did have to run low 40's cold tire pressure to keep them off the sidewall which isn't really ideal, so for next set of autocross tires, I will get a tire with atleast an 87 load rating and maybe 89. Do you do many miles with this truck? If so you probably want to just get another set of rims and switch to those for autocross. The really sticky tires don't last that long on the street, plus you'll probably be heat cycling them in the summer so they'll give less grip before they wear out. I bought bfg sport comp2's for the Focus for my summer tire and they lasted 3 summers ~20k miles and maybe 24 events, which is pretty good, plus they had great wet grip. But they are still street oriented and not the best tire for autocross in any class though, and I could've been running the 200TW autocross specific tires in my class if I was serious about trying to win. I suspect with your suspension mods you'll be in a class where you can run even lower than 200TW tires and none of those are cost effective to run on a DD between wear and heat cycling. I'd look up what tire load rating 4200lb sporty cars have and stay there atleast, just to make sure the tires will work well at reasonable pressures.
 
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Originally Posted by IndyIan
It is illegal most places to run tires with lower than OE load ratings on the street. That said, other than getting the tires installed on the truck, no one will check. I ran 85 load rated tires on my focus(87 is OE) but they had a 150mph speed rating so they had no issues on the street. For autocross though I did have to run low 40's cold tire pressure to keep them off the sidewall which isn't really ideal, so for next set of autocross tires, I will get a tire with atleast an 87 load rating and maybe 89. Do you do many miles with this truck? If so you probably want to just get another set of rims and switch to those for autocross. The really sticky tires don't last that long on the street, plus you'll probably be heat cycling them in the summer so they'll give less grip before they wear out. I bought bfg sport comp2's for the Focus for my summer tire and they lasted 3 summers ~20k miles and maybe 24 events, which is pretty good, plus they had great wet grip. But they are still street oriented and not the best tire for autocross in any class though, and I could've been running the 200TW autocross specific tires in my class if I was serious about trying to win. I suspect with your suspension mods you'll be in a class where you can run even lower than 200TW tires and none of those are cost effective to run on a DD between wear and heat cycling. I'd look up what tire load rating 4200lb sporty cars have and stay there atleast, just to make sure the tires will work well at reasonable pressures.
That's a good suggestion. Haha I was hoping to run it all around though, and go with a 300-400 tw. I know the mods don't do me any favors class-wise between lowering and significant offset changes, but the idea of a all around fun vehicle interested me as it's already surprisingly well sorted stock. Without dumping significant money this vehicle is not going to be competitive in the class it'll need to run in, and I'm fine with that! I did check other similar or even heavier sportier cars and they seemed to still go for 97 load ratings, which is probably because of the significant payload rating differences. Not that that solves the legality aspect. Thanks for the input!
 
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It might be good if we knew both tire sizes - the OE and what you are going to use when autocrossing. Second, I am pretty sure you will that you will need to use more inflation pressure when autocrossing. Plus, there is a phenomenon called "Saturation", where the tire behaves non-linearly at the limit - and the larger the load carrying capacity, the better that is.
 
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Firehawk Indy 500's seem to have the edge on BFG sport comp 2's. Don't really know how they compare on wear but either will be quite the upgrade over all-season tires! Be aware that all the sticky tires have some sort of low temperature restrictions, like cracking rubber, so you may need winter tires anyways. I haven't really paid attention to see if anyone is running high performance all-season tires but my assumption is that they won't like alot of heat and will get greasy near the end of the run.
 
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Stock is 255/65-17. I'm still thinking on what size I want to put on, but ideally I will be close to 25.5 inches tall. Maybe something like 235/45/17 to match up to an 8.5 in width. I'm sure it will look a little funny, but I think the gearing should still be fairly driveable, and I have an ECU tune that can account for the significant size difference speed-wise. And I'm in Austin, TX, so I am not too concerned with running summer year round. My commute(if things ever go back to normal) is 0.4 miles and I don't drive that much otherwise.
 
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Originally Posted by wafflenator
Stock is 255/65-17. I'm still thinking on what size I want to put on, but ideally I will be close to 25.5 inches tall. Maybe something like 235/45/17 to match up to an 8.5 in width. I'm sure it will look a little funny, but I think the gearing should still be fairly driveable, and I have an ECU tune that can account for the significant size difference speed-wise. And I'm in Austin, TX, so I am not too concerned with running summer year round. My commute(if things ever go back to normal) is 0.4 miles and I don't drive that much otherwise.
Down 6 sizes?? Whoa!! I think you need to reconsider this. As I mentioned in an earlier post, there is a thing called "Saturation" - where the tire handling characteristics can become very non-linear, making the vehicle unpredictable at the limit. You would be there! How about going up in wheel diameter? And have you considered that you might have to modify the fender openings to clear the tires?
 
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235 sounds a little small, but I guess since you are buying to an overall diameter, you choices will be limited. Since you are buying rims, figure out what's the widest your tire can use as tires usually work better mounted on rims near the upper limit of width for the tire.
 
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Originally Posted by CapriRacer
Originally Posted by wafflenator
Stock is 255/65-17. I'm still thinking on what size I want to put on, but ideally I will be close to 25.5 inches tall. Maybe something like 235/45/17 to match up to an 8.5 in width. I'm sure it will look a little funny, but I think the gearing should still be fairly driveable, and I have an ECU tune that can account for the significant size difference speed-wise. And I'm in Austin, TX, so I am not too concerned with running summer year round. My commute(if things ever go back to normal) is 0.4 miles and I don't drive that much otherwise.
Down 6 sizes?? Whoa!! I think you need to reconsider this. As I mentioned in an earlier post, there is a thing called "Saturation" - where the tire handling characteristics can become very non-linear, making the vehicle unpredictable at the limit. You would be there! How about going up in wheel diameter? And have you considered that you might have to modify the fender openings to clear the tires?
Unfortunately I am a little limited on wheel widths with the Colorado bolt pattern. I am not sure I am following the saturation comment though. If I were overloading the vehicle then sure, but I am still well within the load limit for the sizing. Yes I lose some width compared to stock, but because I am not going with as tall of a tire I don't want it rolling over on the bead in a tighter turn. I know it' not apples to apples, but say we compare to a 2010 BMW 750li. The BMW is 600 lb heavier, but does have a slightly taller and wider (245/50/18) tire than what I am looking at. It does have a higher load rating slightly, so maybe going up to an 18 or so to get that 100 load rating vs 97 could be a good idea for me. And I don't mind making adjustments, but the 1:1 height to width is the key metric for me here. Hoping this doesn't come off as argumentative, I'm just trying to make sure I understand! And I don't believe the fender openings will need modification. Since I'm shrinking, even though the offset goes up, it's going to have a fairly large wheel well opening for clearance. This is the stock wheels with the lowering kit installed: [Linked Image]
Originally Posted by IndyIan
235 sounds a little small, but I guess since you are buying to an overall diameter, you choices will be limited. Since you are buying rims, figure out what's the widest your tire can use as tires usually work better mounted on rims near the upper limit of width for the tire.
Yeah I looked at that, but many of the wheels available for the platform are 8.5 or 9 (with 9 being more prevalent at 18+). Going over 235 on an 8.5 wide wheel may not be the best choice with a shorter tire unfortunately. I was hoping to keep things light since this is unsprung weight, and going with this was a significant weight savings while still being a reasonable size.
 
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As an additional note too, It looks like the kit did better than I expected for overall height. Measuring now puts me right at 66 in high, though I should probably get the exact in case any SCCA official wants to be certain. With that alongside my other plans I only need to drop 1.7 in additional height, for a change of 3.4 in shorter tire vs 5. Little more reasonable looking!
 
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Ok. Sorry for the multiple posts, I don't see an edit option now on my previous posts. So assuming my actual height measurement is accurate, I only need a 3.4 inch additional drop. If I stick to the 8.5 in wide wheel then I can go a bit wider and taller now. This gives me a bit more options now on load rating. Discount Tire has the Nitto NT555 G2 in 255/45/17 with a 102 load rating. I know that's a fairly low treadwear, but I have used it in the past on some of my "fun cars" and had pretty good results. It's still a 4 inch shorter shorter overall height, but maintains same tire aspect width so I'll be within the allowable ratio per regulations!
 
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Let me explain about Saturation: Taking your BMW and Truck comparison: When both of those go around a corner, there is weight transfer to the outside tires. The amount of weight transfered is a function of the center of gravity, which would be much higher for the truck. So the truck is going to be much more prone to unpredictable behavior because of the added load on the outside tire, even though the vehicles weigh the same. Watch this video:
Machines such as this measure the forces and the moments (torque) generated when tires are cornering. You get plots like this: [img]https://www.researchgate.net/figure...on-on-dry-wet-asphalt-and_fig1_325966391[/img] Notice how the blue lines (either one) peak then drop off, sometimes pretty sharply. For the vehicle to driving predictably, you want the curve to look like the red one. (OK, the plots aren't exactly about what is going on, but the principle is the same.) Using larger tires is better in preventing the lines looking like the blue ones. Notice how much reserve load carrying capacity is in the tires at the specified pressures compared to the GAWR's. That reserve is there partially to avoid saturation.
 
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Originally Posted by CapriRacer
Let me explain about Saturation:.
Thanks! That was helpful. I was looking at the research papers from Google and it was not working on my head.
 
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