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View Full Version : Undersized drive pulley issues


benebob
06-23-2005, 09:21 AM
We put an undersized drive pully on the race car this past week and have found that the steering has now gone to boarderline acceptable for track conditions. VERY TIGHT! I'm assuming that if we put a smaller pulley on the PS pump our issue will be solved but wanna see if anybody has done this and how small did ya go to return it to a decently assisted steering box. Also, where'd you snake the pully from?

dcarrb
06-23-2005, 12:49 PM
If you've downsized the pulley on one end, seems to me you'd have to upsize the other one to restore the proper drive ratio.

dcb

Phast SVX
06-23-2005, 12:58 PM
If you've downsized the pulley on one end, seems to me you'd have to upsize the other one to restore the proper drive ratio.

dcb

the ratio will not change, but i have a ahard time believing that the pulley has created little flow. remember, the pump dosnt create pressure, jsut flow. All the pressure is created in the rack itself with the seals etc.
phil

benebob
06-23-2005, 01:39 PM
Rack is as leak free as a 92 SVX original rack can get. No fluid has been added since we got it and there are no visable leaks. Remember it is spinning about a 1/4th less so there is a 1/4 less flow so pressure should be down from that. It worked just fine the day before only difference was the pulley.

The pulley on the PS would have to go smaller (against your logic but at least your thinking). Imagine it like this. Smaller crank pulley means it is spinning the belt less per revolution of the engine. Going bigger on a sub pulley like the ps pump will cause the ps pump to spin even slower as the drive belt is moving slower and now the bigger pulley has a larger circumference but if this belt is smaller it will in turn spin faster with less rev. from the engine. Here endith your physics lesson for the day. Just think, if all us car nuts had physics questions like this we'd all be rocket scientists like Bill. :D

McTaff
06-23-2005, 02:33 PM
Hey champ, that's a good summary, but I'll break it up a little to explain a little more. This might help others to decide what they want.

Drive pulley on a belt-pulley system:
We will call the source pulley of the power the "Drive" pulley. This is the pulley that will supply the force to turn the belt.
We will call the receiving pulley of the power the "Slave" pulley. This pulley is turned by the belt and thus powers secondary devices.

Each slave pulley in the system can be treated seperately as a lever and fulcrum system. Force applied is at the Drive end, and work done is at the Slave end.

====
On a per revolution basis, increasing the size of a drive pulley will increase the number of times the belt turns for each revolution of the drive pulley.

This decreases mechanical advantage (like moving your fulcrum closer to the applied force), but increases the distance travelled (the other end of your lever moves further). This means it will react less favourably to high load situations, as the transmitted force has less mechanical advantage, like trying to use a shorter lever. (Not really a problem with drive belts in cars, though)

Decreasing the size of the drive pulley will decrease the number of times the belt travels.

This increases mechanical advantage, but decreases the distance travelled.

Now remember that doing this to the Drive pulley changes the "fulcrum" for ALL YOUR SLAVE PULLEYS.

To re-adjust each slave pulley INDIVIDUALLY, the opposite is now in effect, because you are changing the other side of the lever. (By making them smaller, it is like a "Claytons making the drive pulley bigger".) This means you need to reverse the formula.... i.e:

Making the Slave smaller has the same effect as making the Drive bigger, and vice-versa.
====

You need to work out the ratio of Drive change and apply it to the Slave and bring it back towards 1:1 maybe half way. If you went from circumference X to circumference Y on the Drive, work out the ratio X:Y and reverse-apply that to the Slave pulley.

That will give you the size of the Slave pulley you would need to return it to the original specs.

All you need to do then is to pick a size maybe halfway between the stock one you have, and the size you'd need. (Unfortunately this is a black art, and it's rather like inventing a recipe... you have a fair idea what the ingredients do, but no idea what the end result would be like.) This would be a rule of thumb. But I'd say halfway between them would balance it out - if it's close but not quite perfect you'd already have a fair idea because you've already crunched the numbers.

Hope that helps the people who like to fiddle.

Earthworm
06-23-2005, 02:53 PM
I just think of a bicycle chain :D

Beav
06-23-2005, 03:37 PM
If you've downsized the pulley on one end, seems to me you'd have to upsize the other one to restore the proper drive ratio.

dcb

Uh...no. Keeping the same ratio is done by reducing the size of both pulleys. Increasing the diameter of the p/s pulley would only provide a further underdriven ratio.

Beav
06-23-2005, 03:51 PM
the ratio will not change, but i have a ahard time believing that the pulley has created little flow. remember, the pump dosnt create pressure, jsut flow. All the pressure is created in the rack itself with the seals etc.
phil

Without adequate flow, e.g. a slow turning pump, pressure can be hard to come by. You have to maintain a certain amount of flow before pressure can be built and maintained.

R&P p/s works by having pressure on both sides of the rack piston and depleting the pressure on one side when the steering wheel is turned. This allows for quicker response than applying pressure to one side when assist is called for. No pressure = no assist. Low flow doesn't allow for very quick pressure recovery. Rule of thumb:since he did x, y has become an issue. More than likely x was the cause of y.

oab_au
06-23-2005, 05:53 PM
I'd just put the bigger crank pulley back on. :confused:

Harvey. ;)

Earthworm
06-23-2005, 06:17 PM
Now did you replace OEM or OEM-size lightweight?

benebob
06-23-2005, 06:39 PM
Now did you replace OEM or OEM-size lightweight?

OEM size to a cheap aftermarket one (the kind that Hocrest say's he's seen pictures of melted). It came with LeakingOil's SVX and she really doesn't have a need for it or desire for it so we thought we'd give it a wirl to see if it does anything or hurts (which so far I think the later is the answer but I'm not giving up on it just yet).

McTaff
06-24-2005, 01:09 AM
Uh...no. Keeping the same ratio is done by reducing the size of both pulleys. Increasing the diameter of the p/s pulley would only provide a further underdriven ratio.

Which is what I said, right? :confused:

I don't even remember what I wrote. I think I need a kebab, and to stop working at work (and concentrate on forumtime)

svxhunter
06-24-2005, 03:39 AM
We run an underdrive pulley on the SVreX. Never noticed any difference in the steering. :confused:

dcarrb
06-24-2005, 06:09 AM
As I've already demonstrated that I know nothing about this particular topic, I figure it can't hurt to ask: How exactly is an underdrive pulley supposed to enhance the performance of the engine?

dcb

b3lha
06-24-2005, 08:20 AM
As I've already demonstrated that I know nothing about this particular topic, I figure it can't hurt to ask: How exactly is an underdrive pulley supposed to enhance the performance of the engine?

dcb

The theory goes that the engine produces a fixed amount of power. Some goes to drive things like the alternator and steering. The rest goes to the wheels. If you change the pulley to divert less of the power to the alternator and steering, then more will go to the wheels.

But then you may have problems with heavier steering and poor charging. You could possibly fix those problems, but in doing so you might just be reclaiming the power that you diverted to the wheels. :rolleyes:

SVXRide
06-24-2005, 10:46 AM
snip* Just think, if all us car nuts had physics questions like this we'd all be rocket scientists like Bill. :D



:D :D :D :D :D :D
-Bill

dromano
06-29-2005, 05:05 PM
As I've already demonstrated that I know nothing about this particular topic, I figure it can't hurt to ask: How exactly is an underdrive pulley supposed to enhance the performance of the engine?

dcb

I have an Unorthodox Racing billit aluminum under drive crank pulley, the marketing pitch is: the billit aluminum pulley weighs 5 lbs less than the oem pulley, the gain in power to the wheels is in the weight savings of the pulley.
Plus mine is a cool anodized red :D :D :D

SpoonChucker
06-29-2005, 06:55 PM
The gain is in efficiency, which allows more power to be transferred to the wheels. The simple equation is that K=1/2*I*w^2, where K is kinetic energy, I is the polar moment of inertia, and w is the angular velocity. Furthermore, I=1/2*m*r^2 for a cylinder rotated about its axis, where m is the mass, and r is the radius. So the diameter and mass removed from the outside of the pulley (cross-drilled pulleys) have a larger effect on the energy than just the weight reduction. This is however is energy and not power, power is energy per time, so its effect will only be noticed during acceleration. The effect on the power steering and alternator is function of the angular speed of the crank pulley (ratio of the diameters) so if the diameter is held constant the performance of the power steering and alternator remains unchanged.

b3lha
06-30-2005, 02:08 AM
The gain is in efficiency, which allows more power to be transferred to the wheels. The simple equation is that K=1/2*I*w^2, where K is kinetic energy, I is the polar moment of inertia, and w is the angular velocity. Furthermore, I=1/2*m*r^2 for a cylinder rotated about its axis, where m is the mass, and r is the radius. So the diameter and mass removed from the outside of the pulley (cross-drilled pulleys) have a larger effect on the energy than just the weight reduction. This is however is energy and not power, power is energy per time, so its effect will only be noticed during acceleration. The effect on the power steering and alternator is function of the angular speed of the crank pulley (ratio of the diameters) so if the diameter is held constant the performance of the power steering and alternator remains unchanged.

So you are saying that it is better to keep the same radius of pulley but make it lighter. Particularly towards the outside. What about the slave pulleys. Surely the same should be applied to them also?

How much gain in power to the wheels are we talking about here, percentage wise? We're splitting the power from the engine between the alternator/steering and the driveshaft. But what is the initial ratio? Surely the force needed to accelerate the pulley is insignificant compared to the force needed to accelerate a 1.5 ton car. I'm confident that I could spin the alternator and power steering pump with one hand. But there's no way I could turn the driveshaft and make the car move by hand.

Suppose the force needed to turn the driveshaft was 50 times the force needed to turn the accessories (just a random figure). If you could reduce the force needed to turn the accessories by half (quite a lot), then would you gain 1% in power? Or am I over-simplifying the situation? Obviously I'm just making these numbers up, but it seems to me that the gain in actual performance of the car must be very very small.

svxhunter
06-30-2005, 04:22 AM
Sure you're oversimplifying it! Spoonchucker is not! :)

The slave pulleys don't have exactly the same effect- they are not generating any power, they are only being driven. They are not contributing to power gains, they are detracting.

Rotational mass is a fairly significant consideration in engine design. That's why high performance engine internals are both light and strong.

Lightened flywheels and wheels are the 2 easy after market changes that we all can do and feel the difference.

No one has ever had huge gains from a lightened pulley, or we'd all know about it and be running out to buy them. But it is rotational mass, and it does make a difference, even if it is a small one.

I run a lightened underdrive pulley on my SVX racer. We think it makes a difference, but we have no dyno tests, etc. to prove it. I run a lightened flywheel on my 5 spd. I KNOW it makes a very large difference.

McTaff
06-30-2005, 11:34 AM
I think that there also needs to be a clear distinction on what you are trying to do.

As I pointed out in my original post in this thread, if you change the size of the crank pulley, you are changing the ratios for ALL of your pulleys, and thus you get a greater gain across the board. However, this lessens the amount of revolutions transferred to the Power Steering, Air Con (if any) and Alternator.

If you change the size of one of the driven pulleys, then you only change the ratio to that device (but for a lesser gain).

Ideal setup would probably be: lightened Drive/Crank pulley, smaller and lighter Air Con, lighter Alternator and (slightly) smaller and lighter Power Steering.

As stated before, the lightening would probably do little, and the change in size with the pulleys would help a little. To be honest, although it is fairly cheap, there are probably better things to spend your money on elsewhere in the engine bay? I'm not sure how much custom pulleys would cost you in the US of A, but here they aren't cheap.

benebob
06-30-2005, 12:52 PM
I run a lightened underdrive pulley on my SVX racer. We think it makes a difference, but we have no dyno tests, etc. to prove it. I run a lightened flywheel on my 5 spd. I KNOW it makes a very large difference.

Yeah I agree it makes some difference, even if it is just "feel". I think ours though is making the difference the other way with issues with the PS but if we can solve that we'll keep it. Every pound helps. :D

SpoonChucker
06-30-2005, 04:01 PM
For mine I kept the radius the same, I used one from Cobb Tuning (weighed in at 1.25 lbs.). It is difficult to quantify any gains, a typical dyno plot of hp versus rpm would look identical to one with a stock pulley. In theory, a plot of hp or rpm versus time would be shifted to the left (quicker acceleration). I do not notice much improvement from the pulley alone.

Companies such as AEM have made lightened power steering pulleys for the WRX. It’s not a bad idea, but like it was mentioned earlier the improvement isn’t much and might not be worth the cost. When I get some more time, I’ll design and mill out a set (alt/ps).

UberRoo
06-30-2005, 07:43 PM
I wish my power steering pump was electric like the XT6s. If I crank the wheel too quickly in a parking lot, I run out of pressure. I can actually feel the pump playing catch up. This only happens when my foot is off the throttle. If the engine is over about 1500 RPM the pump seems to be able to keep up with me. Very annoying though - especially when you think you're going to easily make a turn and avoid colliding with something very expensive, but suddenly learn that you're going to have to forgo the luxury of a phone booth to change into your spandex and cape, and instead summon all of your Herculean strength on the spot to avoid missing said unspecified expensive object with only inches to spare.

The problem: The 1500 RPM 'sufficient speed' is almost twice the idle speed. It would require a significantly undersized power steering pulley or oversized crank pulley to fix my problem. The power steering pulley would probably not be adequate to handle the power, or the crank pulley would cause the alternator to spin so fast you could build a housing around the alternator cooling fan and use it as a supercharger. That, or the centrifugal force would make the alternator windings explode and fill the engine compartment with copper spaghetti.

benebob
06-30-2005, 08:00 PM
I wish my power steering pump was electric like the XT6s.

More incentive to get on the drawing board :D . I too love the 6 steering! Just not the cost of the liquid gold fluid it requires.

Beav
06-30-2005, 08:39 PM
Ubie:

It sounds as though you might first try flushing (or having it flushed) the p/s system. Old fluid and/or gunk can cause that feeling. Also, a restricted line or a failing pump could do the same. Flushing isn't just changing the fluid in the reservoir, it includes using a cleaner designed for the job followed by several exchanges of clean, new fluid. It takes about 20-30 minutes with the right equipment or an hour or so (and probably a bit of a mess) to do at home.

UberRoo
07-01-2005, 10:54 AM
It sounds as though you might first try flushing (or having it flushed) the p/s system.There's probably a lot of truth to that. I've had the same problem with quite a few vehicles, so I suspect the primary cause is that I simply have a tendency to flick (or attempt to flick) the wheel from lock to lock more than the average driver. ...but any improvement at all would be worth something. With your suggestion, the current problem is now a matter of apathy. :)