| Author | Message | ||||||
     matt wawak (Chicagorovers) New Member Username: Chicagorovers Post Number: 3 Registered: 09-2003 |
Id like to fit 235/85R16 tires on my 1998 Disco. I was planning on using OME 751's in the front and 762's in the rear. Will I have to make any other changes??? Does this work??? | ||||||
| p (Pfb) Member Username: Pfb Post Number: 45 Registered: 08-2003 |
Much info in a recent thread here: ../2/31680.html"#333333"> | ken nishikawa (Scubaman99) Member Username: Scubaman99 Post Number: 88 Registered: 06-2002 |
Truck: 95 DI, OME MD springs & Shocks (10mo old), RN Sliders, Factory brush bar w/LR hella 4000's, QT diff guard on rear, rear swaybar disconnected OUT: Michelin XPC on Castor wheels IN: Goodyear MT/R 245/75/16 on stock LR steel wheels. ON ROAD 1. speedo now reads dead on (measured against GPS) where before with the "stock XPC's" the speedo would read aprox 10mph fast 2. MT/R's feel a little less "on track" truck has a tendency to feel like its wandering 3. eventhough it took a lot of lead to balance the tires, they feel nice and SMOOTH 4. MT/R's are just slightly louder then XPC's are at 50% tread 5. truck no longer fits in garage with new MT/R's. thus i will be swapping them out with my old XPC's on castor wheels... looks like these MT/R's will be my "off road set of tires" OFF ROAD... 1. for the last 4yrs of wheeling my disco i have ALWAYS used the stock XPC's. the difference between the two is NIGHT AND DAY... Much more then i had ever anticipated. flexi trails where i would get crossed and spin the old XPC's... i now dont have that problem. i think the extra inch of tire size coupled with the tremendous amount of traction have a synergistic effect and i was truly suprised by the difference. 2. Having run 245's now, i dont believe Anyone who says you can disconect your rear sway bars AND put 245 MTR's on your rig AND not need to trim your rear wheel well's. My personal exp has shown me that disconnected rear sb, running on relatively new OME MD springs and shocks (only 10mo old) and meaty new MTR's... will chew up your fender well BAD, when you run FLEXI trails(trails where you get extream amounts of articulation). my suggestion, if you get new 245 and you wheel Flexi trails... CUT your wheel wells BEFORE hand, or else risk tearing up your fender well sheet metal. the only thing i can think of for those who do run 245's on MD or stock springs is that your not running really flexi trails, cause 15min into Hollister i could already hear the "pop pop pop pop pop" of the lugs hitting my fenders. by the end of the day, i ended up tearing the sheet metal, ouch... 3. air down. "on road" i run the MTR's at 55psi (tires are rated to 80psi) initially i left them there and spun the wheels suprisingly easy. so i stopped and aired down to 30psi, the tire handled a much better but still had a little wheel spin here and there. stopped again and aired down to 25psi almost no wheel spin what so ever. my gut tells me i could have done 22PSI +/- a few pounds in either direction and would have been fine, but i didnt want to give up to much tire height, so i left it at 25PSI as a nice compromise THings learned... MTR's are good on road and GREAT off road, but they will make short order of your fenders if you dont trim or "volumize them". hope this experience/info helps!!! KEN btw: check us out at:http://autos.groups.yahoo.com/group/norcalrovers | ||||
| Milan (Milan) Member Username: Milan Post Number: 241 Registered: 10-2002 |
Ken, I run my 245s (also rated at 80 for max load) at 44psi rr and 32 frt. I'd imagine the 235/85 have similar if not bigger volume of air in them, thus requiring less pressure than 55psi even on the road (unless, of course, you carry more weight than stock). | ||||||
| Joe M. (Little_joe) Member Username: Little_joe Post Number: 173 Registered: 11-2002 |
Crap, what am I doing wrong! I run 245/75 MTRs Load Range E at 26 and 34 pounds and they're still rock hard! I did the chalk test and this is optimal contact patch. Gas mileage has remained constant and no funny wear............ Those high pressures seem crazy to me? joe | ||||||
| Ho Chung (Thediscoho) Moderator Username: Thediscoho Post Number: 309 Registered: 02-2002 |
joe, it's a jeep thing, we wouldn't understand. Ho Chung | ||||||
| Joe M. (Little_joe) Member Username: Little_joe Post Number: 174 Registered: 11-2002 |
Huh? joe | ||||||
| Ho Chung (Thediscoho) Moderator Username: Thediscoho Post Number: 310 Registered: 02-2002 |
"WRANGLER" MTRs...  Ho Chung | ||||||
| Rob Davison (Nosivad_bor) Dweb Lounge Member Username: Nosivad_bor Post Number: 907 Registered: 02-2003 |
what does the side of the tire say? 26 seems way too low for an E rating rd | ||||||
| Joe M. (Little_joe) Member Username: Little_joe Post Number: 175 Registered: 11-2002 |
""WRANGLER" MTRs... " Duh, sorry. I am so dim sometimes lol! Rob, I haven't even looked at the sidewall. Max pressure is 80psi. I used chalk to find optimum onroad contact patch, starting at Rover's recommended psi for the XPC's. I don't carry heavy loads so this pressure works, based on contact patch. FWIW I run 24 and 20 onroad on my Load Range C MTR's on my other truck - the sidewalls are incredibly stiff on this tire. joe | ||||||
| Milan (Milan) Member Username: Milan Post Number: 242 Registered: 10-2002 |
I did mine by calculating the lb/psi load of the stock tire on a stock vehicle. I then calculated how many psi it would take on the new tires (with different lb/psi ratio) to carry the same weight. Viola. | ||||||
| p (Pfb) Member Username: Pfb Post Number: 47 Registered: 08-2003 |
Huh? The number of pounds your tire supports is only dependent on vehicle weight and psi. Load rating or tire design has (measurably) nothing to do with it. If your Disco weighs 4,500 pounds and your weight distribution is 25% on each tire (which it isn't), each tire supports 1,125 pounds. At 80 PSI, that equals a 14 inch contact patch. At 40 PSI that equals a 28 square inch contact patch. At 20 PSI, a 56 square inch contact patch. Doesn't matter if you are running light weight p-metric tires or LT E-range. Doesn't even matter what size tires you are running, although wider tires will change the shape of the contact patch. Larger contact patches can improve traction, but lower pressures can cause more sidewall rolling. Like most things in life, a tradeoff. | ||||||
| Milan (Milan) Member Username: Milan Post Number: 244 Registered: 10-2002 |
If a 235x70R16 tire carries 2300lbs at 50 psi (40 lbs/psi), it can be run at less psi than a 245x75R16 that carries 3000lbs at 80 psi (37.5 lbs/psi). So the smaller tire can carry more weight per psi even if it's load rating is less. Now, if 235x70R16 needs to be at 26 frt/36 rr psi to carry my disco, then it carries 26x40 lbs frt/36x40 lbs rr, or 1040 frt/1440 rr. This means that 37.5 lbs/psi tire will need 1040/1440 : 37.5 or 27.73/38.4 (with rounding up 28/39) psi to carry the same weight. These numbers here do not correspond to the actual tire figures, so the results are different than what I actually run. But the calculation is the same. I never did mention load rating had anything to do with anything but ultimately a tire with the higher load range can carry more (at max). Also it's not the pressure that carries the tires but the volume so typically bigger tires can run less pressure to carry the same load. But this only works if they have same rating and especially if the bigger tires are rated at more load at same or very similar pressure to the small tires. But I like to go by the numbers on the sidewall as I trust the people that built the tire to know how much it can carry at what pressure (given the tire's volume X). | ||||||
| p (Pfb) Member Username: Pfb Post Number: 48 Registered: 08-2003 |
I see what you are saying, but I don't agree with your conclusion that a higher load-rating tire needs higher pressure than a lower rating tire to carry the same load. It's an interesting argument, but I don't think you can extropolate those max psi/max load numbers to create a meaningful new metric: Max pounds per psi. As demonstration, consider this example: If you pumped up your stock 235/70 front tires to 31 psi (31X40=1240) and lowered your rears also to 31 (31x40=1240) would you then have perfect 50/50 weight distribution? No. You'd have a bigger contact patch in front, smaller contact patch in rear, and the same 60/40 weight distribution you had before you modified the tire pressures, with the fronts holding more weight with the same psi and a bigger contact patch. Is your position that to do this would exceed the manufacturers spec for the tires? | ||||||
| Milan (Milan) Member Username: Milan Post Number: 245 Registered: 10-2002 |
I had a long reply typed up but did not get to post it before my machine crashed. Basically I believe your example does not exceed manufacturer's recommendations and no the weight distribution doesn't change but you'd end up wilth larger contact patch in the rear than the front. Simply because you now have less volume (and thus pressure) to carry the same weight. (In your example you have more weight in the front? and thus the patch would be bigger in the front I guess - not sure if this was a typo or what) My position just has to do with comparing apples to apples. If one tire has load rating X lbs at Y psi it's not the same as W lbs at Z psi and therefore you have to take the ratings into consideration when calculating the pressure for a specific load. I only used the stock tires to come up with some weights as I had no idea how many actual lbs the new tires needed to carry. Bigger tires typically have bigger ratings as they have more volume capacity and thus can carry the same weight with same amount of air as little tires but at lower pressure. The 245 is a heavy tire with thick sidewall so probably similar internal volume as the 235, thus needing similar pressure to carry the same load. Because of it's construction, it may need more air for a given load (I'm not sure why they are rated the way they are, I was shocked at the numbers as I expected the same load - over 3000lbs at less psi than the stock tire), but being tough it can take more pressure and thus has a max load higher than the stock tire. | ||||||
| Joe M. (Little_joe) Member Username: Little_joe Post Number: 176 Registered: 11-2002 |
I'd like to see your contact patch - is it flat, feathered, bowed? The math is spiffy, but if you're riding on only the center lugs due to overinflation (based solely on your calculations), those tires won't be good for long. Also Milan, another question - how can you compensate for differences in composition and material of the tire? A stiffer sidewall should require less pressure to maintain load X where tire y required higher pressure. This is an interesting discussion. I hope my piece of chalk comes out of this ok, all this math hurts my head. ;) joe | ||||||
| Milan (Milan) Member Username: Milan Post Number: 246 Registered: 10-2002 |
I compensate for the composition of the tire by reading the load ratings on the sidewall as I can't possibly know what composition they are or how that composition affects the load carrying capacity of the tire (other than the obvious number of plies and material of cords). I was surprised that the 245 has to be inflated to 80psi in order to carry just a little more than the 235 at 50 (or whatever those numbers were). Stiffer sidewall could also just mean more durability, less flex (= less heat build up), more resistance to punctures and ability to hold air at higher pressure without deformation. After all it's not really the sidewall carrying the load but the air in the tire. Hence my relying on the numbers rather than a rule of thumb. I don't really care about the contact patch much - unless it's extreme in one direction or the other. By looking at the tires the contact patch is fine . Just by feel, I went from 35psi in all 4 (put in by the tire shop) to 32 and 44 and the ride has improved. It became softer over bumps (obviously) and not as squirmy in the rear around corners (not that it was bad with those stiff 245s). Overall it became more like the ride on stock tires, only these stick to the pavement better. After the test drive I stopped worrying about the pressure being seemingly "high". FYI, I use the chalk method myself, but I typically just calculate how much pressure I should run based on the load rating and based on the weight of my vehicle. My 35" MT/Rs should supposedly be run at 16-18 psi to carry my Jeep but I run them at 22-24 as I'm "saving" the outside lugs. I've had them for almost 3 years and 15000km (mostly off-road abuse) and they're still 80% there, wearing quite evenly. Those that know me, are aware that I actually prefer low pressures. But I'm not going to ruin a tire by running it too low on the road. Heat kills the inner cords and allows for belt separation from the inside without any visible damage before a blowout. Rock hard tires are in my opinion bad from ride and performance stand point (though stiffer is beter for faster vehicles) but safer than underinflated ones. I'm not advocating anything, I'm just suggesting people look at the tire load rating and particular application before just pumping the tires to some pressure based on the tire size. I'm sure the chalk method is more accurate than using tire size. | ||||||
| Chris W. (Dcwhybrew) Member Username: Dcwhybrew Post Number: 86 Registered: 05-2003 |
"P", I think your example is reversed(??). Given equal tire pressure, the bigger contact patch would be in the rear. You'll notice that the owners manual calls for greater air pressure in rear rather than the front. It's my understanding that the discos are heavier in the back. Correct me if I'm wrong. | ||||||
| p (Pfb) Member Username: Pfb Post Number: 49 Registered: 08-2003 |
Right, I reversed it. The point was that contact patch is directly related to psi and weight, and that the psi is the only variable you can control. Max PSI or max load can't change this, and you can't shift weight by altering psi. Disco is rear heavy, so with equal psi in all tires, a larger contact patch in the rear. | ||||||
| Milan (Milan) Member Username: Milan Post Number: 247 Registered: 10-2002 |
Now I see what you meant. As long as you're under max load and max pressure, I believe you should adjust the pressure based on the load. This should result in optimum contact patch for the road. | ||||||
| ken nishikawa (Scubaman99) Member Username: Scubaman99 Post Number: 90 Registered: 06-2002 |
hey all the reason i run such high pressures on my MTR's is because 55psi "on road" is the pressure that gives me the "road feel" that i want... it seems the difference between 50 and 55 is (unscientifically) very big... 50 feels "squishy" and when i drive at hwy speed steering feels loose. when i air up to 55 it feels more like the stock XPC's. off road the diff between 40 and 25 is hugh... no wheel spin. sorry im not as "technical" as you guys... i never measured with chalk... i just go by what "feels right". so according to your calculations... what should i be running on road and off road? | ||||||
| Sergei Rodionov (Uzbad) New Member Username: Uzbad Post Number: 36 Registered: 08-2003 |
eek.. 50 on road would feel like you riding wooden wheels actually  . Out of fun i tried yesterday to run 38/44 and it felt like washboard on local paved road. Aired it down to 32/36 - and decent ride is back. In fact if you go into owners manual there are suggested pressures for tires, and bigger tire - lesser pressure (if i remember correctly 235/70 already suggested to be like 36 in rear). Thing is - you can air up to whatever your tire would be able to handle, and improve mileage, but then so would be to run on non-iflatable tires As of off-road it depends again on kind of terrain , and type of tires. I normally run 18/20, 16/18 (rarely) on XTerra with 31x10.5.x15, and with new tires on Rover (still waiting on my 235x80x16 , they got rerouted) - it will be about same. But thats pressure for local Colorado trails. 20/22 would be a bit harsher, but thats what i got on my Rover's XPCs to get test ride on small uniproved road here, and it was acceptable. Not cushy, but then if i wanted cushy i wouldnt sell my crappy GrandCherooke Rule of thumb for me is - try to avoid slippage on trails b/c of overinflation, as it tears them up, and then they got closed It is enough for all those irresponsible ATVs to make mess... Tread lightly | ||||||
| ken nishikawa (Scubaman99) Member Username: Scubaman99 Post Number: 93 Registered: 06-2002 |
eek.. 50 on road would feel like you riding wooden wheels actually.... Nah, thats where if finally stops feeling "squishy" at highway speeds. less then that, my rig "wanders" all over the hwy | ||||||
| ken nishikawa (Scubaman99) Member Username: Scubaman99 Post Number: 94 Registered: 06-2002 |
better said... the MTR's at 50-55 "feel" like XPC's at 33-35 (my daily driver tires which i swap out after wheeling) im not sure but i think maybe why you can run your Xterra at such different pressures is the weight difference between Discos and Xterras (not really sure, as i dont know how much Xterras weight, but i think they are lighter then Discos | ||||||
| Joe M. (Little_joe) Member Username: Little_joe Post Number: 177 Registered: 11-2002 |
Ken, there'll always be disagreement over this, so run what you feel comfortable with. If those pressures are what feels best to you and since they are safe, run them - but understand that it may lead to premature wear of the center lugs / no wear on the outers. Milan has given us great info, take the time to digest it and it may help you. I prefer a nice flat contact patch with even wear across the treads, so I use the chalk method. I accept the tradeoffs, but frankly I don't notice any squirming or squirelly handling. The psi I run seems to be safe as well. Offroad psi is a whole other endless argument. At a minimum, I would suggest that a flat contact patch and possibly some bulge would be preferable - that'll let the treads and sidebiters work for you. Re weight differences, it does play a huge role. On my Toyota p/u w/ 33" MTRs, offroad at 12psi is the sweet spot. But that truck is so light, that's what it takes to get the contact and bulge and ride I prefer while still holding the bead. Especially if you have onboard air, it's a great idea to experiment and make adjustments to see what works best. joe | ||||||
| Milan (Milan) Member Username: Milan Post Number: 248 Registered: 10-2002 |
I run my Jeep at 6-8lbs off-road. But it's on 35s and weighs 500lbs less than the Disco. The Disco I ran at 12lbs with the XPCs but we'll see what the MT/Rs will want. | ||||||
| Sergei Rodionov (Uzbad) New Member Username: Uzbad Post Number: 39 Registered: 08-2003 |
They weight about same actually.. (Actually i think X is wee bit more now, as it got heavier M/Ts, rack, skids and sliders) If you got that "wandering" feeling it normally caused by worn suspension and steering, rather than tires | ||||||
| Rob Davison (Nosivad_bor) Dweb Lounge Member Username: Nosivad_bor Post Number: 924 Registered: 02-2003 |
it depends how loaded down the truck is, bumper winch , sliders, plates, big ass tires , rims it all adds to it. my futura 235/85's i used to run were approved to 90psi with the manufacturer. squishy feel was reduced and rolling resistance seemed better. | ||||||
| Brian Dickens (Bri) Senior Member Username: Bri Post Number: 692 Registered: 08-2002 |
The wandering feeling has to do with a TON of things. Of which warn suspension components and toe are a couple. Expect to get wandering feeling increased with a lift due to loss of caster. There is a ton of stuff on that in the archives. |
