What fanbelt for Alternator

[Bill Goddard]

What fanbelt should I be using for my new Alternator? Bill G

[machine girl]

depends on the pully size and mounting kit, the stock belt works with our kit?

[jmeckel]

I use and recommend the coged belt sold by Napa. It is listed in their book for a Model A Ford any store can look it up.

[hardtimes]

Originally Posted by Bill Goddard

What fanbelt should I be using for my new Alternator? Bill G

Hey Bill,
The 'notched' Goodyear part #BX40, is a good belt.

[Kevin in NJ]

If you plan on driving a lot keep a spare under the seat.

The smaller diameter pulley eats the belts faster as the original type belt is not designed for this use. The guys doing the Alaska trip went through several.

[Bill Goddard]

Sponsored Links (Register now to hide all advertisements)

Originally Posted by Kevin in NJ

If you plan on driving a lot keep a spare under the seat.

The smaller diameter pulley eats the belts faster as the original type belt is not designed for this use. The guys doing the Alaska trip went through several.

Kevin, Do you think they used a cogged belt? Bill G

[Ron Lachniet]

I have over five thousand miles on my coged belt and it still looks fine. They are better suited for the sharper bend that you have with an alternator.

[Kevin in NJ]

After years of reading posts I or my brother notice certain trends.

My brother noted that some have had belt failures while using alternators.

Stands to reason the original belts may not like the very tight turn around the pulley. There are certain min radiuses that the belts are designed to go.
If these cogged belts do the job then you are set.

I am not up on all things Alt as they are no gain over the factory gen unless you change the pulley system. Physically the small diameter pulley is not capable of transmitting the HP needed to get more than around 15 to 20 amps continuous out of the alternator.

[CarlG]

Actual user here! The BX40 belt works great for me. Others in our club have used a BX42 which is a little longer. Their reasoning for that is that by forcing the alternator to swing out a little further, it grabbs more of the belt in the pulley.

[Ron Lachniet]

I try to do as much on my car as I can myself, but when it comes to electrical components like generators and starters I am clueless. The first year I owned my car I had my generator in three times for repairs and it finally quit completely. At that point I was getting gun shy of generators and decided to try the alternator because we drive our car a lot and long distances. I have not had any problems with my alternator and it turns out more current than I need. I don't get any negative comments from purists either because I don't sit in parking lots at shows with my hood up.

[Kevin in NJ]

Originally Posted by PCreech

KEVIN after looking at past posts, this seems to be your soap box theory BUT ACTUAL USERS opinions differ, Just saying

Which opinions are you talking about?

The alt will produce enough juice to keep the A happy. That is a fact, not and opinion. Even with the belt so loose you can spin the pulley with your finger and get a couple revs out of it. I have personally seen this.

The diameter of the alt shaft is only capable of transmitting enough energy to produce something in the 20 amp range. Depends on various factors and I just do not have an alt with the pulley to do tests. It does require a fairly taught belt, which you never see on any A. That is just basic math, go look it up on the web. Just facts, not my opinion.


The only A alternator set up that I see that works is with the A/C conversion kit. The belt system has to be set up different so that the 4 HP or so can make it into the A/C compressor. It needs the extra current from a properly set up alt to work.

Please do not confuse facts from opinions. I know I like to make informed decisions, but it seems many prefer to just do what everyone else does. Seems to work for Lemmings.

Please take some time to verify what I say. It is truly quite astonishing how little power is transferred to the alt shaft because of the pulley's small size.

[CarlG]

Pulley size is only relational to how fast the alternator turns in relationship to the engine RPM. Power generated is only relational to the speed of the alternator shaft. It makes no difference if the alternator is part of the A/C setup or not, the same size pulley on the alternator is going to have the same relationship to the engine RPM in either place.

[FirstA]

I used a replacement pulley and a stock belt. Works fine.

[Jim/GA]

Originally Posted by CarlG

Pulley size is only relational to how fast the alternator turns in relationship to the engine RPM. Power generated is only relational to the speed of the alternator shaft. It makes no difference if the alternator is part of the A/C setup or not, the same size pulley on the alternator is going to have the same relationship to the engine RPM in either place.

True. Until the belt starts slipping. Then the alternator shaft is not spinning as fast as it theoretically could. The more of the V-belt side surfaces you can get in contact with the alternator pulley, the more power that belt can transmit to the alternator pulley and shaft before it starts to slip (coefficient of friction between rubber and steel, contact area, normal force, and all that).

Depending on the belt routing, for the same alternator pulley diameter, you can get just a short bit of belt contacting the pulley or you can get quite a bit of the belt wrapped around the pulley. That is the difference Kevin refers to, I'm sure, and it does limit the power you can get out of the alternator being driven by the pulley.

I don't know which A/C system he is referring to, so no comment about that.

[CarlG]

Originally Posted by Jim/TX/GA

True. Until the belt starts slipping. Then the alternator shaft is not spinning as fast as it theoretically could. The more of the V-belt side surfaces you can get in contact with the alternator pulley, the more power that belt can transmit to the alternator pulley and shaft before it starts to slip (coefficient of friction between rubber and steel, contact area, normal force, and all that).

Depending on the belt routing, for the same alternator pulley diameter, you can get just a short bit of belt contacting the pulley or you can get quite a bit of the belt wrapped around the pulley. That is the difference Kevin refers to, I'm sure, and it does limit the power you can get out of the alternator being driven by the pulley.

I don't know which A/C system he is referring to, so no comment about that.

This is why my friend opted to go to the BX42 belt. By doing that, it requires the alternator to swing out further, therefore the belt contacting more of the pulley (about double).

That being said, I don't have a problem with my BX40 belt slipping on the 2" pulley. Just lucky, I guess!

[Kevin in NJ]

There are two factors.

You need the alt turning fast enough to make power.

As the alt supplies more power it take more force to spin the alt.
Let me give an analogy.
You have a 1000HP drag car. Are you going to be able to transmit that much energy to the pavement with the 14" tires from your car? No you need a large diameter wide tire to connect to HP to the ground.

The problem is the smaller diameter pulley has less friction and can not transmit as much force (horsepower) into the alternator. This is a big thing to understand. That little pulley combined with the as typically installed belt tension will not allow enough HP to be transmitted into the alt to give full output.

Here is a quote I found on the web, that manual is kind of large to go through---

That said: The Gates Heavy Duty V Belt Drive Design manual shows that a "A" V-belt section will transmit about 2.2 HP per belt for a minimum pulley OD of 3" at 1750 RPM. Derate non-linearly for smaller diameter. By non-linear, I mean a 2" pulley will transmit about 1 HP and a 1 1/2" pulley will barely transmit 1/3 HP. A belt with an AX designation has a notched inner surface that greatly enhances belt flexibility and improves performance over smaller pulleys.

The above will have some assumptions on proper wrap as the more contact area the more friction and the more HP you can transmit. It is likely those numbers are for 180 degree wrap- a lot more then what you get on the A.

So here is what it takes to get 60 amps:

website with data click me

So it takes around 1.4 HP to get 60 amps. Basic physics.

So with a 180 degree wrap the 2.25" pulley would get you close to the HP you need for 60 amps. But it is not 180 degree wrap and you will not have it tight enough. So you are below the required HP to get 60 amps.

Here is a picture of a Mustang belt set up. Notice that the alt gets its own belt and has 180 degrees of contact.

Mustang belts

Make sense??

Of course you all know alternators are not rated for 60 amps 100% of the time, just short bursts. Once it warms up it drops off to half the rating.

The big thing to keep in mind is this above HP rating assume a fairly tight belt tension. That can not happen on the A. So the HP transfer is derated more.

So you are back to getting basically what you get from a gen. You do get a regulated output, but you can do that with the gen too.


Now there is a guy who makes a Air conditioning kit for your A. He changes your belt and how it is tensioned to allow for proper transfer of the energy into the alternator and AC compressor.

Simple basic facts on the physics of how the stuff works.

All I am pointing out is for all the work you do you do not get what you think. You see 60 amp alt and you do not get 60 amps. Depending on how tight you want to run the belt you get something +- of what the gen gets you.
So for all the work and money outlay you get no real advantage, just a perceived advantage.

There are also known issues with alt not being as reliable and some (I stress some) one wire units have a constant current drain letting you battery go dead.

Keep in mind the alt works fine with all the above physics. That is because it just does not need much current to charge most peoples cars. You are NOT getting a real 60 amps and you would be in for a surprise if you really needed it.

So now do you understand the facts I am bringing out?

[1931 flamingo]

Is the alt turning with your belt?? Are you showing a charge?? Are your lights bright without a discharge??
?????????????????????????????????????????????????? ???????????

That's all you need. Why are we constantly trying to re-invent the wheel??

Paul in CT

[CarlG]

Originally Posted by 1931 flamingo

Is the alt turning with your belt?? Are you showing a charge?? Are your lights bright without a discharge??
?????????????????????????????????????????????????? ???????????

That's all you need. Why are we constantly trying to re-invent the wheel??

Paul in CT

I guess the best answer to this is one my Dad use to quote me all the time: "Don't confuse me with facts, my mind is already made up". I'll let it go at that!

[RobertB]

Who makes the A/C set up? I live in the desert and use the A/C in my cars every week of the year.

[Herb Concord Ca]

Contact Ken Davis, 817-540-1513 he lives in Colleyville TX. He has made AC kits for Model A's for 20 years. I installed one of his kits in 2010, it works great.