Hub dimensioning

[PerKr]

So, having a car with combined rotor/hub (an '85 fiero), I'm looking to upgrade the brakes and might have to manufacture new hubs or at least modify the existing rotor/hub. What do I need to know in order to avoid making huge (dangerous) mistakes? How do I approximate the forces on the hub?

 

[evelrod]

Just a suggestion. You might consider one of the propriety racing brake mfgrs. There were several race setups for the Fiero chassis in the 80's as I recall, a couple of the Indy Fours and a GT. I would suspect that a simple search on Google or a peek at one of the several Fiero club tech sites would be a start. No sense in re inventing the 'wheel'(pun intended).

Rod

 

[PerKr]

I've been looking for info on fiero brake upgrades for quite a while now and it seems that most use the hub from the old rotor, which means an increase in track width. This seems to have been accepted on the forums as the only way to do it. I might be doing that in the end, but first off I want to look into the possibility of making a separate hub which will not give an increase in trach width (besides, it's a fun little project). So, basically, what info do I need? What friction coefficients are normally used?

 

[GregLocock]

0.3

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[PerKr]

ok, so would that be between the road and the tire or between the pad and the rotor? Can I assume that the greatest moment occurs when locking the wheels, just before the wheel starts skidding?

 

[GregLocock]

0.3 pad to rotor, 1.2 tire to road

Yes, roughly.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[RossABQ]

I used to have an 85 SE, fun cars, especially if you like going sideways!

The whole front end is taken off another GM car, so you might just look for what that was and see what options those cars had over the years. You may also want to consider a spindle swap from a midsize.

But increasing the front brake forces will just make that heavy rear end want to go around it even more!

 

[red67chev]

Perkr,

I have done the exact thing you speak of, but for other applications. Hubs have been designed. manufactured, and are running.

Gary King P.E.

 

[PerKr]

Seems I won't be getting any measurements of the existing hubs this year, so I'm currently looking into whether or not the fiero 9,72" rotor could be used as the hat for a 2-piece rotor.

How many g's should I be calculating for?

With a mid-engined car, would locking the rear wheels be preferred over locking the fronts? From what I've read it's generally preferred to lock up the front brakes, but then most cars are front engined so maybe that will be a factor as well?

 

[GregLocock]

"How many g's should I be calculating for?"

For what?

"With a mid-engined car, would locking the rear wheels be preferred over locking the fronts? From what I've read it's generally preferred to lock up the front brakes, but then most cars are front engined so maybe that will be a factor as well?"

Lock the front's first.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[PerKr]

"'How many g's should I be calculating for?'

For what?"

for heavy braking (in an emergency situation or on the track). A rough estimation is that the fiero pulls about 1g braking from 100-0 (stopping distance about 40m and I was assuming an average speed of 50kph, which is probably way off). To be on the safe side of things, I would be calculating for a bit more, but how much?

 

[GregLocock]

Under braking I'd use corner weight *1.5*1.2 as a likely force at the CP.

But, this pales into insignificance when compared witht he force that is generated when you hit a pothole or kerb.

So it rather depends which bit you are designing. The calliper and rotor will only see the braking load, the rest of the spindle and suspension will se pothole and kerbing loads.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[PerKr]

so for my 215/50R17 wheels I am looking at a moment of about 8kNm divided between the front wheels (the car should have a weight of about 1265 kg and we're assuming the front wheels are doing all the braking just to be on the safe side of things)?

next thing then is to find out what material is likely to have been used for these rotors...

 

[NickE]

Rotors are commonly made from Class 35-45 Grey Cast Iron.

The UTS is ~35-45ksi
The yield is nonexistant (or only a small bit less than UTS)
The Modulus is ~29,000ksi


Nick
I love materials science!

 

[PerKr]

I guess I will need to read up on these things...

What literature would you recommend me to look for?

 

[PerKr]

Been trying to do some calculations on this. Unfortunately I don't know where the center of gravity is or the deceleration when stopping or the centrifugal acceleration when turning, so I'm still not going to be spot on. Assuming I would have a deceleration of 2m/s2, a centrifugal acceleration of 0.5 m/s2 and a center of gravity which is about 0.75m off the gound I have a force of 11930N acting on one of the front wheels (since the rear is now airborne I suspect I would be off the road by now?). Assuming the tire has excellent grip, I multiply this by 1.2 and 0.3234 (radius of the wheels that are currently on) and get a moment of 4624Nm. Multiplying by 1.5 I get 6936Nm.
Curb-weight * 1.5 * 1.2 results in a moment of 7202Nm.

Will continue this thought a bit later...