Official Baja SAE Forums Homepage
Forum Home Forum Home > General > Design Discussion
  New Posts New Posts RSS Feed - Tuning for jumps
  FAQ FAQ  Forum Search   Events   Register Register  Login Login

Tuning for jumps

 Post Reply Post Reply
Author
Message
jarmumd View Drop Down
Organizer
Organizer
Avatar

Joined: Oct/17/2008
Location: Huntsville, AL
Status: Offline
Points: 178
Post Options Post Options   Thanks (0) Thanks(0)   Quote jarmumd Quote  Post ReplyReply Direct Link To This Post Topic: Tuning for jumps
    Posted: Nov/16/2008 at 1:00am
I read some posts on the SAE forum about air shocks and coil shocks, this thread really has nothing to do with that I think both will work.  But it would be interesting to see of teams whose cars "jump well" what their CG's are.  My point is that I think that shocks will only get you so far, sometimes if you want to go over a jump without pitching forward, you need to correct the cg location.  Thoughts?
Back to Top
dillon_b12 View Drop Down
Baja Godfather
Baja Godfather
Avatar

Joined: Nov/15/2008
Status: Offline
Points: 781
Post Options Post Options   Thanks (0) Thanks(0)   Quote dillon_b12 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/17/2008 at 12:00pm
Case-Western's mid-engine car supposedly has a 30:70 F/R weight ratio.  Having a heavy rear end helps more than almost anything. 

South Florida thought that their wing helped a lot and that car did jump well but I think it was more that they had a big counterweight(the wing) hanging off the back of the car than it working as an actual wing.

Designing to make a car jump well is not as necessary as most people think.  There is no great advantage to a baja car that can jump 50 ft. when you have to sacrifice handling characteristics to achieve it.
Back to Top
jarmumd View Drop Down
Organizer
Organizer
Avatar

Joined: Oct/17/2008
Location: Huntsville, AL
Status: Offline
Points: 178
Post Options Post Options   Thanks (0) Thanks(0)   Quote jarmumd Quote  Post ReplyReply Direct Link To This Post Posted: Nov/17/2008 at 1:53pm
Unfortunately it is the first thing that teams tune for most of the time I think.  I suppose my thought was that people always seem to attribute jumping performace with a certain type of shock or suspension type, and I don't believe that the shocks contribute as much as people think.  Well, I suppose that they do, but that you would have to have a really large range of springs and dampeners, which may make it pointess (due to turning).  But if you could maintain more of your turning performance by modifying the CG location for jumping performance?

I remember at midwest 2005 there was a car that had all aluminum a-arm, a blue frame, and an engine almost outside of the rear of the frame.  That car was almost always level off of the jumps, and I believe that the last lap, the hit a jump way to hard and broke all the a-arms.  dillon_b12, we always tuned for turning performance first and jump performance second, so I definitely agree with you, but it does depend on the course to.  If you can stay level you can hold more speed over jumps.  I believe we always shot for a 40/60 ratio ourselves. 

Since you generally don't want to pull weight off the front end, if you did, I wonder if it makes sense to incorperate more concepts like rear-steer to help re-gain turning performance?  Or maybe to incorperate a front sway bar with a heavy rear end to re-load the front tire?
Back to Top
dillon_b12 View Drop Down
Baja Godfather
Baja Godfather
Avatar

Joined: Nov/15/2008
Status: Offline
Points: 781
Post Options Post Options   Thanks (0) Thanks(0)   Quote dillon_b12 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/17/2008 at 6:12pm
The complication and added weight of adding those components to make a car that both jumps well and handles well is not worth it IMHO.

I think the main thing these cars have going against them for jumping is the short wheelbase and lack of power.  Typically ATV riders accelerate to a jump, coast for a second, and then bump the throttle as they go over the jump to pull the front tires up slightly.  We just don't have the power to do this.

Course design is another big factor.  A proper approach ramp would have something like 2-3 times the length of a baja car's wheelbase and no lip at the end.  If you notice, baja cars fly very well over table top jumps.

Long, gradual approach ramp:


Short, steep approach ramp(notice the rear-end kicking up):



Edited by dillon_b12 - Nov/17/2008 at 6:12pm
Back to Top
jarmumd View Drop Down
Organizer
Organizer
Avatar

Joined: Oct/17/2008
Location: Huntsville, AL
Status: Offline
Points: 178
Post Options Post Options   Thanks (0) Thanks(0)   Quote jarmumd Quote  Post ReplyReply Direct Link To This Post Posted: Nov/17/2008 at 10:19pm
Yea, I am not disagreeing with you there.  But from experience, many people think (including me) that they know what a baja car will do and what is "silly" to do, which is why if you have never tried moving the CG of your car (in testing) you should.  Even if you prove yourself right, you would be learning something you may not have been really sure of before.  But if not, if the car surprizes you, you may find that you want to change things up.  I HATE stagnation in design from "I know what will happen," which is why I'm being so enthusiastic about doing testing like this.   And why I hope that more teams can post up about the weight split of the car and how well it jumps.
Back to Top
dillon_b12 View Drop Down
Baja Godfather
Baja Godfather
Avatar

Joined: Nov/15/2008
Status: Offline
Points: 781
Post Options Post Options   Thanks (0) Thanks(0)   Quote dillon_b12 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/18/2008 at 11:39am
Our team is one that hates stagnation in design as well.  That is why this year we implemented trailing arms(a team first), tilt/telescope steering, and a few other goodies.

I think our split is around 40/60 as well.  Our car jumps pretty well,depending on the jump of course.  I think that playing with rear shock damping could significantly improve jump performance but again I'm not sure at what cost to handling.

I would think that having stiff(slow) rebound damping on your rear shocks would help significantly by keeping those rear wheels from tossing you forward as you crest a jump.  However, our shocks don't have adjustable rebound so...who knows.
Back to Top
karman1970 View Drop Down
Milling Master
Milling Master


Joined: Nov/18/2008
Location: Ft Worth,TexASS
Status: Offline
Points: 51
Post Options Post Options   Thanks (0) Thanks(0)   Quote karman1970 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/18/2008 at 4:14pm
One arguement I've heard is to actually keep the nose heavy, thus creating a high polar moment of inertia in front. In theory, the nose will be slower to react when coming off a jump and allow the rear to catch up. It's a theory, anyway.
Back to Top
jarmumd View Drop Down
Organizer
Organizer
Avatar

Joined: Oct/17/2008
Location: Huntsville, AL
Status: Offline
Points: 178
Post Options Post Options   Thanks (0) Thanks(0)   Quote jarmumd Quote  Post ReplyReply Direct Link To This Post Posted: Nov/19/2008 at 1:22am
in my mind, if you had a really heavy front (think all the weight) then the rear would be very responsive - but I think that is the problem, it's so responsive that it pitches the rear end up (pitching the nose down).  Which is why I was saying that to keep the car level over a jump, you would be.....  mass-dampening (can I use that as a term?) the rear of the car, keeping the nose from pitching downward.

I think the key may be that by moving the CG back, you use the rear shocks to translate the mass of the car as opposed to rotating the inertia of the car.  Think of the two extremes, one with all the mass lumped above the front suspension, and one with all the mass lumped above the rear suspension.   Both cars have the same MOI.  In the first case, as the front tires leave the ground, the rear suspension reacts mostly with the MOI, causing it to pitch.  In the second case, the rear suspension reacts mostly with the mass of the vehicle, and being close to the CG (and MOI), imparts a much smaller moment to the car - causing it to leave the jump more flat (so you don't endo!).
Back to Top
karman1970 View Drop Down
Milling Master
Milling Master


Joined: Nov/18/2008
Location: Ft Worth,TexASS
Status: Offline
Points: 51
Post Options Post Options   Thanks (0) Thanks(0)   Quote karman1970 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/19/2008 at 4:34am
Our primary concern has always been little jumps like at Florida in '06. We decided the best solution was to use the brakes! Ivan "Ironman" Stewart said the sign of a good driver is not kowing when to drive fast, but when to drive slow.
Back to Top
adrive7 View Drop Down
Baja Godfather
Baja Godfather
Avatar

Joined: Oct/19/2008
Location: Lancaster, CA
Status: Offline
Points: 711
Post Options Post Options   Thanks (0) Thanks(0)   Quote adrive7 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/20/2008 at 10:35pm
Our 2006 car, (which was a total POS) had a longer wheelbase and a weight distribution of around 20/80. It still didn't fly very well, AND it couldn't turn. And when I say "couldn't turn" I mean complete and total understeer at full speed. Some of that is suspension design (Bad) some of it is weight distribution (also bad).

The main problem with jumping bajas is lack of power, which has been mentioned. That quick power blip on a bike/quad is everything. Although I think slow rebounding would help it some. It certainly does on a Mountain Bike.
-Joe
Chief Organizer, Baja California 2016, 2017, 2019
Ohio State Baja 2005-2009
Co-Host, Science... Sort Of
http://www.sciencesortof.com
Back to Top
jarmumd View Drop Down
Organizer
Organizer
Avatar

Joined: Oct/17/2008
Location: Huntsville, AL
Status: Offline
Points: 178
Post Options Post Options   Thanks (0) Thanks(0)   Quote jarmumd Quote  Post ReplyReply Direct Link To This Post Posted: Nov/20/2008 at 10:58pm
But I've seen baja cars jump well (well = flat , not pitching nose first), I don't remember if I've seen one jump and turn well...   I think that power could make up for poor dynamics, but think about the moment arm that the suspension has on the cg vs the moment arm of the tractive power from the tires...  You get at least twice the moment arm about the cg from the suspension than you do tractive force. 

My point is, until I see someone's before and after pictures of a jump with exactly what they changed, I will maintain that the problem is very poorly known - that everyone knows band-aids, but not the actual cause.
Back to Top
blue2kss View Drop Down
Welding Master
Welding Master
Avatar

Joined: Dec/23/2008
Location: USF
Status: Offline
Points: 169
Post Options Post Options   Thanks (0) Thanks(0)   Quote blue2kss Quote  Post ReplyReply Direct Link To This Post Posted: Dec/23/2008 at 3:08pm
I personally believe that the rebound damping in the shocks has a lot to do with pitching over the jumps.  The faster it is, the more pitch you are going to get (in the rear).  This is the reason why we tried the wing, so we do not have to slow our rebound damping down so the car jumps well, but that is not what we wanted throughout the rest of the course.

As far was the wing goes, it actually did work as a wing.  The wing itself weighed 2-3 lbs (filled with Styrofoam and one layer of E glass).  I really don't think that is enough for use as a counterweight.  The downforce it made at 40 mph was 33 lbs, and with the moment arm of 3' to the CG, it translated to a 100 lb torque.

Another reason that it worked so well was the surface area was enormous (three square feet).  We have tried just running and holding the wing on foot, and that amount of real estate has quite an effect on felt down force.
Dustin Bride
University of South Florida SAE Alumni/Consultant
Mechanical Engineer - Naval Surface Warfare Center, Marine Corps. Counter IED Development
Back to Top
schmicj View Drop Down
Bolt Sorter
Bolt Sorter


Joined: Oct/23/2008
Status: Offline
Points: 10
Post Options Post Options   Thanks (0) Thanks(0)   Quote schmicj Quote  Post ReplyReply Direct Link To This Post Posted: Jan/14/2009 at 2:32pm
My take on jumping performance is course design. The power is also a big factor that Baja cars just don't have. Also depends on luck. Most ATV/MX courses jumps are probably all designed in the same way as to approaches and lips. Which typically just don't jive well with Baja cars. Almost all courses that have been on MX courses with jumps usually end in pain with the exception of Aztalan (sp?) in Ohio was it? I can't remember, in 04. That course was an absolute dream for our 04 car. But many others, like RIT in 07 were horrible. I think the only race where I requested to come in early due to physical discomfort to put it best. Had to slow for a variety of obstacles. Which is why I believe alot of it comes from luck and what you built your car for. Most MX bikes and ATVs have very similar characteristics between them so the course affects most in the same way. Riders have alot to do with at as well as you can shift your weight around. You can tune for jumps but feel the majority of tuning should be for on dirt handling as its where you spend most of the time. But on certain tracks being able to take the jumps and whoops will allow you to go faster. I feel its a toss up unless you can test on the track prior to the race. Which was excellent in Wisconsin 06.
Back to Top
 Post Reply Post Reply
  Share Topic   

Forum Jump Forum Permissions View Drop Down



This page was generated in 0.078 seconds.