Selecting Gear Ratio's, Weddle Gear Calculator

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SeanRitchie

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https://weddleindustries.com/gear-calculator

https://weddleindustries.com/sites/weddleindustries.com/files/downloads/Current Gear Availability 4-1-21.pdf

There are a few ways to sort out what gear ratio will work for your specific application. Most trans builders tend to work with a "common" set of ratio's that they know most people are happy with when using common engines and power numbers, but you the individual can make some determinations on what might work for you car the way that you drive it and where it is mostly used.

Most typical sand cars will never see more than 100 MPH. If you want the most performance from your engine, you can start by figuring out what 4th gear will give you the desired top speed, at the max RPM of the engine. Using our gear calculator on our website, you will have to enter in R&P ratio, tire size, engine RPM (This should be the max RPM where the torque curve starts to fall off, and is the RPM where you will typically shift), then enter 1st-4th gears. 1st-3rd ratio's will simply be place holders until you figure out what 4th gear gives you the desired top speed.

Once the desired 4th gear is chosen, 1st through 3rd can be worked out to give even splits by using the "Percentage of Pull" box on the chart. When the percentages are even to each other, this means that each gear will feel like it has the same amount of acceleration after each shift. This is how most race cars are set up so there is no lag when shifting through the gears. 

Another factor to consider is if you want to have a 2nd/3rd gear that is useful for duning, but have a longer 4th gear that offers an "overdrive" type gear to run lower RPM's on the longer flat runs. Then you would focus on the max speed for 3rd gear (which for most might be between 70-80 MPH in the dunes), then stretch 4th gear out a little bit to offer that overdrive effect. Be careful not to make 4th gear to "tall" and have too big of an RPM drop after shifting from 3rd as this can bog the engine and lower powered cars may not be able to pull that tall of a gear.

A few key terms when figuring out gear ratio's:

  • High number on the gear = lower gear ratio, faster to accelerate, less top speed in that gear.
  • Lower number on the gear = higher/taller gear ratio, slower to accelerate through the RPM, higher top speed.
  • Percentage of Pull = even numbers offer equal acceleration feel with each shift. Lower number means shorter/faster accelerating gear. Higher number means taller/higher speed gear.
  • RPM drops: Don't let these get too large towards 4th/5th gear or suffer a "bogging" gear with lower powered engines.
  • Tire size: Taller tires equal taller/higher overall gear ratio's. Shorter tires can use taller gears. Taller tires need lower ratio gears to compensate for drive ratio.
  • Gear strength: Lower ratio gears are weaker gears. Higher ratio gears are stronger. Gear strength needs to be balanced between R&P and 1st-4th gears to best overall trans reliability, especially with VW transaxles.




 
This is one of the best tools I have used, many times. Now that the trans is where I want it I will refer to this to see what tire changes do to my set up. Very helpful. Thanks.

 
This is good info. A few questions about percentage of pull. First when you say as even as possible is that +/- 5% or more/less. And what is a target percentage of pull? And what is the seat of pants relation to the increase or decrease in percentage of pull?

 
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Jtmoney714 said:
This is good info. A few questions about percentage of pull. First when you say as even as possible is that +/- 5% or more/less. And what is a target percentage of pull? And what is the seat of pants relation to the increase or decrease in percentage of pull?
Click to expand...
If you want even acceleration through every gear up to top speed, these numbers would have to be all the same. 2-3% difference from one gear to the next is typically not going to be noticeable. 5+% different and you will start to feel a difference. This is all relative to how the car is being used though, and I would only recommend this gearing for a race application.

Most sand cars would need to base their gearing off of actual vehicle speed in the dunes they are driving in, so I tell people they should think of 3rd gear as their top speed gear (usually between 70-80 mph at top RPM), and 4th gear is just the extra gear when running the flats. This will usually create a difference in percentage on our calculator, so it's typically not the important detail to make perfect, but should be paid attention to so there are no "gaps" in the gearing.

A lower percentage number will be a lower ratio gear, so the car/engine would accelerate faster. A higher percentage number will be a higher gear, and would slow down the acceleration of the car. 

 
This is one of my favorite tools - the amount of info it provides is fantastic

The right ratios can make or break the way your car performs. 

Not a knock on Weddle, but with the increasingly limited number of ratios available at any one time has made me "re-jigger" my favorite ratio combinations 

I am always looking here after Seam posted this ...  been buying gears as they become available for the next build

Screen Shot 2022-04-04 at 4.45.04 PM.png

Right now it looks pretty good  Unless you Need R&P's

Screen Shot 2022-04-04 at 4.45.39 PM.png

 
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I wouldn't go by what is on their website.

When I had a S4 built, in the middle of COVID, I reached out to Sean about ratios and this table you posted.

At the time, Sean said that table hadn't been updated in a while.
I would always reach out to him directly.  IMO

 
That "Current Gear Availability" sheet is not the most accurate these days (yes, I know it says "current", it once used to be pretty accurate). This sheet is updated every few weeks or so, and cannot keep up with how fast we get these gears in stock and shipped out before they are out again.

It is best to check availability of the specific part numbers for the gears on the website. The "In stock" or "Call for Availability" listed for each part number is live to our in house inventory system and updates daily. 

Most of the sequential gears are pretty backlogged for backorders and trans kits right now. Meaning we have more orders for gears than we produce in a single batch, so unless you are early enough on the backorder list to get a gear from that batch, you will be waiting for the next batch. Backorders are filled based on the date they were placed.

I direct people to that Current Gear Availability sheet when they are considering gear ratio's, so they can see all of the ratio's that we make in one spot. Someday if we ever get caught up again, that sheet will be accurate enough to choose the ratio's based on if it says they are in stock or not (kind of hoping that day never comes to be honest, that means we will be slowing down).

 
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unfortunately i had my previous megasand gone through twice. 2 different gear sets, so i knew exactly what gearing i wanted, right between those two gear sets. when i ordered the weddle hv24 mid engine i got it right for two diff dunes at two diff altittudes.  i did look at the gear chart and it looked like a helpful tool.  

 
One thing to consider: top speed in each gear won't likely be realized top speed unless you're flat-landing it.  In the sand there will always need to be some wheelspin to accelerate quickly, so if you plug in 100mph on the limiter in 4th, it might take a bit to get there unless you're headed down to Duner Diner.  Not that many of us are hitting 100mph in the dunes, but something to consider even at the drags.

 
The calculator link takes me to a malfunctioning calculator. Is there an update?
 
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