Brake caliper bracket mount - single shear bolt question

[71dsp]

I'm installing a set of front brake calipers on my track car, and the mounting brackets are made of aluminum (7075).

What is the better mounting scenario - Thread the mounting bolts directly into the aluminum, or use a through hole mounting method with a proper bolt and nut combination? The bolts are orientated perpendicular to the brake disk, that is, they would be loaded in shear.

If I thread the bolt into the aluminum mount, the amount of tension I can preload the bolt with depends on the aluminum, as I'd expect the threads to strip before the bolt yields. If I use a through hole, I can tension the bolt properly.

If I mount the bracket using a through hole, would the bolt need to fit very tighly on both the knuckle and the mounting bracket? Is there an issue with applying to much tension to the bolt and causing the bracket to deform?

I should mention that the OEM caliper mounting bolt is M12 x 1.25 x 21mm and goes through the mounting tabs (10mm) on the knuckle and threads into the bracket. So it only engages about 10mm to 11mm worth of threads on the bracket. That is the main source of concern for me, especially in an aluminum bracket. Finding longer bolts of equivalent quality has proven quite difficult (if anyone has a source, please let me know, I need something about 25mm to 30mm in length, partially threaded, 10.9 or greater, flanged head preferably).

The alternative mounting scheme would be to go with an AN or NAS bolt, 7/16" in diameter. I would use a 1/2" bolt, but I do not want to bore the mounting holes in the knuckle. I would make an appropriate bushing for the knuckle that would be a light press fit with a 7/16" ID. I was also considering making the bushing with a flange on it so it also acted like a washer. I could then either thread the 7/16" bolt directly into the aluminum mounting bracket, or I could use an appropriate nut on the other side of the bracket. Another possibility would be to make the bushing extend into the bracket and make it a press fit into both the bracket and the knuckle. Keep in mind the wall thickness of this bushing would only be approx. 0.021", so I don't think there's much strength in the bushing itself.

I am likely overthinking this, as the caliper is a radial mount with 3/8" studs that thread into the bracket, but since the bracket has yet to be machined, I'd like to make it as stiff and as safe as possible.

BTW, if anyone has equations that would help me, I would certainly appreciate it. ME is not my forte, but I'm certainly not afraid of any mathematics. In fact, I would welcome it, as I really want to understand the forces involved, and I'd like to convince myself that the mounting scheme I choose is safe.

Thanks!

 

[FredGarvin]

I would take the thru hole option and consider taking it one step further. I would, if space permits, oversize the thru hole and press in a steel insert or bushing. That way you can provide the full amount of preload on the bolt and you will not get any form of deformation in the aluminum due to the shear loading. If you can make it large enough, you can also make a head on the steel bushing to take the full head of the bolt and nut so you don't deform the aluminum directly under the bolt and nut head.

 

[71dsp]

Thanks for the reply. I really like your suggestion, but I'm unsure if there is enough material on the mounting tabs to bore out the bolt holes. Here are a few pictures:
http://www.marcuccimotorsports.com/billy/brackets/5.jpg
http://www.marcuccimotorsports.com/billy/brackets/8.jpg

The bolt in the pictures is the OEM bolt, M12x1.25 flanged head. The hex is 19mm, but I don't have the diameter of the flange handy. I can measure it tonight, though. I can also measure the mounting tab itself and post the dimensions.

I'd also need to find a machine shop that could do the work, but I don't think that would be terribly difficult.

 

[Topher925]

I would stick with the OEM's method for this. Not that there's anything wrong with Fred's suggestion, but its probably overkill. A centimeter of thread is probably all that is required for the bracket since its only be loaded by a shear force. Just make sure that if you decide to go with a bolt instead of a screw, you match or can do much better than the grade of the OEM fastener, which like you said can be dam hard to find. Its a common mistake to go down a grade or two in such places and have a resulting failure. I saw this happen a lot back in high school when some friends thought they were genius mechanics.

 

[Ranger Mike]

caliper bolt has shear force on it..not tension..where is this preload stuff coming from?? you could race it finger tight ifin you wanted to,,( safety wired in of course), stick with the stock theads..OEM bolt is way over kill for requirements to hold it...helicoil if you must..but don't go kobbling on a good known set up ...we run dual piston calipers on th formula car andthey are heldon with 3/16 grade 8 or 9 allen bolts...

 

[71dsp]

Thanks for the advice, I really appreciate it. It's projects like this that make me wish I went and did an ME degree instead of Mathematics and Finance! :)

I was thinking that when you properly tension the bolt, there is enough frictional force between the bracket and the knuckle that the bolt doesn't take the full load in shear. Is this an incorrect assumption?

What kind of formula car? Mine is a Honda that I race with NASA in the PT class. Nothing near as fancy or as fast as a formula of any sort (FM, FA, FV, etc)!

Someone did tell me that aluminum wasn't a proper material for the bracket, not even 7075, unless the bolts are through hole or steel inserts are used. I found that to be an interesting comment, as all of the radial brackets I've seen that were made out of aluminum use neither method of attachment, the screws and/or studs just thread directly into the aluminum. Now, I haven't seen all the brackets on the market, but is there an issue using a radial mount bracket such as the one above made out of aluminum without using steel inserts?

If anyone would care to look at the design for the caliper before it's machined, I'd certainly appreciate any feedback!

 

[Ranger Mike]

SCCA class Formula Continental..1988 Reynard British works car..cosworth 2000 cc mill
we have al. brackets on all 4 corners..wht NASA class u run don't have gummy enuff tires to impact the caliper situation to the point u need inserts or thru bolts,,,but..make sure u safety wire ..loc tite just gums u pstuff on aluminum..
i think u r good to go and will get a checker soon

 

[Ranger Mike]

btw ..the caliper bolt is in double shear which makes a hige difference..i hate single shear cantilever mounts..for tractors only...no business on a race car..just like farm bolts from SAE..

 

[71dsp]

Thanks again Mike! I'll have to drill the heads for safety wire. I was thinking about that. When I worked on helicopters, everything was safety wired. I did safety wire the rotor bolts on my two piece rotors, so I was thinking it would be prudent to do the same on the bracket bolts.

I could run nonDOT slicks if I wanted to, but I take a huge penalty for doing so. I just stick with Hoosier R6 DOT race tires. NonDOT slicks open up a whole new can of worms with oil starvation, fuel starvation, brakes, etc.

Can you post some pictures of your caliper setup? I'd certainly like to see it. I think I have enough clearance to make my setup double shear. It's something that I considered a long time ago, but I didn't pursue it much since all of the production car brakes I see use single shear mounting bolts.

 

[Ranger Mike]

you prob ok on present mounting bolt since the OEM engineers build in so much safety factor on brakes..but u are right on exploring double shear..i got to dig up the old poloroid to photo the brakes..will do it soon...
any time you can safety wire it is best..our driver is aviation mechanic for 30 plus years..he can saftey wire in his sleep..does a work of art with the wire..cheap insurance..in racing ..if it can go wrong..it will...
now get up on the wheel and go for the green flag, racer!

 

[FredGarvin]

I was thinking that when you properly tension the bolt, there is enough frictional force between the bracket and the knuckle that the bolt doesn't take the full load in shear. Is this an incorrect assumption?

That is exactly correct. That is one thing a lot of engineers and all back yard mechanics do not understand about a properly designed joint. In a properly tensioned fastener, the lion's share of the shear load can and will be taken up by the frictional contact between joint members, not the fastener itself. If you must use the fastener to completely take the shear load, you should employ a shoulder bolt. The preload will also help prevent unwanted loosening due to vibration and cyclic heating and cooling.
(I'll get off my soapbox now)

However, one thing that has been mentioned is that if the original set up works well, why go changing it? I say that even though IMO it is horrible practice to put a fastener directly into aluminum, especially if repeated removal and reinstallation of the fastener is going to happen. At least use a threaded insert.

BTW...what helicopters did you work on?

 

[71dsp]

Chinooks in an aviation unit in the Dallas/Ft Worth area.

The reason why I was thinking of changing it was due to the difficulty of finding a screw that had the right dimensions while engaging more threads in the bracket. From what I've seen for steel, 0.8 to 1 times the diameter in thread engagement is acceptable. However for softer metals like aluminum, I recall reading something like 1.25 to 1.5 times the diameter is acceptable. Right now, I have about 0.9 times the diameter in thread engagement.

On most of the M12x1.25 Honda bolts that I have seen, the shank OD is slightly less than OD of the threads. For example, I have one in my hand that has a thread OD of 11.79mm, and a shank OD of 11.06mm. On the bolt I want to replace, the shank OD is 11.9mm, and the thread OD is 11.87mm. It's a much better fit in the mounting tabs on the knuckle.

I did find that there are some Acura automobiles that use slightly longer caliper mounting bolts. I also ordered a few caliper bolts from larger Honda automobiles hoping one of them will be longer.

I think I will take your advice and use steel inserts to help strengthen the threads in the aluminum for sure. Thanks again everyone!

 

[Ranger Mike]

some pics of when i was making brake ducts to streamline the inside of the wheel.tried running moon discs but the tech inspector had a fit..

 

[FredGarvin]

Chinooks in an aviation unit in the Dallas/Ft Worth area.

Are you telling me we have a 67-Uniform in our midst? I was in hooks from 88-92 in Germany and Ft. Lewis (among other places).

 

[71dsp]

some pics of when i was making brake ducts to streamline the inside of the wheel.tried running moon discs but the tech inspector had a fit..

Very nice! Beautiful car. I'd love to step up the ladder to a purpose built race car one of these days!

Are you telling me we have a 67-Uniform in our midst? I was in hooks from 88-92 in Germany and Ft. Lewis (among other places).

Actually, I was a 67-Fox Trot, spark chaser with the 2nd of the 149th in the TXARNG. I was actually quite jealous of all the 67Us that had the opportunity to turn wrenches on the aircraft, but I had my share of fun with the electrical systems.

BTW, I'm really starting to think that I'm over analyzing this whole project. I just want to make darn sure that I'm not the cause of a brake failure!

 

[Ranger Mike]

yer good to go
IMHO the texas Nat Guard could whip the entire Iran army in two days..ifin they wanted too
welcome , troop!
Fred you good! too many Sh---hook.. rides from the tall grass with u guys..

 

[71dsp]

I have some bolts on order that are M12x1.25 that should be a bit longer than what I'm using now. If these work out fine, I'm going to go the route of using threaded inserts. I'm leaning towards Timeserts, but I'm not 100% sure yet.

However, I have one other question. What would be the best aluminum to use? I was set on using 7075-T651, but I thought I should ask those more knowledgeable! I also considered 2024-T351, but it appears that 7075 is stronger.

 

[Ranger Mike]

6061 T-6 billet works great for calipers..