Lurch, you may be correct about Ivan Stewart using IRS on his SCORE truck. But you have to remember that he is not racing on a closed course track like MX racing where fast cornering is a must. Keep in mind that SCORE is very high speed desert racing, not closed course competition. Also, I urge everyone that is in favor of IRS to watch a SCORE event and notice what happens to the trucks in the few turns that they take. There is so much roll in the chassis that it looks like they would tip over. The only thing that is saving them is the fact that they are using shocks with 3 feet of wheel travel. Now, one could argue the fact that the 35" tall tires that they use for ground clearance out in the desert would contribute greatly to the excessive chassis roll. However, this is compensated for by running zero-preload type shocks on these trucks. OK, now that we have that cleared up, let's compare apples to apples. If you really want to compare a race truck to a MX quad. Then you need to check out some CORR races. This is basically MX racing for trucks. The front ends on these trucks(either twin I-Beam or A-Arm) are very similar to quads. To be competitive in this type of racing, fast cornering is a must and it is very difficult to achieve that with an IRS truck. That is why most(if not all, I am not positive) don't use IRS on CORR vehicles. These trucks are so similar to MX quads it is really uncanny, they are setup using the same principles and they are driven using similar techniques. Now, for MX racing, I honestly do not believe that an IRS machine can be made as light as a solid axle without severely compromising strength. However, an IRS bike would definitely be less harsh on components than the solid axle, so maybe that is what Mr. Loscerbo is banking on. There is a lot of stress on the chassis of an MX bike especially in the rear end with the solid axle. The axle(which are generally longer than stock) have much more leverage during cornering. This forces the swingarm into a twisting motion at the pivot. This is the primary reason why 88-89 250R frames break at the swingarm mounting tabs. The 86-87 frames didn't use a solid swingarm so the twisting of the swingarm was less harsh on the frame in that area. Now, if you think about it IRS will alleviate this problem simply because that stress is placed on the shocks but at the expense of chassis roll. Unless you use shocks(like PEP ZPS)which work almost exclusively on the stiff main springs of the shock. Now, consider this. PEP ZPS shocks have an extremely low ride height to compensate for the excessive chassis roll of the bike. However, there is a trade off. The PEP shocks are so damn stiff in the rough that it is tough to hang on. You have to ride so fast to make them work correctly, that for most people they will hate the performance. Now, getting back to the real purpose for IRS; the system is supposed to smooth out the ride because it eliminates the straight axle. But because IRS requires very stiff shocks to counter chassis roll, it loses a lot of it's performance due to such stiff shocks. If you don't believe me, then go back to the article in question(there is a link somewhere in this thread) and read where the designer states that you have to ride really fast to make the bike work. For the average racer, they aren't going to ride fast enough to utilize the IRS. Now, let's look at the components and examine this from a weight point of view. The IRS uses a jackshaft, cv joints, a-arms and shocks. The combination of those parts seems(in my mind) to be a bit heavier than a straight axle. You have an extra shock, and an extra brake caliper. Plus a lot more moving parts that can break. From a reliability standpoint, this is not good for an MX machine. Now, I am not saying that solid axle suspension systems are invincible, because I know they are not. It just looks as though they would be a bit more reliable and cost effective. Cost effective especially from a standpoint of production and sales. An IRS high performance bike would be so expensive and I guarantee that an OEM won't be using ZPS shocks and chromoly tubing. OEM's know that of course these are higher quality, but from a cost perspective OEM materials and components cost half as much. Anyway, to sum up a really long post, I don't really agree with the IRS on a high performance bike. For straight line riding it is a great idea, but once you throw in high speed cornering the idea is shot. But hey you are all welcome to try and convince me otherwise.

 

Wow, it looks like 86atc250r and I are seeing eye to eye on the IRS issue. I just wish I would have read his post before I spent so long on mine!

 

I read your post, it was well thought out and worthwhile...