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Can someone show the math on how the brg load is increased with a reduced bs or an increased bs or use of adapters/spacers? I doubt any adapter/spacer under a couple inches will increase the load on the brgs to the point it will fail let alone a 1/4" spacer.
Can someone show the math on how the brg load is increased with a reduced bs or an increased bs or use of adapters/spacers? I doubt any adapter/spacer under a couple inches will increase the load on the brgs to the point it will fail let alone a 1/4" spacer.
That's part of the discussion. But without the math comparing the two loadings, oem wheel vs use of spacers/bs'g, its all speculation on how it affects the brgs and if the change is significant enough to cause excessive wear.
That's part of the discussion. But without the math comparing the two loadings, oem wheel vs use of spacers/bs'g, its all speculation on how it affects the brgs and if the change is significant enough to cause excessive wear.
I don't know the exact math off the top of my head but I do know that anytime you cantilever a load the force goes up exponentially. I will try to find the math and post it here. #becausephysics
The actual math for cantilevered loads on bearings. This is entirely academic however unless you know the specific load ranges of the bearings you are using.
The 2:1 principle for linear plain bearings and the resulting binding is a product of friction.
Working through the following equation will explain why this is a product of friction:
P = force being applied
L = distance out from shaft that P is being applied
s = center to center spacing of bearings
f = resultant force on bearings by shaft
F = friction force on each bearing
µ = coefficient of friction (about 0.25 when not moving)
Balance the Moments: f * s = L * P
L / s = f / P
Compute Friction Force: F = f * µ
Note: Total friction force pushing up is 2 * F. To lock up the slide, the total friction force must be equal to (or greater than) P.
P = 2 * F = 2 * f * µ
Substitute for P: L / s = f / ( 2 * f * µ ) = 1 / ( 2 * µ ) = > L / s = 1 / ( 2 * µ )
Note: The forces drop out of the equation
Assume static coefficient of friction is 0.25 (µ = 0.25) then L / s = 2. That is the 2:1 ratio.
There are many other factors that add to the braking effect, but the coefficient of friction is the main cause.
Note: Added lubrication can help to drop friction and extend the 2:1 ratio.
I am a giant physics geek and love anything that has to do with the coefficient of friction. My next tattoo actually has the formula for it included.
The end result is that any time you move a wheel past where the bearing was designed to handle the load you will increase friction, increase bearing wear, and shorten bearing life however in practical application using a reasonable sized spacer is likely going to produce such a minor difference as to be unnoticeable. Really, how many people here are putting 100k miles on their C3 to where that might become an issue?
Last edited by PainfullySlow; 11-09-2017 at 09:53 AM.
When a load acts eccentrically on a bearing, a tilting moment occurs. Double row bearings, e.g. deep groove and angular contact ball bearings, can accommodate tilting moments, but paired single row angular contact ball bearings and tapered roller bearings arranged back-to-back, are more suitable
When two tapered roller bearings are are pre-loaded, such as our rear wheel bearings, then loading characteristics are different. The math is beyond my comprehension these days but maybe you take a good look. A really good article for mechanical engineers.
I'm looking at it this way. If the roller brgs are preloaded, then the load at the hub which is transferred by the weight of the car through the wheel attached to the hub, may not matter what the offset of the wheel is because the load is still at the hub/wheel interface and by design, the bearing preload can sustain an added 'moment' on the bearing created by offset.......up to a point of course. A few inches either side of the hub may not matter i.e moment loads.
On 15x8 rims a lot of people seems to be running 255/60. Does this require the parking brake bracket to be relocated? Or 255 still fit without moving it?
Can someone show the math on how the brg load is increased with a reduced bs or an increased bs or use of adapters/spacers?
Assuming the same width of wheel in all cases:
Changing from one wheel to another wheel with the same backspacing doesn't change the static loading on the bearings.
Changing from one wheel to another wheel that uses a spacer to create the same backspacing doesn't change the static loading on the bearings.
Changing from one wheel to another wheel that has a different backspacing does change the static loading on the bearings.
I generally don't like backspacing because the backspacing number is useless by itself. When talking about changing the wheel width, I DESPISE using backspacing. As an example, changing from a 7" wheel with 3.75" backspacing to a 8" wheel with 4.25" backspacing both put the center of the wheel at the same location relative to the axle flange. The backspacing did change yet the loading on the bearings did NOT change.
I prefer offset. Offset is the measured difference between the axle flange and the wheel center line. Positive offset means the axle flange is towards the outside of the wheel with negative putting the axle flange towards the inside of the wheel. In the above example, both wheels have the same offset = -0.25".
Originally Posted by resdoggie
I doubt any adapter/spacer under a couple inches will increase the load on the brgs to the point it will fail let alone a 1/4" spacer.
No spacer will cause bearings to fail. A large enough change in the wheel offset might cause bearings to fail.
Once bolted together, you can consider axle flange and wheel to be a single solid component. So, how the material between the axle flange and the wheel hoop is configured simply does not matter and has no effect on the static load the bearings see. Only how much the wheel hoop center line is offset from the axle flange matters when considering static bearing loads.
The way dynamic loads such as hitting bumps or cornering forces are transferred into the bearings can be affected by both the wheel offset and the wheel width. These are the forces to actually worry about because they will be much higher than any static force. Hitting a large bump while turning puts way more force into the bearings then driving straight on a smooth surface.
A wheel with both more width and more negative offset will impart much different forces into each bearing when the tire hits a bump on the outside edge compared to the wheel it replaced with less width and less offset.
Generally speaking, I'd be more concerned about spindles or axles bending or breaking then the bearing failing if I was making large changes in wheel width and wheel offset.
Last edited by lionelhutz; 11-09-2017 at 01:26 PM.
Great conversation and I can't help but put my .02 in. The hub bearings are subjected to a multitude of stresses and are designed to keep a shaft spinning around a static axis relative to the trailing arm. I find it easier to look at the stress on the end of the axle (shaft) in the bearing area. The shaft is the connection between the u-joint and the wheel and can be under tensile, compression, torsional and shear stresses. Moving the tire contact patch center point away from the centerline of the bearing section of the shaft will increase the stresses on that part of the shaft and the bearing.
But many of the things that have been discussed in this thread will do the same thing. Increasing the ability of the car to accelerate laterally will increase these stresses as well.
In my opinion using a spacer to bring the contact patch back to the original area will not affect bearing performance, using one to move the tire patch out from the original orientation will. But anything that increases the ability of the car to laterally accelerate (turn) probably adds more stress than a spacer would. Making the car turn better and moving the contact patch away could result in a multiplication of stress.
On 15x8 rims a lot of people seems to be running 255/60. Does this require the parking brake bracket to be relocated? Or 255 still fit without moving it?
I ran that tire size for YEARS and nothing needs to be "relocated".
A move up to a 275 might......and beyond that you probably need the offset rear arms.
The short answer is that 17/18 inch tires will make a HUGH difference in the way the car not only handles and steers BUT will actually ride BETTER than the crap tires available in the 15 inch size. Anyone who says the change is not that noticeable is either sleeping or put on a junk 17 inch tire in place of the BFG radial TA or another S/T speed rated 15 inch tire.
The 17 inch Z rated tire is made with modern design and technology, will have a lower aspect ratio, will have generallly softer rubber for superior traction in the dry and wet, and is MUCH stronger than the typical 15 inch select few tires available for a C3.
I have upgraded/changed every single component in my C3 suspension over the last 30 years and finally gave up on the 15 inch wheels since I could not find a decent 15 inch tire that was not a raised white letter standard passenger car tires offered on a Toyota Corolla that had even moderate performance...in fact the modern corolla has BETTER tires than a BFG TA 255/60/15 tire.
I now have 255/45/17 ZR tires that are fantastic AND offer a superior ride to my former BFG TA'a.
If all you want is the looks of the ballon 255/60/15 with zero performance, a wandering steering and bouncy ride, then stick with the 255/60/15's. If you are looking for a noticeable improvement in performance traction, dry and wet braking, superior steering response, and a BETTER ride.....GO 17/18 ultra high performacne ZR/W/Y rated tires NOW!
Or get some Mickey Thompson 255/60r15 ET Street S/S drag radials for the rear and 245/60r15 sportsman S/T (better than BFGs) for front, and you can skip the harsh ride and mismatched looks of larger wheels with skinny side wall tires on your C3. Nascar and other race cars/venues still use 15 inch wheels/tires for good reasons.
I went from 15 inch BFGs to 255/50r17 (Nitto Invo) on my 68 due to traction issues, but I did not notice a significant difference in side wall flex/swaying when cornering. Yes it handled better, but mostly due to the grip of a modern tire design; The tire/wheel swap was a trade off of a harsher ride for better traction. By the way, I bought and built my Vette not just for looks; I have a 400whp LS/T56 swap with 3.73 gears, and about 500lbs in weight reductions, and love to push the limits. After having the 17s for a few years, I then wanted more traction and tried 295/45r18 Nitto NT05 on the rear; they did stick very well, but the very noticeably harsher ride and extra rotating weight (slower acceleration) quickly got to me.
There is always going to be different opinions on wheel/tire looks, but I personally think old school on old school looks the best. Compare my 17/18 inch set-up with this classic 15 inch set-up. I think they both look good, but the 15 inch looks/fits better IMO. By the way, I was born over 10 years after my 68 Vette, but I have never been a fan of the contemporary trend of big and flashy aftermarket wheels.
Whilst changing to an 18" rim has its issues - like for like and size for size vs a 15" tyre the lower profile 18" 40/45 section tyre carcass will have a much increased contact point with the road surface. This may create issue with fender lips and chassis rails. The ride will be noisier and less comfortable. On the 'plus' side the car will feel as though its had a complete suspension change. Cornering will improve dramatically - as will traction and braking - keep those dinosaur rims and tyres for another day.
Whilst changing to an 18" rim has its issues - like for like and size for size vs a 15" tyre the lower profile 18" 40/45 section tyre carcass will have a much increased contact point with the road surface. This may create issue with fender lips and chassis rails. The ride will be noisier and less comfortable. On the 'plus' side the car will feel as though its had a complete suspension change. Cornering will improve dramatically - as will traction and braking - keep those dinosaur rims and tyres for another day.
Yeah, but you can find some good "dinosaur" tires with a good contact patch and which stick even better than modern 17-18in tires; Mickey Thompson's new Street S/S and R drag radials perform like a street tire, but with a much stickier compound. I would actually enjoy driving my Vette around more by not having to cringe at every bump in the road with having skinny and stiff sidewalls with 17-18 tire sizes; Plus you get worse acceleration and braking due to a heavier wheel/tire combo with more rotational mass. Its more about the tread compound/design rather than the wheel size that makes the traction difference (handling, braking, acceleration). Plus if you want good straight line traction, your much better off having some give in the sidewall with a 15 inch tire to help transfer the weight and power, rather than spin too easily on a stiff tire.
Even in a 17" tire that is the same diameter (27") as my 15s there are not many choices. 255/50R17 is the same and 245/50R17 is 26.6". Very few choices and I really don't want to go to 18".
One of the choices is Nitto. Anybody have anything good or bad to say about them?
Nitto makes a good tire. I have the 255/50r17 Invo on the rear of mine and it grips good, and might be the most comfortable 17 inch tire (according to their site). I was contemplating going with the NT555 G2 due to the slightly better grip and looks, but I cant say anything about those from my own actual experience.
Or get some Mickey Thompson 255/60r15 ET Street S/S drag radials for the rear and 245/60r15 sportsman S/T (better than BFGs) for front, and you can skip the harsh ride and mismatched looks of larger wheels with skinny side wall tires on your C3. Nascar and other race cars/venues still use 15 inch wheels/tires for good reasons.
I went from 15 inch BFGs to 255/50r17 (Nitto Invo) on my 68 due to traction issues, but I did not notice a significant difference in side wall flex/swaying when cornering. Yes it handled better, but mostly due to the grip of a modern tire design; The tire/wheel swap was a trade off of a harsher ride for better traction. By the way, I bought and built my Vette not just for looks; I have a 400whp LS/T56 swap with 3.73 gears, and about 500lbs in weight reductions, and love to push the limits. After having the 17s for a few years, I then wanted more traction and tried 295/45r18 Nitto NT05 on the rear; they did stick very well, but the very noticeably harsher ride and extra rotating weight (slower acceleration) quickly got to me.
There is always going to be different opinions on wheel/tire looks, but I personally think old school on old school looks the best. Compare my 17/18 inch set-up with this classic 15 inch set-up. I think they both look good, but the 15 inch looks/fits better IMO. By the way, I was born over 10 years after my 68 Vette, but I have never been a fan of the contemporary trend of big and flashy aftermarket wheels.
Both cars look great in my opinion...I give the nod to the blue one because of all the modern mods...Cars got to be a blast to drive....
Question,how tall is that rear tire on the blue Vette? I run 3:70s with a 4 speed.....I know with a taller tire the car will drop a little in RPM on the highway....Not a lot, but every little bit helps..
Both cars look great in my opinion...I give the nod to the blue one because of all the modern mods...Cars got to be a blast to drive....
Question,how tall is that rear tire on the blue Vette? I run 3:70s with a 4 speed.....I know with a taller tire the car will drop a little in RPM on the highway....Not a lot, but every little bit helps..
Thanks, yes the car is a blast to drive around solo through curvy roads, and then blow away every import wanna-be racer that Ive run into on the street (fingers crossed). The rear tire height on the blue Vette in that picture is 28.25 inches (Nitto NT05 295/45r18). They are pretty much the tallest tire you can go with, and I still had some rubbing issues. I now have my 255/50r17 Nitto Invos (27.0 inches tall) tires back on the rear, and no rubbing issues at all.
11-09-2017, 08:49 AM
