Your experiences: Scrub Angle

[tiagra]

Hi guys...

 

putting on different rims and tires changes quite a few things, among them is the so called "scrub radius" (see http://en.wikipedia.org/wiki/Scrub_radius ). Since the stock value is not known (to me), I have drawn this little picture with the actual values for camber and KPI (kingpin inclination) from the factory service manual and the stock tire size.

 

The result looks like that:

 

As it looks like, the stock scrub radius is already positive - as expected for such an old car. What worries me a bit is the fact, that when increasing the track width to my likings, I'd almost double that value. Bigger tire has some effect, but since the KPI is not very big, the effect is small. Sadly.

 

So the question is: How does it drive? Anyone running greatly increased track width? How does it run on the open road... bumpy roads...?

 

All that under the assumption, that the wheel bearings won't mind. After talking to several race specialists I'm slowly adjusting to the fact (?), that the wheel bearings are pretty strong

 

Thanx for your thoughts,

 

.martin

 

P.S.: Just noticed: The topic should be called RADIUS... not angle. Too late :>

[Klas_B]

Hi,

 

I have used 38.5x14.5 R16 on my TGB-13 C304 6x6 for approx 3 years now. Rims are 2" wider + 1" beadlock outwards, in total 3" outwards. Dont know the actual ET, but for sure much different than stock.

 

The only downs I have noticed so far is

 

•  "force feedback" in steering wheel is a bit worse due to larger swept radius. This is noticeable when I enter a rock/stone or boulder non perpendicular to travel direction. Best to keep ones thumbs out of the steering wheel.... On-road this is not a problem.
• On road stability is a bit affected due to the larger swept radius. I have compensated this with 40 mm spacer on the back seat of the leaf spring, increasing the caster angle slightly. This had a major impact on stability, runs now better than stock. Not ideal for front prop shaft vibrations, but so far I havent seen any kind of lifetime problems on joints or bearing. On-road I never go with with front axle drive anyway.
• Parallel tie-rod life may suffer due to larger forces (swept radius again). This is mainly a problem when I reverse over a large obstacle that introduce pushing force in the parallel rod. Normally, it just bends upwards (like 100 mm off axis). This is pretty easy to correct in field with a offroader friend with winch on his vehicle, or by myself with a simple hand wire winch and some steel bars (called "spett"). For the future I plan to replace the parallel rod to an oversize dimension. Stock size will only survive a limited numbers of bending back operations.

But in general, I have no problem living with the facts above. Using wide 38.5" tires and low pressure (like 0.5 in front and 0.2- 0.3 in rear) is such a offroad performance boost which I cannot live without.

 

Best

 

Klas

[tiagra]

Hey Klas,

 

from what I read, you are a pretty serious offroader and therefore probably have a pretty high tolerance level when it comes to on-road stability etc., right?

Nevertheless, the effects you describe are pretty serious, even though you can live with them

 

It seems to me, that your term "swept radius" is just what I described as "scrub radius", so we're on the same side here. If you really widened stock rims by 3 inches to the outside only, your ET would work out to be roughly 68mm, bringing us to a scrub radius of approx. 110mm, which is almost 1.5x over stock.

 

I'd really like to hear your experiences, if you fitted a steering damper - that's clearly an upgrade you could use in that situation. I wonder how much it would roll back the effect of increased scrub angle, that you called "force feedback". Any plans in that direction? Maybe only temporarily for testing purposes?

 

Thanx for you insight, very much appreciated,

 

.martin

 

P.S.: The width of your tires of course increases the effect greatly, when hitting obstacles on the outside. Maybe 12.5" wouldn't be that crazy then....... hm.

[Klas_B]

Hi again Martin,

 

nice with fast comments on this.

 

Regarding on-road behaviour >> Yes, I think Im very tolerant regarding bad effects of my mods. The 6x6 with 38.5:s is modified with improved offroad in first priority. However, I have a stock TGB131A running as well to compare the modifications with. Both vehicles are used on road like 150 km:s during each offroad meeting occation. And they can be once a month or more, all seasons.

 

Regarding steering damper. Havent thougt about fitting one yet. I have a power steering upgrade in mind (Mercedes GW unit), so no efforts putted on this soo far. But adding a damper will of course reduce force feedback offroad. On-road on bumpy roads a damper may reduce wobbling that may occur due to worn lower spindle bearings cups (pos #15 on your sketch). But if you keep track of play on the portal housings, this should not be an issue.

 

Next upgrades (in random order) for my rig will most likely be:

 

• Coil spring conversion for the front axle. Improved flex, more linear spring ration with less hysteresis and smoother ride on very stony ground is in focus.
• GW powersteering. Consuming smaller breakfasts offroad mornings would be nice...
• Transfercase reduction ratio gearing. I think my vehicle goes a bit to fast on low 1 when driving very stony ground. I aim for approx 1.42 reduction for both low and hi on top of stock gearing. If Im successfull, this will be an bolt on option so transfercase and engine can stay in their stock locations. Today I use hi 3rd gear on road, making approx 70-80 km/h with decent revs. Would be nice using 4th gear as well...
• Rotate the 3rd axle portal housing 45 degrees, making space for 44" Boggers. This implies a set of wider rims as well, beadlocks, cab/frame mods etc. for clearance. ET getting closer to zero...
• Engine swap to turbodiesel (Toyo 1HD-FT or MB OM606.96x) with like 400 nm or so... This is a must using the 44" boggers (or similar) in mud. I need power enough to create mud arces in swamp areas, ie. getting rotational speed for clearing tire thread pattern.

BTW, just love the pics of your C304. I use them as desktop background on my computer, gives med offorad pleasure every day on work!

 

Best Klas

[tiagra]

Hiya Klas,

 

quick replies are really nice, I'd second that!!

 

I think you should add "steering damper" on your list as #1 - it's a very cheap and easy to do upgrade and I suspect the effect to be overwhelming!!

 

Other than that: My TGB is still somewhere south with his previous owner, as I haven't gotten around to finding a new place after moving to another city. He has lots of space and it's dry and warm for the TGB Eager to start with it........ engine is 12H-T from Toyota, fitted into the frame, but not running, yet

 

I'd like to have 17" rims, but can only find some with ET35. That would double the scrub radius and according to your experiences I might not be happy with it.... though running only 12.5" tires could lead to almost the same overall vehicle width as are running - that's why I'm keen to know how much a steering damper would help. Apparently you don't have problems with the wheel bearings...

 

But as in any case, you are using your TGB differently than others... most certainly also than myself. If I travel 3000km of rocky crap road in Tibet or Mongolia, fully loaded... the wear and tear is MUCH different from a few kilometers of hard offroading in the forrest. MUCH different. I broke all 4 wheel bearings and all ball joints in my syncro during the last trip to asia - just because of extended crap roads and pretty high verhicle weight. It just breaks everything. There you have the actual reason for my concerns

 

Thanx,

 

.martin

[Klas_B]

Hi Martin,

 

I think I now have an idea of your aims with your vehicle. Reliability used with high loads on crap roads will be in focus, right?

 

The first things that crossed my mind was maybe to increase the ET closer to stock, if wheel bearing life is an issue. Cutting out the rim centers and weld in new ones is not too complicated. Laser cut rim centers with your hole pattern and externally suited to the rim inner diameter at desired ET is pretty straight forward. New centers are inexpensive and will have better tolerances than stock rims. But I´m not sure if you plan to improve stability of your truck by increasing the track width?

 

Another idea that I have is to increase wheel bearing strength by fitting a small help-bearing (axially floating) between the hub end and the portal housing. It requires some machining of both hub end and housing, and some additional parts. Going to higher loads, larger and wide rubber, more drivetrain power etc will sooner or later point out the weakest spot in the transmission. I have seen some broken portal gears most likely due to bad or improper adjusted wheel bearings. But that was due to extreme shock loads with like 38" x16" wide boggers leaving ground at full throttle....

 

On the other hand, the Volvo portal axles are engineered for the heaviest truck in the C30x series, namely the TGB20 (C306) with a total maximum wheight of 5.7 tonnes, running both on and off road at military conditions. This fact tells me that there is a margin in terms of component strength and life that can be used for modifications (like smaller ET, larger and wider tires) if you are below those maximum axle loads.

 

Anyway, your Toyo 12H-T is an excellent choise for "infinite" engine life on an expedition truck, as what I have heard!

 

Best

 

Klas

[tiagra]

Hi Klas,

 

the thing is: The truck will be registered in Germany, so each and every component needs TÃœV approval. IF I could cut/weld/bla rims, I'd not be thinkiong about ET35 and crap, I'd just build a 9x17ET85 rim But welding rims and stuff is illegal over here and my last chance (which I thought would still be possible) just closed YESTERDAY. So basically: No more welding within Germany. Unless you let a certfied rim manufacturer do it, whcih we also have - or resort to France, they have BOUZINAC for sure. Buuuut.... neither of them can source 9x17 hoops with proper speed/load certificate, which would be the basis to built a legal rim upon. I have been searching THE PLANET fpr 4 months now, but no chance. I can find 9x17 in the US, but they cannot certify a certain load in combination with a defined speed. They simply state load, and that's it. German TÃœV guys think that's ridiculous, since the load capacity decreases with increasing speed, but of course, nobody cares in America So here we are, thinking about stupid ET35, just because there is this DOTZ Hammada alloy rim available in 9x17ET35.

 

In terms of usage, think of stony wash board "roads" for an extended period of time, with 4+t truck weight. Extended "a few thousand kilometers" each time, before you hit civilisation again.

 

E.g. like that - (my g/f) on the way to Mt. Everest, Tibet 2012:

 

Ciao,

 

.martin

[Pajjen]

Do you think that they know how a stock rim should be on these cars in germany?

If its seriously done i doubt they will know if its "home made" or real certified rims... Its not legal to do own rims here but we do.. on old cars, because they dont know if its certified stuff or not.

So i would say that at least try it

 

edit: i have made the extra coilsprings in front and they tought it was original

[tiagra]

Yes true, but I will convert to disc brake which will result in Toyota stud pattern.... so the rim is definately not stock Volvo )) A stock Toyota rim has a very shitty offset... e.g. 35 That's why I calculated with 35 in the picture above......... Plus: I want to switch to 17", because mich better tire selection there If I stick with 16", i don't have a problem at all.........