MEMORANDUM **

Jeffrey B. Workman appeals pro se from the district court’s judgment for defen*597dants in his 42 U.S.C. § 1983 action alleging excessive force, due process violations, and various state law claims. We have jurisdiction under 28 U.S.C. § 1291, and we affirm in part, and vacate and remand in part.

We review de novo the district court’s grant of summary judgment. Devereaux v. Abbey, 263 F.3d 1070, 1074 (9th Cir. 2001) (en banc). On appeal of a judgment following a court trial, we ordinarily review the court’s findings of fact under Federal Rule of Civil Procedure 52(a) for clear error. See Fed.R.Civ.P. 52(a)(6). Where, however, the district court fails to make findings of fact, we review whether the findings that are made are “sufficient to permit meaningful review.” FTC v. Enforma Natural Prods., Inc., 362 F.3d 1204, 1212 (9th Cir. 2004) (citation omitted); see also Swanson v. Levy, 509 F.2d 859, 861 (9th Cir. 1975).

1. The district court properly granted summary judgment in favor of the School District defendants on Workman’s excessive force claim because there was no evidence, and thus no dispute, that any of these defendants touched Workman or had any control over the police officers who allegedly touched Workman. See Jones v. Williams, 297 F.3d 930, 934-35 (9th Cir. 2002) (requiring a showing of personal participation in the alleged rights’ deprivation in order for a person to be liable under § 1983).

2. The district court properly granted summary judgment in favor of District Superintendent Dallabetta because Workman did not raise a triable issue of fact as to whether Dallabetta played an instrumental role in suspending Workman from school or denied him due process in connection with the suspension. See Preschooler II v. Clark County Sch. Bd. of Trustees, 479 F.3d 1175, 1182 (9th Cir. 2007) (holding that a supervisor is liable for the acts of his subordinates only if the supervisor participated in or directed the violations, or knew of the violations and failed to act to prevent them); Goss v. Lopez, 419 U.S. 565, 579-84, 95 S.Ct. 729, 42 L.Ed.2d 725 (1975) (describing minimal due process requirements for students suspended for 10 days or less).

3. The district court also properly granted summary judgment in favor of the Sheriffs Department defendants on Workman’s state law claims for false imprisonment, malicious prosecution, and wrongful arrest because Workman failed to show that the officers lacked probable cause for the arrest. See Gasho v. United States, 39 F.3d 1420, 1427 (9th Cir. 1994) (explaining that under Arizona law probable cause is an absolute defense to a claim of false arrest and imprisonment); Bradshaw v. State Farm Mut. Auto. Ins. Co., 157 Ariz. 411, 758 P.2d 1313, 1319 (1988) (noting that a malicious prosecution claim requires proof that a legal action was brought without probable cause).

4. The district court entered judgment in favor of the Sheriffs Department defendants after a court trial on Workman’s excessive force claim. The substantive inquiry on such a claim is whether the force that was used to effect a particular seizure was reasonable. See Forrester v. City of San Diego, 25 F.3d 804, 806-08 (9th Cir. 1994). Here, however, the district court failed to make any findings of fact on this issue. We have “consistently interpreted Rule 52(a) ... to require that findings be ‘so explicit as to give the appellate court a clear understanding of the basis of the trial court’s decision....’” Sumner v. San Diego Urban League, Inc., 681 F.2d 1140, 1143 (9th Cir. 1982) (quoting Irish v. United States, 225 F.2d 3, 8 (9th Cir. 1955) (first ellipsis added) (other citations omitted)). Sumner further teaches that “[w]e *598could, of course, affirm the district court without the need for further findings if the plaintiff could not prevail under any possible interpretation of the evidence.” Id. That, however, is not the case here. On the factual issue of whether any excessive force was used, e.g., whether the arresting officer placed the handcuffs on Workman too tightly, the evidence is controverted. Workman testified that the handcuffs were placed too tightly, that he complained of pain, and that he suffered bruises. On the other hand, other witnesses testified that there was no use of excessive force against Workman. Because we are unable fairly to review the court’s judgment without the benefit of any findings of fact on this issue, we must remand this claim for the making of such findings.1

5. The district court properly entered judgment in favor of the School District defendants on Workman’s due process claim because the relevant evidence adduced at trial is undisputed, and shows that Workman was afforded adequate notice and an opportunity to be heard regarding his five-day suspension. See Goss, 419 U.S. at 581-82, 95 S.Ct. 729.

6. We decline to address Workman’s challenges to the evidentiary rulings he now contests because they have been waived by his lack of objection to those rulings in the district court. See Drummond ex rel. Drummond v. City of Anaheim, 343 F.3d 1052, 1058 n. 5 (9th Cir. 2003) (concluding that plaintiff waived challenges to evidentiary rulings by failing to object).

7. Contrary to Workman’s contentions, the district court did not abuse its discretion by examining witnesses or addressing Workman in its closing remarks following the bench trial. See Price v. Kramer, 200 F.3d 1237, 1253 (9th Cir. 2000) (explaining that trial judge may participate in the examination of witnesses for the purpose of clarifying the evidence and concluding that plaintiff’s allegations regarding judge’s conduct were unfounded). To the extent Workman contends that the trial judge’s conduct exhibited bias, we reject that contention. The facts do not even approach a showing of “actual bias” necessary to sustain reversal.2 See id. at 1252.

8. Finally, Workman contends that costs should not have been taxed against him in favor of the Sheriffs Department defendants because of his modest financial circumstances. Because we partially vacate the judgment in favor of the Sheriffs Department defendants, we also vacate the award of costs in their favor as the prevailing party. The district court will enter a new judgment in favor of the prevailing party after making its findings of fact on the excessive force claim. The prevailing party will have the opportunity to seek costs at that time.3

*599The judgment of the district court in favor of the School District defendants is AFFIRMED. The judgment of the district court in favor of the Sheriffs Department defendants and the accompanying costs order are VACATED and REMANDED as to Workman’s excessive force claim, but the judgment is otherwise AFFIRMED. Each party shall bear his or its own costs on appeal.