Hub to Drum Gap

Anybody know what the official limit is for space between the rear hubs and drums is? Is it OK for no slack on one side of the drum center and some gap at the other side of the hub? I'm thinking this might cause some vibration when braking to a stop. The wheels are 2WD 2021 SR5 non-steel with the flat lugs with washers, so I'm assuming all the measurements around the hub should be pretty well centered and snug.

 

Welcome to the forum

A picture truly is worth a thousand words -- post a few of what your concern is.

 

I'll take pictures when I take the wheels back off. In the meantime, here's some additional explanation that might clarify the question. Several years ago, there was a warranty fix for low speed braking vibration when the axle flange boss diameter was out of specification (4.252 in. – 4.250 in.). In my case, the axle flange boss is in spec, so I'm looking for the specification between the flange boss (hub) and the big hole in the middle of the brake drum, because it appears to be around .02 inches in some areas and .00 inches in others. My assumption is an out of spec drum might cause similar symptoms since the wheels are not lug centered.

 

You are right that the drum should fit snuggly on the hub, I've never run into a hub bore on a drum being an issue, but I have 100% run into drums that cause out of round conditions and the wheel to even shake side to side.

The hub bore being slightly off would not cause a pulsation technically. The shoes would wear into the pattern set by the drum even if a few thousands of an inch out.

The feeling of a drum pulsation I usually describe as a "bucking" feel from the rear. And zero zero zero feed back from the steering wheel.

You can usually simulate it by using the hand brake on its own to slow the truck down and see if the pulsation is consistent.

 

Yes, the same relatively mild relatively high frequency vibration occurs with only the hand brake. It just seems to me that if .002 inches is the maximum for the hub, then a much larger deviation of the drum bore might duplicate the symptoms. That's why I'm fishing for the spec range for the drum bore. If the wheels were lug centered I would be looking elsewhere.

 

Looks like your gap is also greater than .002 inches, so I guess nobody needs pictures from me now. I've now heard that a larger gap than the hub runout range may be required for heat expansion. I think maybe the drum centering is finally accomplished at lug torque time, even for hub centered wheels. I will see what rotating the drums across the lugs might do and maybe even a few other things, but I'm beginning to think Toyota expects Tacomas to be driven on rough enough and bumpy enough roads to where only a few drivers will notice the low speed braking vibration.

 

Rotating the drums across the lugs may have softened the vibe a tiny bit, but not sure. So assuming the wheels are an accurate 108 mm fit on the hub, I now need to be sure that the beveled edge at the end of the flat lug nuts with washers reaches all the way into the lug holes on the drum when going through the wheels. Of course if the lug nuts are too long past the washer, the wheels won't get tight, but if the lug studs are allowed to move in the drum holes because the lug nuts are too short, then the drums can't be accurately centered at the hub if their center bore allows more slack than the other fittings.

So the short (and last) question is: do the beveled edges of the lugs fit into the lug bolt drum holes when torqued to 83 lb.ft. ?

 

If I am reading this right, you are suspecting that a vibration at the rear is a drum moving around on the hub when the wheel is torqued to it, or the lug holes are not deep enough allowing the lug nut to bottom out on the drum, allowing the rim to move. In either case you would see obvious signs of movement between these parts. Metal shavings and shiny areas of metal. Even if the drum centre bore was beyond to 0.02" spec, that is just for centering it initially, it is held together by the large mating surface of the axle and wheel, sandwiched but the torque of the 6 lug nuts and stubs. That thing is not moving after proper assembly. Also the shoes float a bit and will take up mild off centre issues without any issue.

If you are getting a vibration during a service brake application and can reproduce it during a park brake application, then I would suspect you have brake surface or brake shoe issues. These are the things that are moving and mating during these events...

 

more than u ever wanted to know about tolerances and fits

easy-ish source
https://amesweb.info/fits-tolerances/ansi-preferred-tolerances-fits-charts.aspx

real source
ANSI B4.1

 

Have you tried having the drums turned at a machine shop?

 

A drum center bore gap of 0.02" is 10 times the allowed deviation of the hub. That's why I'm thinking the lug nut bevels should exist in order to slip part way into the big gap between the lug bolts and the drum, so long as the wheel can still be sufficiently tightened. If the lug bolts are the only thing minimizing the tendency of the drum to twist during braking, I think the drum will become out of balance as the gap between the drum center bore and hub will shift off center during braking. It may not amount to much, but I haven't seen any vibration at all like this until these drum brakes.

 

Turns out my thinking was wrong and you are correct. The lug nuts do not reach the drum, apparently meaning Toyota doesn't care if the lug bolts ride up against their drum holes and the drum center bore rides on one half the hub and gaps on the other.

Thanks for everybody's thoughts and analyses.

 

I don't understand...

 

There is nothing but lug nut torque preventing the drum gaps from pushing the drums off center during braking.

 

As I stated in my previous post, there is also surface area where the drum makes contact with the axle flange, also the contact to the wheel. And this is all sandwiched together by the torque of the lugs. There is more in play than just the lug torque preventing the drum from moving during braking.

You have obviously removed the wheels and drums, while you were in there, did you see a bunch of metal shavings or any fresh shiny areas on the drum, or the axle where they meet? These are signs to look for, for drum movement.

 

I assume 'more in play' means more than shoes. I don't have shavings or shiny areas. I'm beginning to suspect an almost imperceptible vibration that doesn't seem like an important problem, but happens when coasting at low speed, that could be getting amplified to an annoyance when braking firmly. Another puzzle is difficulty getting the lug nuts to torque evenly. Working up to 83 lb.ft. from 20 to 40 to 60 to 80 to 83 back and forth all around leaves the last 1 or 2 feeling like they're going too far without clicking. Then I have to loosen the nuts and try different patterns until they all feel right. The wrench is not Snap-On but has been matched to a Snap-On. This happens whether the drum has come off or not, and the drums are always all the way on before tightening. I haven't yet verified that the transmission was filled properly at the factory, but the dealer has performed the 6 and 12 month service. Maybe I need to check the front brakes and drive shaft area. The rear wheels look like they're running true when following from behind.

 

You stated you could duplicate the vibration with the hand brake.

Try having the drums turned at a machine shop, then you will know the friction surface is true and round.

 

The first sign of movement or play is shiny metal and shavings. The wheels are a weight bearing component of the truck. If there was movement or play from too much clearance you would see it. So if you don't see those signs... than you are fine there.

Now torqueing your wheels... there is no need to go in steps. Press the wheel into place with your knees, start the lugs by hand, snug them up a bit with a wrench or lightly with power tool in a star pattern. Put the wheel back on the ground and torque the lugs to spec in a star pattern. No need to go up by 20 each time. I always go over them quickly a second time to make sure I didn't miss one... also I always torque the lock nut last and remove first, always by hand tool, never with a power tool, they aren't designed for that type of impact.

Now this slight vibration, it is there even without braking? And then gets worse while braking?

I just read another post about a TSB describing something similar... I will see if I can find it...

Edit: https://www.tacomaworld.com/threads/brake-shutter-at-low-speed-stopping.767836/#post-27409296

 

O.K. I will always put the wheel on the ground before final tightening. The coasting vibration without braking has always seemed quite small and trivial, just because it's a truck. I didn't pay any attention to it until I began to suspect the brake shudder might be emerging from it. If there is something Toyota can do I want to take care of it asap, but it looks to me like the TSB you pointed to is for 4WD only. My SR5 is 2WD. If I find a solution, I will post it to the forum. Thanks again.