What causes heavy steering at racing speeds

[Ron Earp]

AI have come to the conclusion that the gyro effect of that heavy rotor could have been the main reason for the heavy steering and my main reason is because the steering got heavier the faster I went round a corner. Anyway I don't have conclusive proof just happy to have it sorted.Thanks to all that have helped me.
Ross

I'm not sure that is it but I've never experienced it. Might be hard to really nail down without some calculations though. Hmm, maybe I'll bust some physics books out later but I'm not sure that a rotating ~15 lb rotor is going to provide enough difference to cause the steering to be heavy.

For example you go through the corner at 90mph and you get X amount of angular momentum from the rotor turning that speed.

The next time you really haul the mail and you get say 120mph through the corner (and I think 30 mph corner difference is extremely over exaggerated with a trained driver of any proportions) and get Y effect. The delta is only 30 mph and we're going to need that 30 mph difference to provide the heavy steering feel.

Doesn't seem that the difference there would do it but I'm not sure without performing some calculations. Definitely interested in learning more about the effect and what solved it.

 

[Trevor Booth]

spot on Ron,
the gyro effect is not the difference, you are not displacing the axis of the rotor relative to ground plane irrespective of the steering offset. If you do not displace the axis you have no precession (caster effect ignored) no precession = no force

 

[Ross Nicol]

Ah in that case guys I will return to the excessive scrub radius as the cause. To give you an Idea how heavy the steering was, my 2 handed technique on the steering wheel has resulted in a lump under the cover.Even though my last run at Phillip Island ended with motor dramas I did get to run at Sandown with the temp wheels on the front and the steering was much lighter.

Ross

 

[Jac Mac]

Ah in that case guys I will return to the excessive scrub radius as the cause.]
Ross

Good boy Ross, thought I might have to get back on your case there for a while, rim offset plus the adjustment limitations of your previous setup were also factors.

(((( my two handed technique has resulted in a lump under the cover??? ))))

Not sure I can or even want to help with that one

Jac Mac

 

[Chris Duncan]

Ah in that case guys I will return to the excessive scrub radius as the cause. To give you an Idea how heavy the steering was, my 2 handed technique on the steering wheel has resulted in a lump under the cover.Even though my last run at Phillip Island ended with motor dramas I did get to run at Sandown with the temp wheels on the front and the steering was much lighter.

Ross

Glad to see you achieved an improvement.

It looks like you also maintained your turn radius, the upright doesn't look like it's too far inside the rim lip to cause clearance issues.

It's probably irrelevant at this point but one other thing I'm noticing now that you posted pics of your uprights. Looks like an upright off a car with a 4 banger. Here in the states we use a similar unit from a Pinto. The steering arm looks like it might be a little short which might increase effort. It depends on your steering rack ratio. But then you can get problems in the other direction with too many turns on the rack making the steering slow.

Synopsis of this thread. (IMHO). Many GT40 kits have problematic front uprights. The early CAV's are even worse than the RF's, downright dangerous.

This is caused by 2 things.

The necessity of keeping the "look" of the Halibrand (or BRM's) which have limited outer offset. Which with the bigger modern brakes drives the upright further inboard which increases scrub radius.

The availability of affordable "off the shelf" front uprights. Of which there are none that are optimum for the GT40 replica. This leads leads to compromise elsewhere in the front suspension. In house uprights can also be under-engineered the early CAV being a prime example.

From a replica build engineering standpoint the front upright is the single most complex part on the entire chassis. I spent 1 year on the drawing (cad) boards designing my chassis. About 1/4 of that time was spent on the front upright. It went through 5 major design changes, and if it was going to be done again would be tweaked even further. And the end result is something that is not cost effective. (But that might change if it was CNC'd)

 

[Ross Nicol]

Can't believe I wrote that

 

[Ross Nicol]

Kalun
I agree with you, the steering rack has to be fairly quick in a race car and there is a trade off as you say.By making the upright arms longer you reduce steering effort but increase required steering wheel travel, as you have pointed out. I think your'e on the right track regarding the wheels too.Most of us car admirers like to see a wheel well filled with a wheel and tyre that comes close to the inside of the guard/fender.The standard RF front wheels (Halibrands) I had were 7" wide and their offset was such that they had more outer rim showing than modern wheels. When it came time to have my 9" wide race wheels made, I wasn't aware of the best wheel offset to have, even if I was able to do something about it. So the wheels were made with a 4" outer rim and 5" inner rim. They looked great and the tyres came close to the inner guard/fender lip.The wheel manufacturer didn't warn me about possible problems (maybe cause he races a Po...e****) with scrub radius, so blindly on I raced for 4 years with ridiculously heavy steering.The uprights are actually off a 6 cyl Cortina (unique to Australia) and have a beefed up stub axle and is much stronger than the 4 cyl British sourced Cortina upright. So it is highly likely that when the front end of the RF was designed the upper and lower arms were made to a length to suit wheels with a fairly deep outer rim.Totally opposite to modern design.

Ross

 

[Cliff Beer]

In my experience the answer here is the result of a combination of physical forces and principals. The gyroscopic effect is indeed one of them - while it's true that the gyroscopic effect wouldn't increase the steering effort simply because of the fact of the car being in a turn at that moment, it will however, be felt and be quite apparent if the front wheels are being steered aggressively (different steering angles) in the course of the turn to correct the direction of the car and progression through the turn. And, the more aggressively the car is being steered (greater variation and change in steering angle throughout the course of the turn ie. lots of lock and opposite lock and back and forth) then the gyroscopic effect upon steering effort will increase proportionately.

Also (this may have already been mentioned/considered), the steering effort is going to be higher generally in a race car because the lateral loads are obviously higher when cornering than in the family wagon out for a Sunday cruise. The same amount of castor in each will produce dramatically different required steering effort.

Finally, while it's true that steering effort generally goes down once the wheels start turning from a standstill (high steering effort due to the friction of turning a statis tire on pavement) and many modern sophisticated road cars compensate for this with variable rate and variable boost power steering, a race car with wide race rubber/slicks on it will produce greater resistence to turning because of the proportionally greater friction at the tire/road interface. If you don't believe me then try turning the steering wheel of a NASCAR car at rest with slicks on it and then compare that to your parents old Monte Carlo - neither is easy but one is definitely harder than the other!

 

[Johan]

I admit that I have not read this thread, and that said, have you tried a bigger steering wheel ?

 

[Ross Nicol]

Johan
I'll answer your question with a photo.I'm sitting in the car with my feet resting on the pedals. I have just enough room between my knees and the steering wheel to slide my fingers in the gap. Everything is tight in a GT40.

Ross

 

[Johan]

I run 3-31/2 degrees of castor in the T70 coupe and it seems hard to steer and wears out my hands and forearms, but turns in really nice. I ran 17- 18 lbs of air pressure in the fronts, that becomes 20. We switched to running 19lbs. nitrogen and that solved the problem, as it was easier to steer and did not grow.