Geometry and what we are doing when we design. Ever wonder?
Well for this one we’re gonna quote a lot from a source on the internet. Crawlpedia.com has a write up HERE for some reading. Its a lot to take in at first when reading but we’re going to show two differences why we believed in portal axles years ago.
Two things we feel are myths as we call them. One being that portals are not 1:1 realistic and the other is they raise your CG. When the Capra came out we know the internet was loaded with the haters. Like anything new we are going to have the negative nancy’s. Its just human nature to have an opinion and we respect that. What was sad to see were the people being negative towards a company that pretty much put rock crawling in the hobby.
Back when we were putting pieces together from several different vehicles one company stepped up and gave us a box kit. Axial was created by some of the sports most known drivers. It wasnt just a company that said how can we take peoples money. Take a moment and think what did we have before them.
Now they pushed the envelope with something new again. Yes not the first but definitely a better option for many drivers. With the backing from Hobbico now and manufacturing resources Axial gave us more options as drivers. And it was all designed by hobbyist’s that have given years, decades actually to the sport for all of us.
Lets look at the first myth we feel, portals arent realistic enough. They arent popular in the 1:1 world as many said. We have surely seen the up tick in portal usage over the years. Going back to when Poison Spider (first we recall seeing) started advertising them. If you google around you will find plenty of rigs in competition using these types of axles. They have continued to grow in use and the benefits of them truly show in performance. Now dont take that as saying they are the winning setup, drivers are the key element but portals are beneficial.
So lets go to the second myth we feel is out there with portal axles. They raise the center of gravity (CG). We just dont believe it and heres why.
Designing a rig around portal axles takes some out of the box thinking. It is not just simply taking your existing chassis and changing the axles. Something like that then yes, we could agree your CG is going to be raised. Back when we worked on our 2018 truck we knew we had some hurdles and unknowns to figure out. Challenge was accepted and it paid off.
Heres the how/why of our logic on portals vs CG.
We are going to use a 5.75” rock crawler tire that gives us a center of 2.875” to our axle stub, not our axle tube. Now say at ride height your chassis is 3” off the ground. Lets think about what is in a portal. You have the portal gears, an extra stub axle, 3-4 more bearings and the portal itself. Those have to weight something right? What do we have in a normal axle? A ring and pinion, 4 bearings and two axles.
Weigh your portal parts and think about this. Those extra parts, that extra weight is below your axle housing now. You didnt have those parts on a normal axle so now you lowered your weight just stock. Next think about knuckle weights. Let say for numbers we run 90 grams of weight per side. On a normal axle that 90 grams is roughly .375” below the axle housing tube. Now with portals that same 90 grams is close to a half inch lower, probably more. Seeing a trend here yet?
Here comes the fun part. Lets take a traditional shafty rig and again say the belly/skid is 3” off the ground at ride height. Some may think if you added a half inch under the axle then you just raised the chassis a half inch. Wrong to us. We arent changing the chassis height, we are simply moving the axle housing up. Your center of gravity is truly some where around the motor/trans in the middle of the truck. That horizontal line of your CG didnt change in relation to the ground, it stayed the same. You merely leveled out your lower links and added a bunch of weight lower then before.
Chassis designs need to accommodate for this and we did that with the Pneuma. Doing this helped us keep a balance between shock up travel and droop. Now the geometry side of things portals really carry a benefit. For one we all know how we’re use to bending links for clearance. A traditional truck tends to have links angled upward to the skid. We bend them to gain clearance from dragging on the rocks.
Portals bring our links flat and up, out of the terrains way. It also does something beneficial to our geometry. Lets show a picture of our Pneuma if it had traditional axles on it.
First heres some quotes from the linked Crawlpedia.com:
Anti-squat in a linked suspension system determines how the rear end of a vehicle moves under acceleration or upon the rear axle contacting an obstacle at speed. The anti-squat value is determined by the vertical angle of the rear links as they relate to the front axle position and the center of gravity of the vehicle.
- Anti-squat between 140% and 180% works well for drag racing on smooth pavement with heavy rebound valving.
- Anti-squat between 110% and 150% works well for hardcore technical rock crawling and some styles of rock bouncing.
- Anti-squat between 100% and 130% works well for mud drag racing and some hill-n-hole racing.
- Anti-squat between 10% and 50% works well for high speed desert racing.
- Anti-squat between 20% and 80% works well for open road racing and rally racing.
- Anti-squat between 70% and 100% works well for rock crawling and trail running.
Basically this is saying to the same thing but two different ways. AS base point is 100%, thats like your base point. But lets say you cant go below 100% and can only go up. You would probably find a liking in the 110-150% range for AS. But now lets say you start at 100% and cant go higher, you can only go lower. Then you may find your comfortable with negative anit squat, that 70-100% range. We think it will make sense after seeing our pictures.
So lets look at our Pneuma if it had SCX/AX style axles with links angled up. for instance. Take note of where the CG is in relation to the tire center but more to where the AS numbers would fall in.
We are basing our link locations off best guesstimate but notice where your upper and lowers converge. They come together virtually at your instant center (IC). In this configuration we would have AS over 100% and to lower it we would be stuck running high upper link risers. In our opinion also we arent driving enough weight transfer forward for front traction.
Again – not an engineer this is just our opinion from experience.
Now lets look at the Pneuma with portal geometry.
Notice the difference? Now our IC is the same, our CG didnt change because our chassis is still the same height off the ground. But our AS numbers are below 100%.
So our traditional ring/pinion axle we have been using for years would fall more into that 110-150% range due to the link geometry. Where as our portal axles with virtually horizontal link geometry puts us more into the 70-100% range. Both ranges accomplishing, with tuning other things of course, virtually the same effect. Understand what those rear link risers are doing now?
A lot of reading, questioning and trials were put into the Pneuma because we believe in portals. Now thats not to say the Pneuma wont work on the traditional style axle. As you read above both axles on the same chassis can accomplish the same characteristics/feeling. And thats not to say Portals wont work on any chassis thats already out there, they can and are proving do. As we said from the start of the project, for us this was a complete redesign. We took many extra steps when designing from our previous chassis and think its paid off. For a little comparison heres a shot of all 3 of our chassis’s layed over on each other.
Good food for thought we think.