Quote:
Originally Posted by abhijeet080808
Brake dive to a certain extent is beneficial. When the front end dives, weight transfers to the front which increases grip at the front. If there is no front end dive (panic braking where suspension does not get time to load up or using too stiffer spring) the tyre skids. Also brake diving reduces overall wheelbase and makes the motorcycle turn quicker (trail braking with the front brake).
Now, what happens when there is too much dive? The suspension bottoms out. A sudden force can now easily get applied on the tyre, now that there is no cushioning effect. Result is a skid if the rider is careless with the brake! Also, too drastic change in the wheelbase can catch the rider unaware and make the bike oversteer suddenly in the middle of the corner. Also too much oversteer tendency means that the bike will feel twitchy on that long curve and the rider will need to hold the handle tightly! Thus the bike feels as if it wants to fall in the corner. Rider is stressed while cornering.
So, we do need dive. But too much of it is bad, as it can be dangerous for a novice rider! And stiffer spring is more forgiving on the rider.
Stiffer springs (in general and not wrt brake dive) also provide more feedback and helps prevent sudden unwanted weight transfer due to bumps etc. This is the only benificial part.
So, we choose a balance between the two.
|
So far so good Abhijeet with just a couple of exceptions. It is true that we need 'some' dive to allow for 'cushioned' or 'graded' weight transfer onto the front contact patch. And that too stiff a front would be almost akin to a bottomed spring. This was actually the reason for the decline of the once favored 'anti-dive' systems (the Honda TRAC and the Kawasaki AVDS). Too stiff a springing would make the wheel chatter and skitter over small bumps.
It is also true that there is a certain reduction in the wheelbase which increases instability which in turn contributes to some extent in making the bike easier to turn in (more 'flickable). But the major contributor here is the reduction in the bike's 'trail'. See fig below:
The front suspension compression under braking reduces trail (the steering head drops lower and the contact patch spreads rearwards) and this reduction in trail leads to quicker direction changes.
So in conclusion, we tread the middle path by 'tailoring' some amount of dive into the front suspension to get the best of braking without compromising on comfort and inducing too much instability.
Quote:
Originally Posted by pranay
I am not sure on this one, sir. Actually, I meant the R15 has a Linked type suspension which means that it has a link between the swing-arm and the rear suspension. The link acts like a lever for various loads. The lesser the load the higher the lever ratio, and softer the suspension becomes and the whole dampening system is soft. But greater the load, the lesser the lever ratio, and harder the suspension becomes, therefore hardening the dampening system. I don't think this is pre-load as this is more linear whereas pre-load is more incremental. I might be wrong though.  |
Pranay: I'd thought you were asking about the 'pre-load' thing.
As for what you've stated above, the 'link and lever' suspension has links and levers just to multiply the available suspension travel. A shocker/damper compresses just about a couple of inches before bottoming out. To get 4 inches of wheel travel with the same shocker unit requires the use of some sort of linkages and levers. The 'progressiveness' of the suspension comes from what are called 'variable rate' springs. The spring coils, visually speaking, are far apart towards the top of the spring and get closer together towards the bottom of the unit. The top coils get compressed easily under light loads while the bottom coils deflect less and need higher loads to compress. So, on the whole, the coil spring acts like a 'soft' spring under light loads and as a 'stiff' spring under heavy load. More expensive units also provide variable damping but then that would be another story.
Quote:
Originally Posted by abhijeet080808
Diving does reduce the wheelbase. This "temporary" phenomenon helps a bike to turn in quicker in corners and at a higher speed. As we know a shorter wheelbase results in a flickable bike. This particular technique is called trail braking. Also, we do not need to apply full brake to observe this. Lightly loading the front is enough.
|
There seems to be some mis-conception about 'trail braking' here. I'll start with a diagram that I've taken from a book 'Sport Riding Techniques' by Nick Ienatsch.
The orthodox riding technique staees that we finish all our braking and gear shifting while the bike is upright, enter the turn on constant throttle and just past the apex, feed in the throttle progressively. But the preferred method now is using 'trail braking'. Trail Braking is a technique where the rider progressively reduces his braking force as he gets deeper into the turn and closer to the apex. This progressive and smooth reduction in braking is to use a progressively larger share of traction for turning, a need that hits its maximum at the apex, where the braking input is reduced to zero. Trail braking has two major benefits: 1. It allows for braking while turning, allowing the rider more control over his situation. and 2. The rider can carry speed deeper into the turn and by slowing progressively towards the apex, has more reserve traction to trade for the same needed for turning.
Post-apex, progressively openng the throttle makes for gradual rearward weight transfer allowing the rear tyre time to gain traction and transfer power for acceleration.
In practice, the overlap between trail braking and powering out is a smooth blend, always balancing the available traction to the traction needs.
Quote:
Originally Posted by abhijeet080808
I had considered all such factors to be same.
My friend, if the bike suspension is stiff as a brick, will weight transfer occur? No. Weight transfer occurs only if the front end dips down compared to its original position. So, suspensions needs to be soft to cause weight transfer. Of course too much weight transfer due to very soft suspension will result in oversteer as the rear end looses grip.
And it is used in tight corners as well. Nothing to suggest that it can be used in long curves only. The steering geometry change you are talking about is this wheelbase change only - more specifically the trail of the bike changes. (Hence the name trail braking).
Anyways, want to hear what OF has to say regarding this.
|
Ref to the bold part: weight transfer 'causes' front end dive. Weight transfer is NOT the result of front end dive. Again a fig below:
Weight transfer WILL occur even with no suspension springing at all, ex. the bicycle. The bicycle skids (on loose surface) or does a stoppie(on tarmac) because with no suspension to 'gradually' load up the front tyre, a sudden load forces it to stop. If the tyre grips, the whole bicycle rotates around the front contact patch that acts as a pivot. If the tyre does not grip (i.e. the coeficient of friction between the tyre and road is small), the it locks up and skids.
And abhijeet, hope I've managed to clear the air about the 'trail braking' part. And yes, trails braking is a useful skill, whatever the turn is like.
Quote:
Originally Posted by pranay
And with all due respect to OF and other highly knowledgeable people in xBhp, I think discussions from less knowledgeable members with differing point of views should be encouraged because if you can substantiate your views like both of us did, it relates to more thought process required while doing your research about bikes.
|
An incessant curiosity for things not known is anytime welcome at my end.
Ride long and safe...
OF