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differences between 240 and 280 steering racks


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The bushing width on the 280 rack is wider than the 240.

There are also other differences beyond that but I don't think they would affect form-fit-function. Stuff like details in the design of the inner tie rod ends. Different, but would be unnoticeable in application. But the only form-fit-function hurdle I can come up with quickly is the bushing issue.

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9 hours ago, grannyknot said:

Thanks Captn, so the turn ratio would be the same?

Honestly, I'm not sure. It appears that way, but the specs in the manuals are all kinds of screwed up. I dug into it a bunch in this "steering effort" thread:

https://www.classiczcars.com/forums/topic/61176-steering-effort-differences-between-years/

but never completely confirmed most of the numbers. In that thread, we nailed down the lock-to-lock is different on the 240 vs. everything that came after (but just because the L-L is different does not guarantee that the ratio is different). We also nailed down that the 240 used a different pinion and rack gear than everything that came after (but just because the gear and rack are different part numbers, does not automatically require that the ratio is different).

So, I would defer to rturbo.

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I'm not 100% because my brain doesn't mesh with whatever Nissan means with their newer-former-superceded interchange lingo but I think that they're saying "Yes".  Not really sure though.  They left some boxes unchecked I think.

http://www.carpartsmanual.com/datsun/Z-1969-1978/steering/steering-gear/2

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On 3/22/2011 at 2:33 PM, cygnusx1 said:

OK The 76 280Z rack travels roughly 1.5" per turn of the input shaft. There are 3 full turns lock to lock on the 280Z. There you have it. Pretty significant difference that can easily be felt. The 240Z is much closer to what a modern car with power steering has. The 280Z just feels "off the mark" after coming from a modern car, or a 240Z. I shall proceed with the 240Z rack swap into the 280Z.

Rough Measurements:

240Z (1972) - 1.81" of travel per 360 deg of steering.

280Z (1976) - 1.5" of travel per 360 deg of steering

https://forums.hybridz.org/topic/99017-240z-rack-into-280z/

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The Specs tables in the FSM's seem to say that the 280Z got a 15.8:1 rack for the first two years (MY 75-76) before changing back to a ratio similar to that of the 240-260 MY's (18:1 for the MY77 280Z vs. 17.8:1 for the NA-spec 240Z/260Z).  This makes no sense, for a couple of reasons:

A 15.8 rack (just like the Euro-spec 240Z) is faster but requires more driver effort than a 17.8 rack.  Not good for us slow-driving, parallel-parking North Americans.  But then the 280Z comes along and it's designed as safer and more luxurious (and heavier) than the 240Z, so as to further cater to the North American mass market.  The (numerical) steering ratio for the  280Z should have gone up, not down.  And that's exactly what the FSM spec chart says happened -- but not until MY 77 (and not by very much).  However...

If you look again in the 75 FSM, you'll see that the introductory text at the start of the 'Steering' section says that the ratio is 18:1.  That makes more sense.  I wonder if the FSM editor forgot to update the Specs table at the end of the section?  And then forgot to do it again for the 76 FSM.  

Of course, this doesn't explain the differences in rack travel measured by cygnusx1 on hybridz for his 240Z vs 280Z racks.  At 1.5" (76 280Z) vs. 1.81" (72 240Z), his measurements were definitely consistent with a slower rack in the 280Z (i.e. more wheel twirls per inch of output), but they also suggest a difference in ratios of 20%, which means that the 280Z ratio should be 17.8 x 1.2  = 20.6:1.  That's a lot more than 18:1. 

Edited by Namerow
correction to source of rack travel measurements
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When I get a free day (2025 at the rate I’m going) I’ll measure my vast array of steering racks and let you know!

At the very least, I have a known 75 rack that I can measure and tell you about.

I don’t understand rack and pinion ratio’s anyway.   One turn of the wheel produces a certain amount of rack movement. No ratio of nothing.

 

 

Edited by zKars
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Quick look at the internets:

Moog Parts defines "steering ratio" as the ratio of how far the steering wheel turns to how much the wheels turn. For example, if a 360-degree turn of the steering wheel causes a car's wheels to turn 20 degrees, then that car's steering ratio is 18:1 (360 divided by 20).

So you cannot determine steering ratio simply by measuring the distance the rack moves for one revolution of the wheel.

However, If everything else in the system is the same (like the lengths of all the other steering members), then the rack that moves more per wheel rev will have a higher ratio, but I'm not sure (in the Z car realm) that we have nailed all that down for sure.

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2 hours ago, zKars said:

When I get a free day (2025 at the rate I’m going) I’ll measure my vast array of steering racks and let you know!

At the very least, I have a known 75 rack that I can measure and tell you about.

I don’t understand rack and pinion ratio’s anyway.   One turn of the wheel produces a certain amount of rack movement. No ratio of nothing.

 

 

Ratios and proportions were always difficult math for me. I really struggle(d) with math, and yet I pursued a career in engineering (shortly) and then became a mechanic/machinist. 

Go figure.

Anyway, as I understand it, the ratio for something (as in gearsets and pulleys and other mechanical things) is the difference between the movement of one part compared to the other. Or, In mathematics, a ratio indicates how many times one number contains another."

As the good Captain points out, Moog defines the ratio in terms of degrees. 

360 degrees divided by the 20 degrees the wheels turn equals 18, so the ratio is 18:1.

So if the rack moves the wheels 24 degrees for one turn of the steering wheel the ratio would be 14:1.

 

I think I got that correct. A bit easier than calculus, trigonometry or descriptive geometry (the math behind geometry).

 

 

 

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The 75 rack moves 1.51 inches for one pinion revolution.

Feel free to convert that steering angle any way you like.  

 

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